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TRMM FOR EL NIÑO AND LA NIÑA STUDY


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Background

El Niño and La Niña refer to abnormal cold and warm sea surface temperature (SST) events respectivelly, which initiate along the western coast of Peru and expand west to the central equatorial Pacific Ocean.Scientists have found that El Nño and La Niña have profound effects on weather and climate in the whole tropics and subtropics, as well as in the middle and high latitudes.To understand El Niño /La Niña and further predict them is a challenging task, and needs to treat the ocean and the atmosphere above as an integrated system: SST affects the atmosphere mainly by providing latent heat and sensible heat fluxes; and the atmosphere behavior, such as wind and precipitation, in turn, affects SST.Scientists gain their knowledge of El Niño and La Niña by analyzing observed data and carring out the experiements with numeric atmospheric, ocean, or coupled ocean-atmosphere general circulation models.

It is well known that the low-frequency variation of tropic precipitation with an irregular 4-5 year period is mainly corresponding to SST cold/warm events.TRMM data by themselves provide a uniquely accurate rainfall data set, but the sampling becomes inadequate for short intervals and small areas. However, by combinining TRMM data with other satellite rainfall estimates and rain gauge analysis, the product TRMM 3B43, titled TRMM Other Data Source, extends TRMM-like accuracy to finer space and time resolutions.

 

Sample Application

Scientists have shown that TRMM rainfall data document the interannual variability during 1998-2000.The following figure depicts the El Niño and La Niña signature in the TRMM 3B43 data.

 

[rainfall of
            TRMM 3B43]

The top panel is the two-month mean rainfall rate (mm/month) for Jannuary and February 1998 over the tropical and subtropical regions. Heavy rainfall appears over the South Pacific Convergence Zone (SPCZ), South Indian Ocean, and the South America. The bottom one is the difference in the two-month mean rainfall between 1999 and 1998 (subtracting the mean of 1998 from that of 1999).

The difference figure shows a typical ENSO patterns in precipitation field corresponding to El Niño and La Niña events of SST. To see this relation clearly, the SST anomaly (C o) averaged on the 5S-5N belt is plotted on the longitude (horizontal axis) and time (vertical axis) dimension below.

 

[SSTA longitude- time]

This SST anomaly figure depicts the peak of the 1997-1998 El Niño at the start of the year and the entry into the 1998-1999 La Niña by the end of the year. The corresponding changes in the tropical precipitation field are characterized by a dipole pattern: a belt with reduced precipitation from the eastern equatorial Pacific Ocean to the Central part, and an area with increased precipitation over the western Pacific Ocean and thesouthwestern Pacific Ocean.

Scientists at GSFC explore the use of TRMM precipitation data in their numerical ocean model simulation.With TRMM precipitation data as an input, their model reproduces the real SST variations over the Indian ocean, as well as over the eastern equatorial Pacific and the equatorial Atlantic Oceans, for the El Niño period of fall 1997 to spring 1998. TRMM data successfully document the interannual precipitation variation, coordinating the SST changes beneath the atmosphere.

Reference: Murtugudde, R., J.P., McCreary Jr., A.J., Busalachi, 2000: "Oceanic processes associated with anomalous events in the Indian Ocean with relevance to 1997-1998",Journal of Geophysical Research , Vol. 105, No. C2

TRMM 3B43--TRMM and Other Data Source

TRMM 3B43 includes precipitation rate (mm/hr) and its root-mean-square (RMS) error estimates derived from TRMM and other data sources.These gridded estimates are on a calendar month temporal resolution and a 1-degree by 1-degree spatial resolution global band extending from 40 degrees south to 40 degrees north latitude.

The algorithm 3B-43 combines four independent precipitation fields: monthly average unclipped TRMM Microwave Imager (TMI) estimate (intermediate product from Algorithm 3B-42), the monthly average Special Sensor Microwave/Imager (SSM/I) estimate (TRMM product 3A46), the pentad-average adjusted merged-infrared (IR) estimate (TRMM product 3B42), and the monthly accumulated Climate Assessment and Monitoring System (CAMS) or Global Precipitation Climatology Centre (GPCC) rain gauge analysis (TRMM product 3A45).All input data sources are on the calendar month temporal resolution with the exception of the adjusted merged-IR data which is on the pentad (5-day) resolution.To obtain the requisite calendar month average of adjusted merged-IR data, the algorithm 3B-43 averages the adjusted merged-IR pentads that span the calendar month of interest.Also, prior to combination with the SSM/I, adjusted merged-IR, and rain gauge data, the monthly average unclipped TMI data is converted (re-calibrated) to TRMM Combined Instrument (TCI) data using the TMI/TCI calibration parameters from Product 3B-31.After the preprocessing is complete, the four independent precipitation fields are merged together to form the final precipitation rate and RMS precipitation-error estimates.

A complete description of Algorithm 3B-43 is provided in the Algorithm 3B-43 User's Guide, available from the TRMM Science Data and Information System (TSDIS). The file content description for Product 3B-43 is also available from TSDIS.

The data are stored in the Hierarchical Data Format (HDF), which includes both core and product specific metadata, gridstructure, and precipitation rate and relative error.The file size is about 0.2 MB (uncompressed). Sample Fortran and C programs to read this data set are available at Hydrology Online Information.

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Last updated: May 27, 2010 04:50 PM ET
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