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 non-convective regions (Salathe and Hartmann 1997)

-Possible transport to stratosphere of moisture, radiation, and chemical species

High Cloud (p<440mb)in the tropicsis most common over warmest SST,or over land.

(As adopted fromHartmann)

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 FractionJJA 2003→

Mean SST JJA 2003→

WP High Cloud FractionJJA 2003→

Mean SST JJA 2003→

Now, choose equal latitude subregions where the convection is most active (7N-12N for JJA 2003)

←Peak in high cloud occurs at ~210K

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 is slightly warmer than 210K

More low ←clouds than WP or CP

EP (7N-12N,150W-100W), SST Range: 27.2°-29.1°C

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

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Seasonal Variations of Convective Clouds (CP)

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Seasonal Variation of Convective Clouds (EP)

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And, to summarize…

Summary of Seasonal Variations

●In WP and CP, high clouds are colder in DJF than 3-month 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