# Presentation: 056789

## Comparison of TRMM Rain Rate Retrievals in Tropical CyclonesDan CecilMatt WingoUniversity of Alabama - Huntsville

*acknowledgements*:
- Support from NASA TCSP program
- Data provided by TSDIS (via Precipitation Feature database) and Remote Sensing Systems

## Motivation

- Want to use Microwave Imagers to study Tropical Cyclone rainfall, as a function of environmental conditions
- Multiple rain estimates out there - need to understand some characteristics
- Merged Microwave-IR products give a more complete temporal history of a storm, but we want to use estimates that rely on the same physics at all times (not alternating between varying degrees of impact from  IR and Microwave approaches)
- We use Remote Sensing Systems (RSS) products, in order to smoothly extend into pre-TRMM era with SSMI
- RSS uses 0.25° grid - need to account for that when comparing to pixel-level 2A12 and 2A25

## Methodology

- Average the 2A12 and 2A25 rain rates to the 0.25ox0.25o grid used by RSS
- Consider histograms of rain rate and contributions to total from each rain rate
- Use rain estimates from all tropical cyclones worldwide seen by TRMM
- mostly Atlantic results shown here, but others have similar behavior

**Methodology**

## Example- Ivan 2004

- 87 mm/hr peak

- Old RSS

- New RSS

- 2A12

- 2A25

- Large area saturated at 25 mm/hr

- Coast and land flags - data not used

- In the comparisons that follow, only grid points from within the PR swath are used.
- Land and coastal areas excluded.

## Rain Rate histograms

- colors -> different TC intensity classes
- linestyles -> different algorithms
- In TC inner core, variability is more a function of storm intensity than algorithm (that’s a good start!)

**Rain Rate histograms**

- Old RSS algorithm had numerous 25 mm/hr values

- These PR 2A25 plots are artificially capped at 30 mm/hr for plotting purposes only.

- 2A12

- New RSS

## Rain Rate histograms

**Rain Rate histograms**

## Contribution to Inner Core Rain

- As expected, high rain rates contribute more total rainfall for hurricanes (purple and red lines) than for TD (green) and TS (blue)
*The integral of each curve is an estimate of 0-100 km mean rain rate (unconditional)*

**Contribution to Inner Core Rain**

## Contribution to Outer Core Rain

- Old version of RSS algorithm had much more contribution from the high rain rates than any of the other algorithms

- PR has more contribution from the lowest rain rates

**Contribution to Outer Core Rain**

## Contribution to inner core rain

**Contribution to inner core rain**

- Old RSS

- New RSS

- PR 2A25

- TMI 2A12

- TMI 2A12 and current RSS have similar contributions to total rainfall as a function of rain rate - peak contribution from ~10 mm/hr
- PR has more broad distribution of contributing rain rates.
- NOTE: For plotting purposes only, the PR 2A25 plots are capped at 30 mm/hr.

## Major Hurricanes (ATL)

**Major Hurricanes (ATL)**

- Old RSS estimates ~2x as much rain (16.5 mm/hr) in inner core of major hurricanes, compared to other algorithms - too many 20-25 mm/hr rain rates
- PR has ~50% more rain (10.6 mm/hr) than TMI (~7 mm/hr), due to more of the high rain rates

*Areal mean rain rates*

## Cat 1-2 Hurricanes

**Cat 1-2 Hurricanes**

- New RSS has similar distribution as 2A12
*Total* mean rainfall from PR and TMI similar (~5 mm/hr), but with contributions from very different rain rates

## Tropical Storms

- Current TMI algorithms have more rain in the ~3-13 mm/hr range than PR
- PR has more of the very weak and very strong rain rates - gets same areal mean rain rate as TMI, but for very different reasons

## Tropical Depressions

## Variability with radius

**Variability with radius**

## Comparing basins

**Comparing basins**

*(smaller sample size for N. Indian Ocean)*

## Inner core histograms

**Inner core histograms**

- Radar is able to identify more weak, nonzero rain rates due to both its sensitivity and horizontal resolution - a single ~4x4 km radar pixel can give the ~25x25 km grid box a non-zero value

## Histograms with radius

**Histograms with radius**

## Summary

- TMI 2A12 and the current RSS algorithm (using TMI) give similar rain rates to each other
- Other algorithms (PR 2A25 and previous RSS) give very different distributions of rain rate
- Old RSS produced far too much rain, but new version generally consistent in amounts with 2A12 and 2A25

## Summary

*Even when TMI and PR produce similar total rain, it is distributed into very different rain rate distributions  -  NOT just a simple matter of horizontal resolution, since all values here were averaged to 0.25o grid*
- Many related plots on my laptop, if anyone is interested
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