Transcript 040915_AQUA-MODIS_1stYr-Presentation_Kubar

Determination of Tropical Pacific
Cloud Structures using AQUA MODIS
Data
Presented By: Terry Kubar
Advisors: Dennis Hartmann and Rob Wood
Why Are Clouds Important?
-Clouds cool surface by reflecting SW radiation, and warm the surface
by absorbing LW radiation
-Still much uncertainty among General Circulation Models (GCMs)
regarding how clouds will respond to climate change
1. Cess et. al. (1996) showed that runs from several different GCMs
yield cloud feedbacks that range from weak negative to strong
positive
2. Initial conclusion: Clouds are still not well understood or
parameterized by climate models
-Use of remote sensing technique (i.e. MODIS) may help us better
understand current cloud distributions, including cloud radiative forcing
(net radiative impact of clouds at the top-of-atmosphere)
Why Focus on Tropical Convection?
From Ramanathan and Collins (1991), Hartmann and Michelson
(1993), and Soden and Fu (1995):
-Frequency and structure of high tropical clouds determines:
-upper tropospheric humidity
-outgoing LW radiation
-incoming solar flux
-clear-sky greenhouse effect
-High tropical clouds transport moisture to surrounding nonconvective regions (Salathe and Hartmann 1997)
-Possible transport to stratosphere of moisture, radiation, and
chemical species
High Cloud (p<440mb)
in the tropics
is most common over
warmest SST,
or over land.
(As adopted from
Hartmann)
Some Scientific Questions:
1. What are the vertical cloud structures across the tropical Pacific?
2. Do these structures vary from west to east Pacific? If so, is there a
physical explanation of these geographic variations of clouds? Is
SST a good predictor for cloud type (i.e. low or high cloud)?
3. Is there a seasonal variation of clouds across the tropical Pacific?
4. Are convective clouds clustered at a particular temperature?
N.H. Tropical Pacific SST Map
West
Central
East
Brief Summary of Annual SST Distribution across
Northern Tropical Pacific
-West Pacific: Large area of very warm SSTs, known
as the warm pool
-Central pacific: Transitional area, with warm SSTs to
the west of 180E, cooler SSTs to the east of 180E
-East Pacific: Much cooler SSTs in general, except for
a narrow band of warm SSTs (the ITCZ) that slopes
gently northward to the east
Brief MODIS Background
-MODIS stands for “Moderate Resolution Imaging
Spectroradiometer”
-Two MODIS instruments – one aboard TERRA satellite (launched
in December 1999) and one aboard AQUA (launched May 2002)
-Monitor cloud cover, cloud type, tropospheric aerosols, trace gas
concentrations, solar radiation
-Features: wide spectral range, high spatial resolution, and near
daily global coverage
Methodology and Procedure
●Order Aqua MODIS data and check for quality assurance
●Process data; create maps of cloud fraction and optical depth of
each of the three domains
●Look at SST data, and choose subregions in each region where the
warmest SSTs are (i.e. where the convection should be the most
active)
-Make histograms of these subregions for each season (i.e.
DJF, MAM, JJA, SON)
-Examine histograms and focus on cold cloud populations to
quantitatively examine variability (seasonal and geographical) of
cold cloud top temperature
EP High
Cloud
Fraction
JJA
2003→
Mean
SST
JJA
2003→
EP JJA
2003
Warm
Cloud
Fraction
Mean
SST
JJA
2003
CP High
Cloud
Fraction
JJA
2003→
Mean SST
JJA 2003→
WP
High
Cloud
Fraction
JJA
2003→
Mean
SST
JJA
2003→
Now, choose equal latitude subregions where the
convection is most active (7N-12N for JJA 2003)
WP (7N-12N,120E-160E), SST Range: 28.7°-29.9˚C
←Peak
in high
cloud
occurs
at
~210K
CP (7N-12N,160E-160W), SST Range: 27.5°-29.4°C
←Peak in
high cloud
occurs at
~210K
EP (7N-12N,150W-100W), SST Range: 27.2°-29.1°C
←Peak in high
cloud is slightly
warmer than 210K
More low
←clouds
than WP or
CP
JJA 2003 Histograms (WP, CP, EP)
Initial Summary of Cloud Vertical Structure (JJA 2003)
-Peak in high cloud amount near ~210K in WP and CP (only
slightly warmer in EP)
-this is perhaps evidence of the Fixed Anvil Temperature (FAT)
hypothesis
-Rather large population of high, optically thin clouds (cirrus) in
WP and CP, but not in EP
-High, thick clouds (with τ> 20.0) peak at lower temperatures
than other cloud types (1< τ <20.0 or τ < 1)
-Secondary population of clouds are low clouds; more low clouds
in EP, most likely because of cooler SSTs
Seasonal Variations of Convective Clouds-WP
75th
50th
25th
10th
5th
Seasonal Variations of Convective Clouds (CP)
75th
50th
25th
5th
10th
Seasonal Variation of Convective Clouds (EP)
75th
50th
25th
10th
5th
And, to summarize…
Summary of Seasonal Variations
●In WP and CP, high clouds are colder in DJF than 3month periods
-Larger seasonal variation for CP, especially
for thin clouds
-Coldest tropopause temperature in DJF (esp. in CP)
●EP cloud percentiles are systematically warmer than
WP and CP
●Different seasonal pattern across EP; cold clouds
(total, thick, and intermediate thickness) are warmest in
DJF
Future Work
-Composite histograms explicitly with SST, such that a particular
histogram corresponds to a specific SST bin
-Scientific Question: Will East and West Pacific clouds
differ across the same SST? If so, does the spread of
SST explain differences in the histograms?
-Combine MODIS with AIRS (Atmospheric Infrared Sounder) to
gather temperature and humidity profiles
- use reanalysis data to look further into link between cloud top
temperature of cold clouds and cold point tropopause
temperature