Mesoscale Atmospheric Systems
Ocean evaporation
Stephan Pfahl
9 May 2017
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Atmospheric water cycle
The global water cycle following Baumgartner and Reichel (1975).
Annual values are in units of 103 km3 year-1. From Bengtsson (2010).
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Atmospheric water cycle
The global water cycle following Baumgartner and Reichel (1975).
Annual values are in units of 103 km3 year-1. From Bengtsson (2010).
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Parameterization of ocean heat fluxes
Fluxes at the ocean-atmosphere interface:
where
< > denotes the time mean value
Reynolds decomposition: e.g.,
zonal wind
<U> + u
temperature <T> + t
Cp specific heat at constant pressure of dry air
LE latent heat of evaporation
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Parameterization of ocean heat fluxes
Stability theory (Monin-Obukhov): Mean gradients are
universal functions of stability parameter z/L
where
u*, T*, q* are scaling parameters:
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Parameterization of ocean heat fluxes
L is the so-called Obukhov length
- and z/L is the only free (dimensionless) stability parameter in
M-O similarity theory
The so-called similarity functions must be empirically determined
– they are “fits to measurements”, which is particularly difficult
over the ocean during strong winds!
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Parameterization of ocean heat fluxes
In atmospheric models surface layer is only “represented” by the lowest
model level (typically at a height of 10-50 m).
Therefore the surface fluxes must be expressed as a function of the
model parameters at this reference height zR:
z0a
CaN
denotes the so-called roughness length for the quantity a
is the drag coefficient for the quantity a during neutral
conditions, i.e., dθ/dz = 0
(note the alternative notation: <T> + t = T + T’ )
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Parameterization of ocean heat fluxes
For non-neutral conditions the drag coefficient, e.g., for temperature
becomes
where, e.g.,
is the integral of the similarity function
This yields for the oceanic fluxes of
heat
~ CH <U>(zR) [ <T>0 – <T>(zR) ]
moisture
~ CQ <U>(zR) [ <Q>0 – <Q>(zR) ]
à fluxes are large if winds are strong and if large temperature/humidity
difference exists between sea surface and lowest atmospheric layer;
drag coefficients depend on stability parameter z/L (they become
smaller for stable conditions)
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Surface heat fluxes
HL DJF
HL JJA
HS DJF
HS JJA
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Instantaneous latent heat flux (evaporation)
18 UTC 04 Aug – 00 UTC 05 Aug 2008
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Instantaneous latent heat flux (evaporation)
06 UTC 05 Aug – 12 UTC 05 Aug 2008
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Instantaneous latent heat flux (evaporation)
00 UTC 12 Sept – 06 UTC 12 Sept 2008
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Instantaneous latent heat flux (evaporation)
12 UTC 19 Oct – 18 UTC 19 Oct 2008
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Parameterization of ocean heat fluxes
How accurate is the similarity approach?
Fairly OK – but large uncertainties associated with both measurements
and parameterization
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Sensible and latent heat fluxes in cyclones
Study by Persson et al. (2005) with measurements from the
FASTEX field experiment in 1997 onboard the ship RV Knorr
measurements of wind,
temperature, humidity with very
high temporal resolution
è  direct observations of <W>
and w, ...
è  fluxes from both
<wq> (“eddy covariance”)
and <Q>0 – <Q>(zR) (“bulk”)
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Sensible and latent heat fluxes in cyclones
Measurements during 1 month in regions with cold/warm SSTs
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Sensible and latent heat fluxes in cyclones
Unique aspect: measurements during strong winds (> 15 m/s)
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Sensible and latent heat fluxes in cyclones
Measurements during passage of cyclonic systems
à Observations of fluxes and basic meteorological parameters in
warm sector, during frontal passage and in post-frontal air
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Sensible and latent heat fluxes in cyclones
Front-normalised composites of T, q, wind speed & direction
cold sector
warm sector
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Sensible and latent heat fluxes in cyclones
Front-normalised composites of sensible and latent heat flux
Sensible heat cov
Latent heat cov
Sensible heat bulk
Latent heat bulk
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D04106
Sensible and latent heat fluxes in cyclones
Study by Yuan et al. 2009 (JGR), analysing cyclones and surface
fluxes in the southern ocean based on reanalysis and satellite
data YUAN ET AL.: SOUTHERN OCEAN MIDLATITUDE CYCLONE STATISTICS, 2
D04106
negative
Figure 5. Example of the calculation of fluxes associated with
a cyclone,values:
5 Januaryupward
2003, 1200flux
UTC. The
minimum pressure location is indicated with a black dot. The size of the cyclone is indicated with a circle.
(c, d) pressure
Solid lines are positive
(a)composite
The contours are of
4-hPa
isobars. cyclones,
(b) The contour centred
interval is 0.4over
N m!2.low
many
contours, dashed lines are negative contours, and bold lines are zero contours.
minimum
ividual cyclone. An example is shown in Figure 5. Figure 5a
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example of an 8-day cyclone appearing in the lee of the
t the The depth and radius of the storm reach their maximum on
obar. day 7. The modified pressure fields suggest slightly deeper
sted lows along the track except in three instances. They also
w the suggest a longer track, extended by one synoptic period at
ove. the beginning and two periods at the end. The corresponding
their flux integrals are shown in Figure 8. The spatial integral in
ape. Figure 8 (left) provides a quantitative estimate of the contriorm, bution of the storm to the fluxes of momentum and heat into
Study
Yuan
et al.by2009
(JGR),
analysing cyclones and surface
mum the
ocean. By by
dividing
the contributions
the area of
the
neral storm (Figure
8, right),
obtains
an estimate of theocean
average
fluxes
inonethe
southern
based on reanalysis and satellite
rom intensity of the fluxes inside the storm at each step of the
ating storm life
cycle. For example, in Figure 8a, the integrated
data
The
D04106
YUAN ET AL.: SOUTHERN OCEAN MIDLATITUDE CYCLONE STATISTICS, 2
D04106
f the
cean
e on
rom
heat)
cheg the
heat
sentoutonic
as of
oxition
antly
milar
t the
sible
ean,
Sensible and latent heat fluxes in cyclones
g an
the
Figure 5. Example of the calculation of fluxes associated with a cyclone, 5 January 2003, 1200 UTC. The
minimum pressure location is indicated with a black dot. The size of the cyclone is indicated with a circle.
(a) The
are 4-hPa
isobars.
(b) The
contour interval is 0.4 N m!2. (c, d) Solid lines are positive
Figure
6. contours
Idealized Southern
Ocean
midlatitude
cyclone.
contours, dashed lines are negative contours, and bold lines are zero contours.
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8 of 18
Sensible and latent heat fluxes in cyclones
Conclusions
-  In the warm sector differences between the atmosphere and
sea surface in terms of T and q are small, which leads to
weaker fluxes than outside the warm sector.
-  Sensible heat fluxes in the warm sector are downward, latent
heat fluxes upward.
-  The upward LHF in the cyclone region is believed to be
important for the intensification of cyclones, as the moisture
might contribute to latent heating.
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