Investigating Properties of the West African Monsoon Using the QuikSCAT

and Tropical Rainfall Measuring Mission (TRMM) Satellites

April 23, 2008

Taylor Hays

Katie Yoder

Mary Moore

Effect of the Intraseasonal Wind Fluctuations in the West African

Monsoon on Air-Sea Fluxes

Study conducted by:

Semyon Grodsky and James Carton

2001

Background

The West African Monsoon occurs during the Northern Hemisphere summer and is strongest from mid-May until October

During this time, the intensity of the monsoon fluctuates, with periods of heavy rain as well as dry, sunny days

Background

Two well-documented wave-like functions are present during monsoon:

1. African Waves:Zonal Wavelength: 2500-

3000 kmPeriod: 3-5 daysWestward movement: 9-10

m/s2. Second Type

(unnamed):Zonal Wavelength: >6000

kmPeriod: 6-9 daysWestward movement: 11

m/s

Goals of This Research ProjectGrodsky and Carton wanted to investigate a possible 3rd

wave-function:Bi-weekly Oscillation:

Occurs near the equator by the ITCZ around April-June and does not appear to propagate

westward

To conduct this study, wind data from QuikSCAT and rainfall estimates from Tropical Rainfall Measuring Mission (TRMM) were analyzed

QuikSCAT

Overview

• Provides wind speed and direction information over oceans to NOAA

• Purpose is to fill in the gap created by the loss of data from the NASA Scatterometer that was lost in June 1997.

• Launched June 19, 1999

Specs

• Circles Earth at an altitude of 800km • Orbits once every 101 minutes (14 orbits per

day)• Passes close to Earth’s north and south poles • Inclination of 98.6º• Seawinds is the Scatterometer instrument

onboard

SeaWinds• Microwave radar that measures near-surface wind speed

and direction• Rotating dish antenna with two spot beams that sweep in

a circular pattern • Radiates microwave pulses at a frequency of 13.4

gigahertz across broad regions on Earth's surface• Collects data over ocean, land, and ice• Continuous 1,800 km wide band • Approximately 400,000 measurements • Covers about 90% of Earth's surface in one day• QuikSCAT is currently the only US-owned instrument in

orbit that measures surface winds over the oceans

Image from SeaWInds of a tropical storm

Objectives

• Acquire all-weather, high-resolution measurements of near-surface winds over global oceans.

• Determine atmospheric forcing, ocean response, and air-sea interaction mechanisms on various spatial and temporal scales.

• Combine wind data with measurements from scientific instruments in other disciplines to help us better understand the mechanisms of global climate change and weather patterns.

• Study both annual and semi-annual rain forest vegetation changes.

• Study daily/seasonal sea ice edge movement and Arctic/Antarctic ice pack changes

• Improve weather forecasts near coastlines by using wind data in numerical weather- and wave-prediction models.

• Improve storm warning and monitoring.

• Ascending Pass

• Descending Pass

QuikSCAT Storm Page • The storm centered imagery are generated based on storm

center files provided by NRL(Naval Research Laboratory at Monterey).

• These files are updated whenever new information about active storms become available

• Since QuikSCAT 1800 km wide swath on the results in twice per day coverage over a given geographic region.

• Display the current and past wind vector images for the selected storm.

• Brighter means higher backscattered power levels, which in ocean scenes generally implies increased small scale (~2-5cm) roughness on the surface.

http://manati.orbit.nesdis.noaa.gov/cgi-bin/qscat_ice.pl

EPAC qscat ATL qscat

                                                                   

This page contains the most recent wind vector images covering the Atlantic Ocean, Indian Ocean, East Pacific and West Pacific Ocean. It is updated shortly after reciept of new QuikSCAT orbit data files.

QuikSCAT is a polar orbiting satellite with an 1800 km wide measurement swath on the earth's surface. Generally, this results in twice per day coverage over a given geographic region.

QuikSCAT Ocean

http://manati.orbit.nesdis.noaa.gov/cgi-bin/qscat_ocean.pl

QuikSCAT Ice Page• The daily ice image products are generated from

SeaWinds • This page contains the most recent images

covering the Antarctic, the Arctic, the Ross Ice Shelf, South Georgia Island and the Weddell Sea.

• The Antarctic and Arctic products typically span a 24-hour data period, while the other regions span a 38-hour period.

http://manati.orbit.nesdis.noaa.gov/cgi-bin/qscat_ice.pl

Bad News

• The instruments still work, but the satellite that carries it has begun to show signs of age. Some of the sensors that provide orientation have failed and its transmitter is wearing out.

• Because it is now running on a backup transmitter and having other problems, this satellite has the potential could fail at any moment, jeopardizing weather forecasts for potentially dangerous tropical storms.

• In early June 2007, Bill Proenza, Director of the National Hurricane Center in Florida, came under fire for criticizing his NOAA superiors for not pursuing a back-up plan for replacing the capabilities provided by this satellite.

Tropical Rainfall Measuring Mission (TRMM)

Overview• Joint mission between NASA and JAXA• Studies/Moniters tropical rainfall• Launched Nov 27, 1997• First mission dedicated to measuring rainfall

Specs

• Circular orbit at altitude of 350km

• 35 degree inclination

• 91 minute orbit (16 orbits per day)

• Great coverage of tropics of 24 hour period

• Land and sea measurements

Spatial Coverage

Payload

• Precipitation Radar (PR)

• TRMM Microwave Imager (TMI)

• Visible Infrared Scanner (VIRS)

• Clouds and the Earth’s Radiant Energy System (CERES)

• Lightning Imaging Sensor (LIS)

Precipitation Radar (PR)

• Swath width = 250km

• Ground resolution = 5km

• Phased array antenna

• Vertical profiles of rain/snow

TRMM Microwave Imager (TMI)

• Swath width = 880km

• 5 frequencies 10.7, 19.4, 21.3, 37, 85.5 GHz

• Uses Planck’s Radiation Law to calculate rainfall rates

Visible Infrared Scanner (VIRS)

• Swath width = 830km• Picks out cloud features small as 2.4 km• 5 spectral regions from 0.63 to 12

micrometers• Delineates rainfall• Used as standard to other measurements

– POES– GOES

Good News

• Long lifetime of satellite

• Global distribution of rainfall

• Better predictions of rainfall

• Under-sampled ocean and tropical continental areas

• GPMM due to launch in 2013

ResultsFound that trade winds reverse about every 2 weeks (~12-15 days), which was determined

by tracking wind direction changes as well as precipitation

location changes.

Reversal may result from interactions between Africa’s

hydrological cycle, land heating, and tropical

trade winds

Results

Illustration of the wind, rainfall, and land heating cycles that cause the bi-weekly

oscillation to develop

Conclusions

Grodsky and Carton were able to detect the bi-weekly oscillation during the West

African Monsoon with the aid of the wind observations from QuickSCAT and rainfall

amounts from TRMM

Questions/Comments?