# Presentation: 054653

## A Ten-Year Rainfall Climatology Based on a Composite of TRMM Products

***Tropical Rainfall Measuring Mission (TRMM)***

- Robert Adler (U. of Maryland-College Park/NASA Goddard)
- _Jiang-Jiang Wang_ (UMBC/Goddard)
- Guojun Gu (UMBC/Goddard)
- George Huffman (SSAI/Goddard)

## Motivation

- Original TRMM climate objective: How much rain is falling in the Tropics? (especially over the ocean)
- To answer the question: “Bob, I want to validate my model against TRMM precipitation.  Which product should I use?”									                  Issues
- Different estimates of surface precipitation a strength of TRMM, but also confusing to users
- Passive microwave mean values questionable over land, radar estimates affected by boost in 2001; narrow swath vs. full swath, etc.

## Objective

- Develop a 10-year TRMM Composite Climatology (TCC) of _surface rainfall_ over the Tropics using the strength of multiple estimates from TRMM and using the dispersion among the estimates as a measure of error or confidence

## Approach

- Sub-select from among TRMM precipitation products based on known characteristics (strengths and weaknesses); separate for ocean and land
- Climatology will be simple mean (month of the year); range of values will be measure of “error”

- _Ocean:_
- 1) Passive microwave (2A12)
- 2) Radar (2A25) [_Near Surface_-adjusted for boost]
- 3) Combined (2B31)

- _Land:_
- 1) Radar (2A25) [_Near Surface-_*not adjusted* for boost]
- 2) Combined (2B31)
- 3) Multi-satellite with gauges (3B43)

_*Selected TRMM Rain Products*_

## Adjustment of Post-Boost PR (2A25) Estimates  see poster by Wang et al. (and submitted to JGR)

- Adjustment of Post-Boost PR (2A25) Estimates  *see poster by Wang et al. (and submitted to JGR)*

- Ocean Total Anomalies (25°N-25°S) from GPCP

## Anomalies as Function of Pre/Post Boost and Nino 3.4 SST

- _End Result:_ Post-boost _Near Surface increased by 6.0%_ over ocean

## Nine-year TRMM Composite Climatology (TCC) from Multiple Products

- _Ocean-- Mean of:_
- 1) Passive microwave (2A12)
- 2) Radar (2A25)
- 3) Combined (2B31)
- _Land-- Mean of:_
- 1) Radar (2A25)
- 2) Combined (2B31)
- 3) Multi-satellite with gauges (3B43)

- _Range_ of values at each point as a measure of confidence

- mm/day

## Nine-year TRMM Composite Climatology (TCC) from Multiple Products

- _Ocean-- Mean of:_
- 1) Passive microwave (2A12)
- 2) Radar (2A25)
- 3) Combined (2B31)
- _Land-- Mean of:_
- 1) Radar (2A25)
- 2) Combined (2B31)
- 3) Multi-satellite with gauges (3B43)

- mm/day

**Mean Tropical Rain (35N-35S):**
_** **__**2.69 mm/d**_**   +/- 3%**
**2.74 (ocean)  +/- 2.5%     2.55 (land) +/- 4%**

***Estimate of error = range/2***

## TCC Zonal Means and Comparison to GPCP Over Ocean

_**Zonal mean at peak (7°N)   **_**6.5 mm/d   +/- 3%**
_**Zonal mean at 6°S**_** ****	****3.6 mm/d +/- 5.5% **
_**Zonal mean at 23°N**_** ****	****1.7 mm/d +/- 9%**

*GPCP climate values over ocean  driven by Chiu/Chang/Wilheit estimates using SSM/I*

## Eastern Pacific Ocean

- _Ocean-- Mean of:_
- 1) Passive microwave (2A12)
- 2) Radar (2A25)
- 3) Combined (2B31)

- mm/day

**Rainfall Peak in E. Pac.****	**
**9+ mm/d +/- ~ 1.5 mm/d    ****	****(~ +/- 15%) **

- Mean

- Range

- mm/day

## Seasonal Variation of TRMM Composite Climatology

*Seasonal Variation of TRMM Composite Climatology*

- January

- July

- April

- October

## Comparison of Inputs to TCC

- TCC

- 2B31-TCC

- 2A12-TCC

- 3A25-TCC

**Not used over land**

## TCC Compared to GPCP

- TCC (smoothed to 2.5o)

- GPCP

- TCC-GPCP

- 1 mm/d out of ~6 mm/d

- 1.5 mm/d out of ~5 mm/d (30%)

## TRMM Composite Climatology (TCC) compared to GPCP

- Units: mm/d

- TCC (GPCP)

- 25oN-25oS: Close agreement-especially over ocean
- 35oN-35oS: TCC less than GPCP (~7%)

## Conclusions

- A _TRMM Composite Climatology (TCC)_ of surface rainfall is feasible, but must use a subset of products and account for limitations of individual estimates (e.g., impact of boost)
- _Range of estimates (or other statistic)_ can be used as measure of dispersion (and “error”) [absolute error?] (Kwajalein data now useful to estimate bias of TRMM estimates.)
- TRMM (TCC) estimate for global _tropical rain is 2.69 mm/d +/- 3%_ (2.5% over ocean), 3-9% for zonal means (ocean), up to 15% in E. Pac. maximum.
- _TCC confirms GPCP mean ocean estimates in deep tropics_, but _GPCP is significantly higher poleward of ~25o_.  Mid-latitude difference needs attention, even before GPM.   2A12 and 2A25 _may_ both be underestimating

## Future Work

- Redo calculations with _10 years_ of data
- Examine _monthly climatologies_ for clues to explain differences among products or compared to GPCP and other data sets (including validation)
- Examine land and coast issues with regard to gauge analysis impact (satellite only version?)
- Update with improved TRMM products (Version 7)  and  additional years of TRMM