Transcript ccu_nov10_mixlayers - Colorado State University

The Unique Structure of
Atmospheric Mixed Layers over
the Northern South China Sea
Paul E. Ciesielski and Richard H. Johnson
Colorado State University
Presented at Chinese Cultural University on (11 November 2010)
Goals for Talk
1. to demonstrate that after the onset of the summer
monsoon, the characteristics of atmospheric mixed
layers over the Northern South China Sea are quite
different from such observations made in other
tropical and subtropical regions
2. to consider the ramifications of these unique
mixed layer structures
Vertical structure of oceanic, tropical atmosphere
Surface fluxes
3 distinct layers in
atmospheric boundary
layer (ABL)
•Surface layer (10-100m),
strong gradients of winds,
T, and moisture
•Mixed Layer (ML)- (~500
m deep over oceans, can
be much deeper in
daytime over land); small
vertical gradients
•Transition layer
ABL (entrainment zone) - ~100
m deep, separates ML
from free atmosphere
Characteristics of Atmospheric Boundary Layers
• presence of frequent turbulence
• turbulence generated by buoyancy (unstable
stratification - surface warmer than air) and/or
mechanical mixing (due to wind shear)
• one of the main mechanisms for generating positive
buoyancy is radiative heating at the earth’s surface
• this energy surplus is transmitted upward to the
atmosphere by conduction and convection
• convective transfer usually takes the form of convective
plumes
Development of mixed layers over land (Stull 1991)
sunset
sunrise
• After sunrise, the mixed layer grows rapidly during mid-morning, then
slows before noon as mixing reaches LCL and clouds develop.
•Cooling at the earth’s surface results in a noctural stable boundary
layer and less-turbulent residual layer containing former mixed layer
air.
Example of mixed layer over land in a monsoon environment
Pingdong (May 2008)
8 am
8 pm
 increases
with height
indicating a
stable
stratification
Signature of a ML on a SkewT diagram - T curve lies along dry
adiabat ( is a constant) and Td curve lies along a constant
mixing ratio line.
From ASTER dropsonde mission in TiMREX on 31 May 2008
• ABL is the medium through which the land/ocean surface and
the free atmosphere are coupled.
• Accurate prediction of when and where convection will occur
depends on how well models handle energy exchanged at the
surface and its coupling through the mixed layer to the free
atmosphere; this is accomplished by land-surface models and
boundary layer parameterizations.
In this study a subjective technique was used to identify ML tops (zi).
Examples of mixed layer (ML)
identification from TOGA
COARE (Johnson et al. 2001)
θ
•Here zi is the level at which
 exhibits an abrupt
increase and q a sharp
decrease.
•Both of these structures
must be present in a
sounding to be assigned a zi.
q
TOGA COARE was conducted over
the West Pacific warm pool region
from 11/92 – 02/93.
Mean mixed layer
statistics for 4
month Intensive
Observing Period
Station
# / # possible
Mean zi
Ship 5
142/207 (69%)
553 m
Kapinga
351/475 (74%)
518 m
Moana Wave
189/255 (74%)
501 m
Ship 3
244/346 (71%)
491 m
Total/Mean
926/1283 (72%)
512 m
• from ~700 sondes examined in GATE conducted in the tropical East Atlantic,
mixed layers were present in 79% with mean zi of 424 m (Fitzjarrald and Garstang
1979)
• ~75% sondes have mixed layers with a mean zi ~ 500 m
SCSMEX (South China Sea Monsoon Experiment)
Integral part networks were two R/Vs, which had hourly flux
measurements and 6-h soundings.
Summary of Mixed Layer Analysis in SCSMEX
Station
# / # possible
Mean zi
Ship 1
(southern)
126 / 153
(83%)
459 m
Ship 3
(northern)
74 / 153
(48%)
342 m
(northern)
(southern)
• At Ship 1, the frequency and
mean ML depth are similar to
that observed in other tropical
regions.
•At Ship 3, the frequency and
mean ML depth are
considerably fewer and lower
than observed in previous
studies.
•These histograms, which
show the distribution of mixed
layer tops in the vertical at the
two ships, clearly show the
deeper mixed layers at Ship 1.
Time series of ML tops and various other fields
F = S + .61cpTsE/L, F - is sfc buoyancy flux, S - sensible heat flux and
E - latent heat flux
CBH = 125*(T-Td), CBH is an estimate of the Cloud Base Height (m)
Time series at Ship 1 (southern ship)
• Characteristics similar to other
tropical locations being present
83% of time
• Cruise 1: suppressed convection
and light winds, under these
conditions ML variability driven
by radiative diurnal effects. ML
depths were ~60 m deeper in
afternoon and evening soundings
•Cruise 2: enhanced convective activity (P=9.1 mm/day) led to greater
variability in ML depths.
•SSTs continue to rise while Ts slowly decreased likely from convective
downdrafts transporting cool, dry air towards surface.
•Resulted in larger buoyancy fluxes, stronger mixing and deeper ML.
Time series at Ship 3 (northern ship)
Monsoon onset
•Two convectively active periods
over northern SCS, the first was
associated with monsoon onset.
•MLs decreased dramatically
following mid-May rainy peak,
reflecting the presence of
recovering precipitation downdraft
wakes in regions of convective
rainfall.
•Frequency of ML during cruise 1
was 75%, during cruise 2 only 20%.
•Cruise 2: characterized by northward advection of warm, moist air
with Ts > SST and RH > 85% which results in small or negative F,
weak vertical mixing and a lack of MLs.
Boundary layer characteristics at Ship 3
mean with ML
74
mean without ML (stable)
79
zi
•Each profile with a mixed layer was scaled by zi to preserve its
boundary layer structure.
•The main distinction between these composites is that those without
mixed layers have (1) stronger vertical shear, (2)  increases
monotonically with height indicating a high degree of stability at lowlevels, (3) q decreases monotonically with height.
To review:
•GATE (East Atlantic) , TOGA COARE (West Pacific warm
pool), the southern ship in SCSMEX (SCS) had mixed
layers about 75% of the time with average depth ~500 m.
Such mixed layer characteristics were also observed prior to
monsoon onset at the northern ship in SCSMEX.
•Following monsoon onset at the northern ship, winds
shifted to southwesteries allowing warm, moist air to flow
over cooler water. This in turn resulted in weak or negative
buoyancy fluxes and stable conditions. In the presence of
this low-level stable air, few mixed layers were observed
(20% of time) and they were shallow (340m).
To our knowledge, such anomalous mixed layer
characteristics, namely, the lack of mixed layer
structures, or conversely, the high frequency of stable
boundary layers have not been previously observed in a
tropical, oceanic environment.
A few key questions:
•Are these features unique to SCSMEX, or do such
conditions exist in other monsoon regimes and
seasons?
•What are some implications of these stable
boundary layers?
TiMREX (Terrain influenced
Monsoon Rainfall Experiment)
conducted from 15 May – 25
June 2008 had upstream ship
soundings and 15 dropsonde
mission over SCS.
dropsonde
flight pattern
TiMREX period was
characterized by low-level
south-southwesterly flow
advecting warm air over
cooler waters.
TiMREX mixed layer analysis
Platform
# / # possible
Mean zi
Ship
32 / 135
(24%)
40 / 175
(23%)
351 m
Dropsondes
372 m
• Results from TiMREX mixed layer analysis are similar to post-onset
analysis from SCSMEX
• The relative lack of mixed layer structures over the Northern SCS as
observed in SCSMEX and TiMREX may, in fact, be a common
characteristics in the post monsoon onset environment of this region.
Unfortunately the ships in TiMREX did not have any flux
measurements, but there is a satellite product GSSTF (Goddard
Satellite-based Surface Turbulent Fluxes) which provides
estimates of latent and sensible heat fluxes over the ocean.
Some details of this product:
•Uses SSM/I, TMI and AMSR-E retrievals of SSTs, winds and Wb
(where Wb is the PW in the lowest 500 m). This information is
combined with NCEP reanalyses surface air temperature and used in
the bulk aerodynamic algorithms to compute latent and sensible heat
fluxes.
•Version 2b of this product provides these fluxes at 1° horizontal
resolution over the global oceans on a daily basis for the period from
July 1987 to Dec. 2008. (personal communication Chung-Lin Shie)
Time series of various fields at SW
ship of TiMREX
•around May 20, surface winds
switch from a northerly to
southerly direction indicating
the onset of the summer
monsoon.
• warm, moist air advects over
cooler waters (sfc air
temperature > SST)
• in this environment, surface
fluxes drop dramatically
•bouyancy fluxes are weak or
negative
IN SHORT, CONDITIONS SIMILAR TO SCSMEX
A few key questions:
•Are these anomalous mixed layer
characteristics (i.e., high frequency of stable
boundary layers at S3) unique to SCSMEX?
•Do they occur in other monsoon regimes and
seasons? Yes, they also occurred TiMREX.
•What some implications of these stable
boundary layers? (OKAY, so what?)
Time series of wind speed and surface fluxes
during SCSMEX at Ship 3 (observed vs JMA)
onset
•large difference between the
JMA reanalysis fluxes and
those observed at Ship 3 in the
post-onset period suggest:
JMA (7.3)
Obs (1.8)
JMA (104)
Obs (37)
•models had difficulty
reproducing the correct BL
structures under weak stability
and mixing conditions
•models are fluxing too much
heat and moisture into the
atmosphere
NESA
•During post-onset convectively active period model rainfalls are
nearly double the TRMM and GPCP estimates
•excessive model fluxes  too much model rain
Rainfall maps (satellite-estimated on left vs JMA model on right)
Satellite
Model
Onset period
Post-onset
In post onset period (3 - 9 June), JMA analysis has nearly double
the rainfall over southern Taiwan as was estimated by TRMM.
SUMMARY
•SCSMEX and TiMREX mixed layer analyses suggest that
following the monsoon onset northward advection of warm, moist
low-level air over the cooler waters of the northern SCS results in
stable boundary layers with weak vertical mixing of heat and
moisture from the surface.
•Under these conditions of weak mixing, mixed layers are present
~25% of the time tending to be quite shallow (~350 m).
•During SCSMEX, the models appeared to have a difficult time
dealing with these stable boundary layers (and weak mixing) as
evidenced by their excessive fluxes and rainfall.
•Models may need to handle these stable boundary layers
differently to accurately predict convection and lower-atmospheric
variability.
for additional details see: Ciesielski and Johnson 2009, SOLA
Supplementary material
Skew-Ts from SCSMEX
with (left) and without mixed layers (right)
ML
•Low-level T lies along dryadiabat (constant )
•Td lies along constant mixing
ratio line
no ML
•Low-level lapse rate is stable
( increases with height)
•Td decreases with height.
scaled mean ML profiles at the ships
Created by scaling each sounding, which had an observed mixed layer,
by its mixed-layer depth (has effect of preserving the boundary layer
structure).
Ship 1
Ship 3
•small vertical shear
•, and q nearly well
mixed
•slightly unstable near
the surface
•shading in lowest ~50
m denotes some
uncertainty in sfc obs.
•mean profile for S3 is
~1C cooler likely due
to cooler SSTs at this
site.
Dropsonde mission on 4 June 2008 (11 LT takeoff)
SST
5
Only sondes 7 and 8 from this
flight of 12 dropsondes showed a
mixed layer structure; 7 and 8
occurred over warmer SSTs.
7
8
Example of dropsondes without (#5) and with (#7) a mixed layer
structure
5
7