Quarterly Journal of the Royal Meteorological Society
Q. J. R. Meteorol. Soc. (2011)
Nocturnal low-level jet and ‘atmospheric streams’ over the rain
shadow region of Indian Western Ghats
T. V. Prabha,* B. N. Goswami, B. S. Murthy and J. R. Kulkarni
Indian Institute of Tropical Meteorology, Pashan, Pune, India
*Correspondence to: T. V. Prabha, Indian Institute of Tropical Meteorology, Dr Homi Bhabha Road, Pashan, Pune
411008, India. E-mail: [email protected], [email protected]
Spatial and temporal characteristics of a nocturnal low-level jet (LLJ) on the east side
of the Western Ghat mountain range over India’s west coast and processes leading
to the formation of the jet are discussed. The boundary-layer jet has a regional
scale extent, as revealed by high-resolution Advanced Research Weather Research
and Forecasting (ARW) model simulations, and contributes to the formation of
‘atmospheric streams’ of water vapor over the selected land regions. Simulations
indicate that the formation of LLJ is mainly attributed to the baroclinicity of
the valley atmosphere due to the gently rolling terrain, which is assisted by the
persistence of an unstable residual layer above the developing stable boundary layer
in the valley and cooling over the slopes. Prior to the formation of LLJ, the boundary
layer is dominated by deep roll circulations. The LLJ followed a gust front zone
associated with a mountain wave. The low-level flow below the jet is decoupled from
the upper-level flow as a result of strong vorticity below the jet and suppression of
turbulence at the jet core. A conceptual model for the boundary layer interactions,
dynamics of the mountain wave, LLJ, etc. are proposed for Western Ghat region.
c 2011 Royal Meteorological Society
Copyright Key Words: low-level jet; nocturnal boundary layer; moisture transport; atmospheric streams; mountain
waves; gust front; rolls; residual layer
Received 18 May 2010; Revised 12 February 2011; Accepted 3 March 2011; Published online in Wiley Online
Library
Citation: Prabha TV, Goswami BN, Murthy BS, Kulkarni JR. 2011. Nocturnal low-level jet and ‘atmospheric
streams’ over the rain shadow region of Indian Western Ghats. Q. J. R. Meteorol. Soc. DOI:10.1002/qj.818
1.
Introduction
Low-level jets (LLJs) play a crucial role in the transport and
mixing of water vapor and pollutants. The LLJs have different
spatial and temporal scales and are caused by a variety of
mechanisms (Blackadar, 1957; Holton, 1967; Garrat, 1992).
Some of the widely reported LLJ formation mechanisms
include inertial oscillation (Blackadar, 1957; Bonner, 1968),
change in surface and terrain characteristics and diurnal
cycle as in the case of a coastal jet (Smedman et al., 1993,
1995), slope and valley wind (King and Turner, 1997),
large-scale baroclinity by sloping terrain (Li et al., 1983;
Gerber et al., 1989), severe weather conditions (Arrit et al.,
1997), frontogenesis during frontal passages (Whiteman
et al., 1997; Banta et al., 2002; Lundquist, 2003), geostrophic
c 2011 Royal Meteorological Society
Copyright adjustment as in the case of a barrier jet (Parish, 1983; Li and
Chen, 1998), etc. or a combination of these mechanisms.
Nocturnal decoupling of the flow from the surface and an
imbalance between the pressure gradient and Coriolis forces
introduces an inertial oscillation of winds and is considered
to be a major contributing factor for LLJ formation.
Large-scale features of the synoptic jet over the Indian west
coast are studied extensively in the context of the ‘Somali jet’,
which is a climatological feature during the pre-monsoon
and monsoon seasons (Joseph and Raman, 1966; Findlater,
1969). This jet is one of the major characteristics of the Indian
monsoon, with a ‘cross-equatorial current from the southern
Indian ocean to the central Arabian sea’ (Krishnamurthy
et al., 1976), providing a moisture supply over the land
regions, fuelling convection and rainfall (Sikka and Gadgil,
T. V. Prabha et al.
1980). Findlater’s jet characterizes considerable spatial and
temporal variations and oscillations (Goswami et al., 1998)
on an intraseasonal scale coupled to the active and break
periods of monsoon. Findlater’s jet, observed below 700 hPa,
is associated with strong horizontal and vertical wind shear,
but deviates from the definition of a boundary-layer LLJ as
it is typically found above 1 km, in association with synoptic
scale flow. Boundary-layer jets over the Indian region are less
explored. Boundary-layer LLJs are low-level wind maxima
observed below 1 km typically as a nocturnal phenomenon
where shear-induced turbulence predominates and the jet
nose often coincides with nocturnal inversion layers. LLJs
transport moisture (McCorcle et al., 1988; Mitchell et al.,
1995; Parker et al., 2005) and pollutants (Beyrich et al.,
1994; Corsmeier et al., 1997; Banta et al., 1998; Seaman and
Michelson, 2000; Bao et al., 2008) from far-away places and
often contribute to surface-level concentrations by vertical
mixing. Vertical mixing is accomplished by an upside-down
boundary-layer structure attributed to the shear generation
of turbulence at the jet height (Mahrt and Vickeres, 1999;
Banta et al., 2002; Mahrt and Vickers, 2002; Prabha et al.,
2007, 2008; Karipot et al., 2008).
The presence and characteristics of nocturnal LLJ on the
east side of the Indian Western Ghats (WG) is not documented, although flow dynamics and moisture transport in
this region are expected to play a significant role in the premonsoon thunderstorms and also contribute to the diurnal
cycle of convection and precipitation. The need to understand LLJ in this region is manifold because of its importance
in several applications such as weather prediction, weather
modification, moisture and pollution transport assessment,
wind power assessment and aviation safety. This region is
also influenced by the transport of dust from the Arabian
mainland and maritime aerosols. The knowledge of moisture distribution along with that of the aerosols will help
determine the areas of potential convective activity in this
rain shadow region. This information is also instrumental
for an ongoing Cloud Aerosol Interaction and Precipitation
Enhancement EXperiment (CAIPEEX) in the region.
The current study is intended to explore the nocturnal
LLJ formation mechanism and evolution during a dry
period before the pre-monsoon thunderstorm formation.
The main emphasis is on the boundary layer characteristics
leading to the formation of the jet and associated features.
The investigation is based on numerical simulation and
observational data analysis. An attempt is also made to
introduce a conceptual model for the formation and
evolution of the LLJ.
Figure 1. Topographic height in model domains 2 and 3. (PUN, Pune on
the east side, and DAP, Dapoli on the west side of WG mountain range, are
shown. Four valleys in the domain are marked as A, B, C and D.
The Advanced Research Weather forecasting model
(WRFV3.1.1) was configured with three two-way nested
domains (30, 7.5 and 1.875 km resolution). Two nested
domains are presented in Figure 1, showing the surface
elevation that is resolved in the model at respective
resolutions. The lowest model level is kept at approximately
7 m above the surface and the top layer was kept at 19 km.
The boundary layer was parameterized with the Yonsei University (YSU) non-local PBL scheme. The lower part of the
model atmosphere (<1.6 km) was represented by 15 vertical
layers with varying vertical resolution. The PBL scheme was
coupled with the Monin Obukhov (MO) similarity scheme
for the surface layer and Noah land surface scheme (Chen
and Dudhia, 2001) using the updated MODerate Resolution
Imaging Spectroradiometer (MODIS) land use dataset. The
innermost domain has a cloud resolving model and the
outer domains are parameterized with the Kain and Fritsch
cloud scheme. The WRF double moment (WDM-6) cloud
microphysics scheme was used with modified boundary
conditions. The new parameterization of Klemp et al. (2008)
is used for damping vertically propagating waves in the
upper layers. The model is initialized with final reanalysis
products (FNL) from the National Center for Atmospheric
Research (NCAR) and outer boundary updates are done
every 6 hours. Hourly model output is used in the analysis.
Simulations are carried out for the period from 0000 UTC
(5.30 IST; IST = 5.30 + UTC) 12 May until 1200 UTC
(17.30 IST) 14 May and hourly model outputs are analyzed
to investigate the diurnal signature of the LLJ.
The observational data used in this study include hourly
surface observations from India Meteorological Department (IMD), radiosonde observations carried out at Pune
(73.86◦ E, 18.5◦ N, 570 m above mean sea level (msl), surrounded by hilly terrain with elevations reaching up to
710 m). The radiosonde observations were carried out as part
of the pilot observations for Cloud Aerosol Interaction and
Precipitation Enhancement EXperiment (CAIPEEX). Sodar
observations of three-dimensional wind components at the
same location are also used in the study. The sodar (Sameer
inc., India) was operated at a frequency of 1.8 kHz. Observations of all three wind components were averaged for 5 min
intervals from 19 s sampling with an accuracy of 0.1 ms−1 .
Observations were taken at 30 m above the surface to 900 m,
at 30 m vertical resolution and were used in the study.
A wavelet analysis using the Morlet wavelet was applied
on the sodar wind observations to investigate periodicities in
the wind components and associated variances. Continuous
sodar data for only 2 days were available during the study
period, which was insufficient for looking at oscillations
with longer periodicity (more than a day). An extended
study period of 5 days’ data (30 May to 3 June) with similar
dry conditions was used to examine periodic fluctuations
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
2.
Data and method
Nocturnal Low-Level Jet Over Indian Western Ghats
A model verification is done with the help of India
Meteorological Department (IMD) surface observations at
Pune and simulated values from MERRA reanalysis products
(Figure 2) for 2 days. Although the diurnal temperature
variation is reproduced well in the ARW simulation, the
model results show lower maximum temperature compared
to observations. MERRA temperature shows a higher
maximum temperature. Hourly nocturnal temperature
from ARW has a slight warm bias (≤2◦ C), in contrast to
MERRA, which shows a cold bias during the night. A warm
bias at night points to less radiative cooling and more downmixing of warm air from above. A comparison of ARW water
vapor mixing ratio showed a dry bias (2–4 g kg−1 ) against
most of the hourly observations (Figure 2(b)). MERRA
showed reasonable agreement with water vapor mixing ratio
observations during the daytime. Night-time differences
exceed 2 g kg−1 . Wind speed comparisons show considerable
variations in the observations compared with the simulation
(Figure 2(c)). The increase in daytime wind speed on the
second day would mean that there is more turbulence.
However, temperature also increased in association with
this. The nocturnal surface level wind was always above
2 m s−1 , both in the model and observations. The wind
direction showed a clear diurnal variation, changing from
northwesterly during the day to westerly during the night.
MERRA does not reproduce this feature on the first day
of the simulation; however, it gave a good comparison
on second day. The backing of wind with time is noted
here.
The differences in the temperatures and mixing ratio
between the models could be examined with the help of a
comparison of energy balance comparison from ARW and
MERRA (Figure 3). The incoming southwest radiation flux
from the ARW model is slightly higher than that of MERRA
(Figure 3(a)); however, there is no significant difference
between the sensible heat flux of both models (Figure 3(b)).
Sensible heat flux stayed positive throughout the night due
to weak convective conditions in the model. It may also be
noted that the land use is ‘urban and built-up land’ and soil
type is ‘clay loam’. This behaviour is possible because of these
conditions. However, it needs further investigations with
direct observations of energy balance and soil characteristics.
The ground heat flux from the ARW model is large compared
to the MERRA, during both day and night. The ground
heat flux differencesduring the midday period (less heat is
stored in MERRA) are consistent with higher temperatures
in MERRA. It should also be noted that the ARW model
has predicted a lower temperature on the first day, which
is mainly due to a higher ground heat flux on this day
compared to the second day. The latent heat flux played a
minor role in the energy balance (MERRA represented low
latent heat flux <60 Wm−2 compared to the dry conditions
in WRF), the primary influence on the temperatures being
from partitioning of energy into ground heat flux.
The model results at 1130 IST on 13 May 2009 are compared with the radiosonde profile observed over Pune
(Figure 4) and MERRA reanalysis profiles. The simulated
profiles and observations are generally comparable; except
that a jet observed above (at 3 km) the boundary layer
was at a higher elevation in the observations. This jet is an
extension of the Findlater jet. Between the layers 3.5 and
7 km, wind direction changes to northeasterly, indicating
advection from land regions. The radiosonde wind ascent
throughout the 17 km layer lasted approximately 1 hour
(starting at 1130 LST) and showed several oscillations during
the upward flight, indicative of the gravity waves. The
radiosonde flight was conducted during the development
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
(greater than a day) in vertical velocities. MODIS
observations of precipitable water at 5 km spatial resolution
were used in the study. The MODIS level II precipitable
water data at 11.30 IST (Indian Standard Time) were used
to compare with the vertically integrated precipitable water
(IPW) from model simulation in the inner domain.
In addition to the observational data, the Modern
Era Retrospective-analysis for Research and Applications
(MERRA; http://gmao.gsfc.nasa.gov/research/merra/intro.
php) reanalysis data (with a resolution of half a degree
along latitude and two-third degrees along longitude)
was used to compare high-resolution Advanced Research
Weather Research and Forecasting (ARW) results against a
coarse model and also for energy balance comparisons as
observational energy balance data are not available. Hourly
MERRA data for 2 m level air temperature and mixing ratio,
10 m level winds, radiation and energy balance components,
and vertical profiles of wind and temperature at 11 IST from
grid points adjacent to radiosonde observation locations are
used.
The LLJ is identified in this study with a maximum wind
at a certain height with decreasing wind speeds of at least
2 m s−1 both below and above that height (Andreas et al.,
2000). Only jets identified below 1.5 km are considered to
be LLJ in this study. Other higher elevated jets present in the
study domain are considered as an extension of the Findlater
jet and are not given emphasis in this paper.
3.
General characteristics of the study period
The study period chosen for detailed analysis was 12–14
May 2009, which was a dry period over the study
region with northwesterly/westerly mid-level flow and
dry easterly/northeasterly flow in the upper layers (above
5 km), characterizing pre-monsoon dry condition before the
thunderstorm events. The northwesterlies were associated
with the high-pressure system situated over the Middle
Eastern region. The subtropical jet is a characteristic feature
of northern latitudes over the Himalayan region. Wind fields
over the study region veered with height, indicating warm
air advection from the northern/northeastern drylands.
The study domain shows considerable variation in
topographic height (Figure 1(a)) from a few meters
to a kilometer above msl. There are four main valleys
represented in domain 3 (indicated as A, B, C, and D in
the figure), which are oriented westnorthwest to southeast
on the eastern slopes of (WG). The rainfall distribution in
this region is quite heterogeneous, with heavy precipitation
concentrated on the west side of the mountain and a
reduction to 70–80% (of the windward side rainfall) within
a distance of 100 km on the east side of the Ghats (Gunnell,
1997). Seabreeze events typically noted over the western side
of the mountain ranges do not usually penetrate inland to
distances beyond 200 km. A line of clouds is typically noted
near the mountain summit along with an area parallel to the
coastline without clouds on the upwind area, indicative of
the presence of mountain wave and associated subsidence.
3.1.
ARW, observations and MERRA comparisons
T. V. Prabha et al.
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Figure 2. Comparison of simulated and observed 2 m level temperature (a), 2 m level water vapor mixing ratio (b), 10 m level wind speed (c) and wind
direction (d) at station Pune. Respective data from MERRA reanalysis are also shown.
SW_WRF
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Figure 3. Comparison of WRF simulated and MERRA incoming short-wave (SW) and radiation at the surface (a), sensible (H) heat flux (b), ground
heat (G) flux (c), and long-wave (LW) radiation (d) at the surface for Pune.
of the convective boundary layer. A comparison of water
vapor mixing ratio profiles indicated that the dry surface
bias in the water vapor content (Figure 4(b)) pertained
not only to the surface, but extended up to 6 km
(Figure 4(b)). In addition, the water vapor mixing ratio
(Figure 4(b)) in the updrafts/weak downdrafts (Figure 4(c))
was higher than that over downdraft regions, indicative of
water vapor transport through updrafts. Strong downdrafts
(Figure 4(c)) are characterized by dry air (Figure 4(b)).
It should be noted that both profiles (in the updraft
and downdraft) deviated considerably from the radiosonde
observations.
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
Nocturnal Low-Level Jet Over Indian Western Ghats
Wind direction
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Figure 4. Comparison of vertical profiles of MERRA and WRF model horizontal wind speed profile (a) and water vapor mixing ratio (b) derived from
radiosonde observations (RS) at Pune on 13 May 2009 at 1100 LST. Model-derived vertical velocity (c) and corresponding mixing ratios in the updraft
(WRF Updraft) and downdraft (WRF downdraft) are also shown. Wind direction from radiosonde is given in (a) top axis. This figure is available in
colour online at wileyonlinelibrary.com/journal/qj
3.2.
Comparison with sodar observations
4. Temporal evolution of the boundary layer leading to
jet formation
A comparison of sodar observations at Pune and model
results is shown in Figure 5. Weak boundary layer winds
are noticed both in the sodar observations and in the model
(Figure 5) during daytime. Sodar observations indicated
a nocturnal low-level jet with a mean height of 200 m
(Figure 5(a)). Jet height and speed were variable and
showed a close resemblance to the model simulated jet
(Figure 5(b)). Sodar wind profiles at 15-minute intervals
are presented, which showed a clear indication of the nonstationary behaviour of the jet, with considerable changes
in the height and speed of the jet. Simulated results are
presented for hourly intervals and appear smoother than
the observations; however, general characteristics of the jet
such as height and speed, time of onset and jet decay
are simulated reasonably well. It is apparent that the
model has simulated some of the general characteristic
of the observed LLJ. These characteristics over a regional
scale/horizontal extent of the jet will have a significant
effect on the moisture distribution over the region. The
temperature and mixing ratio variations in the simulation
showed development of a strongly stable layer below the jet
and a strong vertical gradient of water vapor (Figure 5(b)).
Daytime boundary layer development showed indications
of alternating updrafts and downdrafts that became stronger
and had a longer period until the jet initiated at 18.30 IST
on both days examined here (Figure 5(c)). The long-period
updrafts/downdrafts are associated with the onset of the gust
front. Indications of this gust front are noted at 1500 IST on
12 May and 1600 IST on 13 May, with a strong updraft on
both the days. The updraft/downdraft pairs become weaker
and disappear as the LLJ sets in, indicating stable boundary
layer development. Overall, WRF model comparison with
observations indicated better agreement with MERRA and
suitability for further detailed analysis.
The discussion in this section is based on intricate details
of the flow images of vertical velocities derived from
simulations at 700 m and 1500 m above msl (Figure 6)
and vertical transects along the Pune latitude (Figure 7). The
characteristics of the convective boundary layer, its evolution
and decay, leading to the formation of the nocturnal jet, are
discussed. The convective boundary layer at 1330 IST is
dominated by boundary layer roll circulations off the coastal
areas over the Arabian Sea (Figure 6(a) and (b) shows a
horizontal distribution of vertical velocity at 700 m and
1500 m and Figure 7(a) shows a vertical transect at 1330
IST). A front is noticed along the coastal areas 73–74◦ E
with high vertical velocities, apparently due to the sea breeze
penetration inland. During the development of the daytime
boundary layer (Figure 5(a)), pairs of strong updrafts and
downdrafts develop over the eastern slopes and inland areas
and transform to roll-type circulations, which are elongated
and aligned parallel to the wind (Figure 6(a) and (b)).
Convection over inland regions is dominated by closely
spaced updrafts and downdrafts which are deeper compared
to that over the slopes (Figure 7(a)); at 1330 IST, the
valley boundary layer has stronger upward vertical velocities
(>1 m s−1 ) and greater vertical mixing (reaching 3.5 km
in height) over a larger volume, unlike over the slopes and
hill tops, with updraft/downdraft pairs reaching 2–2.5 km.
A sea breeze front (note the north–south orientation of this
front parallel to the coastline) is noticed along 74◦ E, but has
not advanced inland. The convective rolls that occupied the
valley boundary layer inhibited the progression of this sea
breeze front. Rolls are responsible for the vertical mixing of
the water vapor and pollutants and ventilating the valley.
At 1830 IST, the organized convection began disintegrating into more disorganized patterns which are more
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
T. V. Prabha et al.
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Figure 5. Time–height cross-section of wind speed (m s−1 ) from sodar located at Pune (a) and wind speed from simulations (b) in color shades, potential
temperature (continuous lines) and mixing ratio (dash-dotted lines). Time–height cross-section of vertical velocity (m s−1 ) from sodar located at Pune
(c) and from simulations (d). Arrows indicate onset of the gust front.
prevalent over the valley atmosphere (Figure 6(c) and
(d)). This was also observed with the help of photographs
of clouds taken over Pune. A comparison of those
cloud patterns and temperature patterns is provided at
http://www.tropmet.res.in/∼majfiles/QJRMS thara/cloudpatterns-valley.pdf. Meanwhile, the vertical cross-section
shows that there are deeper and wider convective cells
(Figure 7(b)) compared to that during 1350 IST. The rolls
subsequently disappear as a stable layer is formed close to
the surface, as revealed in Figure 6(e) at 2030 IST. The roll
circulations are replaced by wide updrafts along the valley
bottom and downdrafts over the slopes and hill tops. A
residual layer is present above the stable layer (Figure 7(c)
and (d)), with characteristics similar to the disintegrated
convective boundary layer, with dominating updrafts;
however, rolls are not noticed in that layer.
A frontal zone noticed along the offshore area has
progressed further inland by 2030 IST (Figure 6(e)).
The WG mountain range is a strong barrier to the
landward progression of sea breeze, initiating mountain
waves (Figure 7(b)) that propagate upwards (Figure 7(c)),
characterizing strong updrafts and downdrafts (>1.5 m s−1 ).
Figure 7(c) shows a classic picture of the mountain wave
propagation. Such a mountain wave formation has been
coined an ‘evening wave’ by Roper and Scorer (1952). There
have been two seminal studies (Sarker, 1965; De, 1971)
investigating the mountain waves over WG using theoretical
approaches, which emphasize that mountain waves over WG
can have a wavelength of 25–75 km; this is also true in our
study. However, these mountain waves were not explored in
relationship to its diurnal course. It may be noted that the
signature of this mountain wave appears earlier; at 1830 IST,
before the convection ceases, a wave gradually propagates
upward, which appears as a standing wave. However, as the
wave moves inland (as the convection stops), the wave also
propagates to greater heights (Figure 7(c)). The mountain
wave propagation excites horizontally propagating gravity
waves upstream, which reach down to the lower boundary
layer (Figure 7(c) and (d)). This causes momentum transfer
in the residual boundary layer (RL) and oscillations in the
stable boundary layer (SBL). The initiated mountain wave
is seen as a gust front at the surface and in the sodar
measurements. The SBL wind speeds behind this front align
with strong updrafts (>2 m s−1 ), forming an LLJ, which
flows downslope, carrying more moist air.
Maximum LLJ speed noted in the simulation was
10–14 m s−1 along the eastern slopes of WG. The gust front
moved at a speed of 100 km h−1 ; it was situated along 74◦ E at
1830 IST and shifted to the valley bottom (77◦ E) at 2230 IST.
This may be viewed as the propagation of mountain wave to
inland locations (see movie file llj.mpg – refer to Appendix
for details of available files). Simulations indicate that the
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
Nocturnal Low-Level Jet Over Indian Western Ghats
(a)
(b)
(c)
(d)
(e)
(f)
Figure 6. Spatial distribution of simulated vertical velocity at three times (1330, 1830, 2030 IST; 800, 1300, 1500 UTC) at 700 m and 1500 m above msl
on 13 May. (Dark gray indicates vertical velocities >−1 m s−1 and white indicates show maximum updrafts > 1 ms−1 ). Convergence lines with high
vertical velocity are noted with dashed lines (bottom right).
LLJ is formed behind the front, follows the gust front inland
and is noticed over the entire model domain. Boundary
layer clouds (BLCs) noticed offshore bring supersaturated
air to the level where the jet is initiated (Figure 8). There
is a second line of clouds along the mountain tops, which
is closely associated with the forced lifting. However, the
low-level flow is channeled along the coastline, making a
barrier between the two lines of BLCs. This cloud-free region
is also characterized by descending motions (downdrafts
associated with the mountain wave). Upper-level flow is
north/northeasterly, showing a veering of wind with height
(thick arrows).
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
5.
Spatial distribution of LLJ
Analysis of model domain 2 with a horizontal resolution of
7.5 km is used in this analysis. Figure 9 shows a temporal
variation of the jet speed (a), jet height (b), 2 m level
T. V. Prabha et al.
Figure 7. Vertical distribution of simulated vertical velocity at four times – (a) 1330 IST, (b) 1830 IST, (c) 2030 IST, and (d) 2230 IST on 13 May – above
msl. (Black/dark gray indicates downdrafts >−1 m s−1 and white indicates show maximum updrafts > 1 m s−1 ). This figure is available in colour online
at wileyonlinelibrary.com/journal/qj
water vapor mixing ratio (c) and Brunt–Väisälä frequency
for stability (d) at different longitudes. There is a clear
indication that LLJ develops early along the coastal areas
only along the east side of WG and propagates inland at a
speed of 10 m s−1 or more (Figure 9(a)). The initiation of
LLJ over locations 300 km inland are delayed by 3–4 hours.
Once initiated, the LLJ lasted until the following day’s
insolation heated the eastern slopes of the WG. The height
of the jet also varied between 200 and 600 m depending
on location (Figure 9(b)). The 2 m level water vapor
mixing ratio (Figure 9(c)) increased at the same time as
jet initiation and seemed to be related to LLJ evolution.
The low-level atmospheric stability showed more stable
conditions developing early at night along locations closer to
the coastline and weak stability over the valley (Figure 9(d)).
For a given time, stability increased significantly closer to the
slopes and the valley bottom was more unstable compared
to the slopes. This can be visualized from Figure 9(e)
showing the vertical cross-section along the Pune latitude.
The presence of LLJ is clearly noted to follow the terrain.
The temperature distribution showed strong gradients in
temperature, leading to the differences in stability conditions
discussed earlier.
(Figure 10(b)) at 1130 IST showed significant influence
from the topographic features. The valley points showed
enhanced IPW compared to summit points. There are four
main valleys in domain 3 (marked A–D in the figure) and
four undulating spatial patterns of IPW are noticed both in
the observations and model simulations. The IPW observations and simulations show enhanced values along the
coastal areas. Over the Arabian Sea there are some pixels
with very low IPW, attributed to cloud. Moisture transport
to the east side of the WG is blocked by high mountain
ranges. However, the spatial demarcation in the IPW is
noted across a very narrow range of distances (approximately 60 km inland). The orographically forced winds,
however, play a crucial role in the transport of moisture to
inland locations through relatively lower elevated ‘gaps’ in
the mountain barrier. There are three gaps as shown in the
inner domain image of water vapor mixing ratio at 700 m
above msl (Figure 10(b); black color indicates the terrain
intersections and dark gray to white indicates high to low
water vapor mixing ratio); and moisture is transported to
inland areas through gaps in the barrier as ‘streams’. These
streams are sharper in the MODIS data than in the model
output. The model output is derived from integrated water
vapor mixing ratio over the whole depth of 20 km vertical
6. Moisture transport by the LLJ
domain. Another important aspect is that offshore convection associated with the boundary layer clouds (noticed as
The spatial distribution of Integrated Precipitable Water alternating gray and white patches over the Arabian Sea)
(IPW) from both the simulation (Figure 10(a)) and MODIS increase the moisture at this elevation through saturated
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
Nocturnal Low-Level Jet Over Indian Western Ghats
Figure 8. The structure of LLJ and an associated gust front ahead of the jet (jet is shown with isosurface of 10 m s−1 ). Horizontal streamlines
through a coastal location (star) at different vertical levels up to 6 km are shown. Arrows indicate direction of streamlines. Clouds along the
coastline and offshore are also shown with white and gray isosurfaces. Thin vectors correspond to lower levels and thick vectors correspond to higher
elevations. Location of gust front inland is shown with dashed lines. A temporal evolution of these features is provided in the movie file located at
http://www.tropmet.res.in/∼majfiles/QJRMS thara/llj.mpg
updrafts. The increase in moisture over inland locations
can also be identified by an increase in IPW in the MODIS
data and in the model results (e.g. small lower-atmospheric
‘water streams’ analogous to atmospheric rivers (Ralph et al.,
2004) of water vapor). Interestingly these patterns are also
noticed in the MODIS aerosol optical depth (not shown),
indicating the aerosol accumulation in the valley points.
Further details of this important aspect are presented for
hourly images of water vapor distribution at 1 km above msl
made from the ARW model output as a movie file accessible at http://www.tropmet.res.in/∼majfiles/QJRMS thara/
streams.mpg. In the presence of LLJ, water streams appear
well along the valleys and four main streams in the study
area are found.
7.1.
Thermal wind
One potential mechanism for formation of LLJ could be
associated with the horizontal temperature gradient between
valley and slopes. The thermal wind equation (Holton,
2004) could be used to explain the formation of the jet
corresponding to this scenario. The vertical shear in the
geostrophic v component could be expressed with the help
of horizontal temperature gradients as follows:
∂vg
g ∂T
=
∂z
fT ∂x
(1)
The WG is considered to produce upstream blocking effects
offshore (Grossman and Durran, 1984). In the lee of this
mountain range, various complex interactions could take
place. A preliminary analysis of various effects such as
the influence of temperature gradients between the valley
and slopes, influence of blocking effects with the help of
Froude number, slope wind influences, and effect of inertial
oscillation are explored in this section.
where f = 2 sin ; g is acceleration due to gravity, is
angular velocity vector, is latitude, T is temperature,
and x is distance between two east–west points (74◦ E
over the slopes, 79◦ E over the valley). The temperature
gradient between the slopes and the valley atmosphere, jet
speed and height, and vertical shear in the geostrophic v
wind component are presented in Figure 11(a). Jet speed
is enhanced when the temperature gradient between the
valley and the slopes increases, and a strong geostrophic
shear (>0.015 s−1 ) is noticed in correspondence with LLJ
speeds exceeding 8–12 m s−1 over the Pune location. Two
important aspects to be noted are that the air column
over the slopes cools faster than the valley atmosphere. A
convective residual layer remains over the valley atmosphere
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
7.
Mechanism of LLJ formation
T. V. Prabha et al.
Figure 9. Variation of jet speed (a), jet height (b), 2 m level mixing ratio (c) and Brunt–Väisälä frequency (d) a vertical transect across Pune latitude
at 18 IST (e) on 13 May 2009 for wind speed (color), water vapor mixing ratio (dashed contour), perturbation potential temperature (contour) in (e);
hatched area shows terrain intersection.
Figure 10. Column integrated precipitable water (cm) from ARW simulation (a) and from MODIS Level II data (b) on12 May at 525 UTC (10.45 IST).
A movie file is provided at http://www.tropmet.res.in/∼majfiles/QJRMS thara/streams.mpg which shows evolution of water vapor streams.
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
Nocturnal Low-Level Jet Over Indian Western Ghats
(a)
Dq
shear
Dqs
UJ
HJ
12
10
-10
8
6
-15
4
-20
600
400
200
0
10
16
1.5
1
7
wind speed
13
20
4
Froude number
8
9
8
1.0
7
6
4
2
0.5
6
0
10
13
20
4
layer averaged u velocity
10
0.5
8
0.4
6
0.3
4
0.2
2
0.1
DT (K h–1)
Velocity (m s–1)
(c)
16
1
7
downslope velocity
Upstream speed (m s–1)
N
x 1e-5 N (s–1)
Froude number
800
2
-25
(b)
1000
Jet height (m)
-5
Jet speed (m s–1)
x10–3 Shear (s-1) , Dq (K)
0
0.0
0
10
16
1
Time (IST)
7
13
20
4
Figure 11. Variation of potential temperature gradient between the slopes (74◦ N) and valley (79◦ N), the slopes and valley atmosphere at 850 m
(θ = θslope − θvalley 850 mb ), slope and valley bottom (θs = θslope − θvalley bottom ) LLJ speed (UJ ; speed at jet maximum) and LLJ height (HJ ; height
of jet maximum) are presented in (a). Temporal variation of upstream wind speed, Brunt–Väisälä frequency and Froude number are presented in
(b). Slope wind and layer averaged velocity vector, heating rate are presented in (c) over Pune location. This figure is available in colour online at
wileyonlinelibrary.com/journal/qj
as the valley atmosphere cools slowly. A combination of these
two effects leads to strong east–west temperature gradients.
As Figure 11(a) suggests, increase in v wind shear due to
the thermal wind could explain the existence of the jet.
The increase in v shear adds a northerly component to the
mean flow, which leads to northwesterly LLJ (see movie file
streams.mpg for wind direction changes during the jet).
7.2.
Blocking effects of mountain
The Froude number (defined as the ratio of upstream wind
speed and the product of Brunt–Väisälä frequency and
the width of the mountain barrier ≈ 100 km) was less
than one, estimated for a 2-day period (Figure 11(b)).
This indicates that on the west side of the mountain
blocking effects due to high terrain dominate. This also
suggests that flow separations and channeling effects are
possible over the west coast. A diurnal oscillation in
the Froude number (Figure 11(b)) is due to stability
changes associated with the diurnal cycle. Low values
(<1) of Froude number do not support amplification
of the lee waves and their horizontal propagation. The
kinetic energy is rather insufficient for excessive lifting and
amplification of downstream propagating mountain waves,
rather supporting the vertical propagation of mountain
waves.
c 2011 Royal Meteorological Society
Copyright 7.3.
Slope flow effects
Another possible cause for the LLJ formation is slope flow
on the eastern side of WG. McNider (1982) suggested
an oscillatory solution for the slope wind, which depends
on the rate of temperature, vertical temperature gradient
and terrain slope. Slope wind analytical formulation from
McNider (1982) is used to calculate the slope wind
component in the layer below 600 m. The downslope velocity
component (us ) for frictionless conditions is given by
us = (Lc /γ sin α)(1 − cos ξ t)
(2)
where, ξ 2 = (gγ /θ0 ) sin2 α, Lc is rate of change of
temperature, α is slope, γ is lapse rate and θ0 is ambient
temperature. The solution does not explain the strong winds
that are noted during the LLJ (Figure 11(c)) on day 1;
however, it produces nearly half the wind speed of the LLJ.
This result suggests that slope winds are important in the
nocturnal boundary layer but are unlikely to be the reason
for the existence of the large-scale nocturnal jet.
7.4.
Inertial oscillation
The role of inertial oscillation is considered a common
reason for the formation of LLJs over several places. To
Q. J. R. Meteorol. Soc. (2011)
T. V. Prabha et al.
corresponding to the Pune latitude is 37.7 hours. In order
to check the inertial periodicity, continuous 5-day data (30
May to 3 June) with similar ambient conditions to the 2-day
case study period was used to derive spectra. As is evident in
Figure 12(a) (lines), the energy corresponding to the inertial
period was not significant at lower levels. However, this
period was found to be significant above the jet core and has
more energy.
(a) -2
ν velocity (m s–1)
4
-3
3
2
-4
7
6 8
5
22
1 023 11
12
910
21
9 10
4
11
8
32
1 023
22
7
-5
-6
5
13
20
14
19
12
18
15
16
17
13
14
21
6
20
19
18
-7
15
16
17
7.5.
Recirculation zones below the jet
A close examination of w spectra shows a suppression
of the variance (and thus less turbulence) at jet height
u velocity (m s–1)
(Figure 13(b)). Spectral analysis for horizontal components
of sodar winds at different heights (not presented) also
(b)
56
22
10 m
7
0
1223
indicated similar semi-diurnal and diurnal periodicities
0
4 21
8
250 m
20
3
and suppression of energy at jet height. The turbulence
9
-1
is maximal in the lower layers and also increases above the
8
19
1011 12 13
14 9
21 3
jet. It is noticed from the simulations that this suppression of
-2
4
1 202
1516 17
22
0
turbulence at jet height is closely associated with production
23
18
7
10
-3
of strong vorticity below the jet (Figure 13(c)). This could
11
13
19
12
6 5
14
be interpreted as due to a hydraulic jump, as described
15
-4
16
by Schär and Smith (1993) for sub-critical upstream
conditions, but needs further clarification from vorticity
17
-5
budget analysis. The presence of less turbulence at jet level is
18
-6
in contradiction to several other LLJ studies where vertical
mixing is accomplished by the generation of turbulence at
2
3
4
5
6
7
8
9
jet height due to increased shear (Mahrt, 1999; Banta et al.,
u velocity (m s–1)
2002; Mahrt and Vickeres, 2002; Prabha et al., 2007, 2008;
Figure 12. Hodograph of winds over Dapoli (a) and Pune (b) at 10 m and Karipot et al., 2008). The increased shear in some cases is
250 m level. Numbers associated with the symbols represent each hour in also found to suppress turbulence by sheltering large eddies
Indian Standard Time (IST). Arrows show the direction of oscillation.
(Smedman et al., 2004; Prabha et al., 2008) from penetrating
to lower layers.
investigate the role of inertial oscillation, wind hodographs
are considered. Figure 12(a) and (b) shows hodographs of 8. Conceptual model for LLJ over the WG
wind (Holton, 1967) at the surface and at 250 m above the
surface, at Dapoli (located on the west coast) and at Pune The nocturnal LLJ that has formed appears to be due to
(on the east side), respectively. In the hodograph, u and v a combination of effects. During the daytime, the moiscomponents of wind velocities are plotted, along with their ture/temperature distribution shows an increase/decrease
time variation at a constant height. The wind oscillations until the top of the mountain barrier. The wind speed in
during the course of the day are described with the help of u the lee side is less than 3 m s−1 , compared to 9–12 m s−1
and v component variations with time at two different levels on the windward side. The boundary layer roll circulations
(10 m and 250 m above the surface). Each point in the figure are the primary mode of convection over inland locations
corresponds to hourly horizontal wind components from (vertical velocity distribution showed alternating updrafts
the model, giving information on velocity variation with and downdrafts which scale the boundary layer depth of
time. The wind oscillations at both elevations are similar. 3–3.5 km at deep valley points). Events after sunset are
Dapoli has a typical anticyclonic (veering) wind vector with summarized with the help of the sketch shown in Figure 14.
time, and a cyclonic turning of wind is noted over Pune, After sunset, the eastern slopes cool faster than the valley
which is atypical, compared to LLJs studied over other bottom and the valley atmosphere. The disappearance of
locations (Thorpe and Guymer, 1977; Ulden and Wieringa, daytime intense convection over land assists the mechan1996; Baas et al., 2009; Bain et al., 2010). This indicates that ically lifted air over the mountain to progress inland, in
inertial oscillation may not be the cause of jet formation over the form of a mountain wave. As the valley bottom cools,
Pune. It is also to be noted that both u and v components of a residual layer remains over the developing stable layer.
These events lead to a strong temperature gradient (of up
winds increased during the oscillation.
Another investigation was carried out with the help of to 14 K; see Figure 11(a)) between the slopes and the valspectral analysis of sodar vertical velocity. The relationship ley atmosphere (point B is warmer than A in Figure 14).
between periodicity of events associated with LLJ and its The valley potential temperatures are independent of height
variance contribution are shown in Figure 13(a). The spectral over a very shallow stable layer. This indicates the presence
analysis for the study period of 2 days is shown with different of a deep, well-mixed, and warmer residual layer. These
symbols for three elevations (90 m, 270 m, 420 m). There are events create a pressure gradient between the slopes and
three peaks in the spectra corresponding to 2 hours, semi- valley to drive the LLJ carrying moisture to inland locadiurnal and diurnal periodicity. The diurnal periodicity is tions. A thermal wind v component is imposed on the
most dominant at lower levels and might be attributed mean flow to make it northwesterly over the region. The
to the mountain wave propagation. The inertial period jet blows from west to east with a northwesterly component
-8
0
1
2
3
4
5
6
7
8
ν velocity (m s–1)
-1
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
Nocturnal Low-Level Jet Over Indian Western Ghats
100
(a)
Diurnal cycle
Semi diurnal cycle
Normalized variance
10
(b) 700
Inertial
period
600
1
Height (m)
500
0.1
0.01
1E-3
400
200
90m
270m
420m
1E-4
1
2 hours
12 hours
24 hours
38 hours
300
100
0
10
Period (hours)
100
0.01
0.1
1
10
Normalized vertical velocity variance
(c) 600 -1E- 4
0
Height (m)
-2E-4
-1E-4
400
-1E-4
-1E-4
0
200
-2E-4
-3E-4
0
-1E-4
-2E-4
-3E-4
0
7
11
15
20
-4E-4
24
4
8
Time (IST)
12
16
20
24
4
Figure 13. Energy spectra of vertical velocity at different heights (a) and normalized vertical velocity variance at peak periodicities (2, 12, 24, and 38 hours)
as a function of height (b). Symbols are used for data during 12–13 May. Time–height cross-section of the vertical component of vorticity (s−1 ) from
simulation at sodar location (c).
Upward
propagating
mountain wave
Low frequency gravity waves
q
U
Downward
propagating
gravity waves
FWC
q
Gu
BLC
us
ng
BLC
d
ce
Jet la
nt
fro
A
ti
lif
yer to
Vortex
B
p
NLLJ
r
Fo
Arabian
Sea
Residual Layer
st
U
SBL
FWC: Fair Weather Cumulus; U: wind speed; Us slope flow component; NLLJ: Nocturnal Low Level Jet;
BLC: Boundary Layer Cloud; SBL Stable Boundary Layer; q: Potential temperature
Figure 14. Conceptual model for the nocturnal LLJ over the WG.
along tributary valleys. This change in wind direction was
also noticed in the IMD surface observations (Figure 2(d)),
as discussed earlier. The mountain wave carries a large
vertical flux of horizontal momentum associated with the
LLJ. In response to the progress of LLJ inland, moisture
is also transported in the lower layers, especially through
selected narrow regions as ‘streams’ where the jet is strong.
This low-level moisture accumulation is noticed behind the
gust front associated with the mountain wave (see movie
file at http://www.tropmet.res.in/∼majfiles/QJRMS thara/
streams.mpg).
Numerical models and reanalysis products have difficulty
in representing LLJs by their speed, height, horizontal and
temporal variations and extent (Kosović and Curry, 2000;
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. (2011)
T. V. Prabha et al.
Cuxart et al., 2006; Steeneveld et al., 2008). These problems
are believed to be due to low spatial and temporal resolution
of models (Anderson and Arritt, 2001; Prabha et al., 2011)
and to inadequate boundary layer physics (Storm et al.,
2009) or influence on boundary layer physics through
sensitivity to land surface parameterization (Prabha et al.,
2011). Our simulations reinstate model inadequacy, while
also illustrating the capability of the model to re-create
some of the observed features. Recent numerical model
verification studies are unraveling the importance of LLJs
and their role in the diurnal cycle of convection and
precipitation (Hu 2003; Wang et al., 2009; Pospichal et al.,
2010). These jets have a significant impact on the water cycle
over the region through atmospheric streams, which are not
represented in coarse resolution models. Most importantly,
the representation of these jets in high-resolution numerical
models also requires improved understanding from observations and simulations, which is inevitable for accurate
prediction of cloud and precipitation processes.
Additional verification studies using more observations
are needed to investigate these interactions further.
However, this study gives some important details of the
boundary layer dynamics and their interaction with the
topography over the rain shadow region. This forms a
crucial step in designing experiments aimed at studying
the boundary layer over the area. Non-local characteristics
of the rolls might imply that point measurements can be
misleading and careful considerations are necessary to make
appropriate measurements and interpretations. The rolls in
the boundary layer reach up to 3.5 km in the valley during the
day and remain as a convective region through the night as a
convectively active residual layer above the developing stable
boundary layer, holding the pollutants until the jet dynamics
and gravity waves fill the stable valley atmosphere beneath
it. This is a very important aspect considering the pollution
dynamics and might play a key role in the accumulation of
dust and pollution during the dry season.
10.
9.
Appendix A: Web Links for Supplementary Material
Conclusions
http://www.tropmet.res.in/∼majfiles/thara.html
A high-resolution simulation with the ARW model has been
http://www.tropmet.res.in/∼majfiles/QJRMS thara/
used to highlight some of the characteristic features of the
readme-movies.pdf
boundary layer over the rain shadow region in the Western
http://www.tropmet.res.in/∼majfiles/QJRMS thara/
Ghats of India. It is shown that roll circulations rooted
llj.mpg
in the surface layer and reaching to approximately 3.5 km
http://www.tropmet.res.in/∼majfiles/QJRMS thara/
height are the primary mode of convection during the day.
temperature.mpg
The study revealed the events that lead to the formation
http://www.tropmet.res.in/∼majfiles/QJRMS thara/
of an LLJ on the eastern side of the mountain range and
streams.mpg
their regional-scale characteristics. Some intricate details
http://www.tropmet.res.in/∼majfiles/QJRMS thara/
such as the importance of mountain wave propagation and
cloud-patterns-valley.pdf
its association with the LLJ, and the relationship with the
residual layer over the valley atmosphere, are revealed for Acknowledgements
the first time.
The following conclusions are drawn:
The Indian Institute of Tropical Meteorology (IITM) and
the CAIPEEX experiment are fully funded by the Ministry of
• There exists a nocturnal LLJ on the east side of the WG Earth Sciences (MOES), Government of India, New Delhi.
mountain range over the Indian peninsular region, Computational support for this study is also provided
which is different from the well-studied large-scale jet through the CAIPEEX program. Sodar is funded by the
over the Indian region: the Findlater jet.
Department of Science and Technology (DST, India).
• The nocturnal LLJ is baroclinically driven as a The authors thank Dr Anandakumar Karipot, University of
result of the temperature gradients between the Pune, for several useful comments and suggestions on the
valley atmosphere and slopes, and influenced by the manuscript. The first author acknowledges discussions on
progression of mountain wave and an associated gust the topic with Prof. Robert Houze and Dr Jimy Dudhia.
front.
The authors also thank two anonymous reviewers and the
• Atmospheric ‘streams’ of water vapor noticed with associate editor for several key suggestions, which improved
the LLJ are an integral part of the diurnal cycle, the manuscript and presentation considerably. Mrs V. V.
which are important over the eastern slopes of WG. Sapre of IITM and Alan Norton of NCAR are thanked for
These features might not be resolved in low spatial efforts in making supplemental materials available online,
resolution models, as details of the topographically with respective web links. This manuscript is dedicated
forced nocturnal jet are not appropriately simulated. to Prof. Anna Mani who made the first Indian wind
• A mountain wave with a diurnal periodicity is noticed, atlas.
which can be recognized in the surface observations
with strongly coherent updrafts and downdrafts
exceeding 1 m s−1 .
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