On the temporal evolution of vertical velocity in tropical convection
Vickal V. Kumar1, Alain Protat2, Christian Jakob1, Christopher R. Williams3 and Peter T. May2
1School
of Earth, Atmosphere and Environment, Monash University, Australia. 2Centre for Australian Weather and Climate Research: A partnership between the Bureau of
Meteorology and CSIRO, Melbourne, Australia.3University of Colorado, and NOAA/Earth System Research Laboratory/Physical Sciences Division, Boulder, Colorado
Overview
Composite response of CPOL reflectivity and Profiler vertical velocity associated with 13 storms
Discussion and Summary
Measurements of vertical velocities, unlike other
Retrieval of vertical velocity from the wind profiler pair
aspects of clouds, have been difficult to ascertain. The
existing
in-cloud
techniques are
LeMone
and
vertical
velocity
(Williams, 2012). The 50-MHz measures vertical velocity
observational
of air parcels (wanted) and Rayleigh scatter from
in situ aircraft penetrations (e.g.
Zipser
1980),
remote
hydrometeors. The hydrometeors signal is removed using
sensing
This peak due to Rayleigh Scattering
1999; Heymsfield et al. 2010), and retrievals from dual
profiler is at a fixed location and samples only a section
of the storm, but at least for 30 mins for the 13 cases
the 20-dBZ ETH at t=0 min, however, it ensures that the
This peak due to Bragg Scattering
airborne wind profilers (e.g. May and Rajopadhyaya
evolution of vertical velocity in convective storms. The
analysed here. By putting maximum height reached by
the 920 MHz returns
measurements from vertical pointing ground based and
A composite analysis method is used to demonstrate the
storms are in-phase despite having a fixed observation
point.
50 MHz spectrum
Doppler scanning radar (e.g. Protat and Zawadzki
1999).
Knowledge of vertical velocity in cumulus
cloud are needed to evaluate and improve the mass
flux schemes in the General Circulation Models.
Here,
we
combine
high
temporal
resolution
The updrafts are found to be dominant in the growth and
920 MHz spectrum
CPOL provides information on convective/stratiform
cloud classification, cloud top height, amount of
water content, drop size characteristics and cloud
tracking
mature phase of storm. Several competing processes are
responsible for the acceleration/deceleration of updraft
speed in cumulus cells. The acceleration in updraft
observations of in-cloud vertical velocities over two
speeds near and above freezing level is due to buoyancy
wet-seasons at Darwin derived from a pair of wind
provided by latent heating from condensations of liquid
profilers with the physical properties (cloud top
drops
heights CTH, convective-stratiform classification) of
Whereas, the deceleration in updraft speeds have been
clouds derived from a C-band polarimetric radar
explained by the entrainment processes and the drag
(CPOL) to provide an observational picture of the
effect caused by water loading.
and
supercooled
liquid
drops,
respectively.
evolution of vertical velocity as a function of storm
lifetime.
In contrasts, the downdrafts mainly occur in the decay
Visualisation of the methodology
phase, and are of much weaker intensities compared to
The profiler is at a fixed location and scans only
updrafts. Downdrafts in cumulus clouds are caused by
vertically. CPOL is a scanning radar and tracks the
precipitation loading, evaporation and melting occurring
growth of precipitating clouds. The schematic diagram
below the FZL, cloud edge evaporation cooling in the
shown below illustrates how storms are tracked over
middle level (5–10 km), and entrainment processes near
the profiler site using CPOL reflectivity volumes.
cloud tops.
Reference
Mature phase (-20 mins to ~+20 mins)
Growth phase
~10–50
mins
before
the
updraft strength increases gradually with
peak in 20-dBZ)
T= 0
Profiler
Two selection criteria were used: First criterion was
the convective system must last of at least 30 min
within the profiler domain. Second, 20-dBZ echo tops of
the convective system must at least once exceed 7 km.
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time. Below FZL, show early onset of
Rapid
CPOL
Updraft strength and altitude of peak
increases
reflectivity
and
appearance
of
in
downdraft.
early
updraft
Reflectivities depict upward diffusion 20-
motion below freezing level
30-dBZ echoes around t=0 mins, suggesting
(FZL).
upward transport of large particles.
Decay phase (+20 to +60
mins)
Mainly stratiform in nature.
Strong downdraft above 10
km, suggesting entrainments
of dry air into the deep
cumulus
progressively
cells.
tower
collapses
that
the
Heymsfield, et al., 2010: Characteristics of deep tropical and
subtropical convection from nadir-viewing high-altitude airborne
Doppler radar. J. Atmos. Sci., 67, 285–308.
LeMone and Zipser, 1980: Cumulonimbus vertical velocity events in
GATE. Part I: Diameter, intensity and mass-flux. J. Atmos. Sci.,
37, 2444–2457.
May and Rajopadhyaya, 1999: Vertical velocity characteristics of
deep convection over Darwin, Australia. Mon. Wea. Rev., 127,
1056–1071.
Protat and Zawadzki, 1999: A variational method for real-time
retrieval of three-dimensional wind field from multiple-Doppler
bistatic radar network data. J. Atmos. Oceanic Technol., 16 (4),
432-449.
Williams, 2012:Vertical air motion retrieved from dual-frequency
profiler observations. J. Atmos. Oceanic Technol., 29, 1471–
1480.