# Presentation: 665281

## Evaluation of the effect of the orbit boost of the TRMM satellite on the PR rain estimates Shuji Shimizu, Riko Oki, Tetsuya Tagawa (JAXA/EORC)  Toshio Iguchi (NICT)Masafumi Hirose (Nagoya Univ./HyARC) 

- The 3rd Int’l TRMM Science Conference

***Evaluation of the effect of the orbit boost of the TRMM satellite on the PR rain estimates****** ****Shuji Shimizu, *_*Riko Oki*_*, Tetsuya Tagawa (JAXA/EORC) ** Toshio Iguchi (NICT)**Masafumi Hirose (Nagoya Univ./HyARC) *

*Introduction*
*Time Series of Global Precipitation from PR Standard Products*
*Evaluation of the Effect of the Orbit Boost on the PR Rain Estimates*
*Summary*

## 1. Introduction

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*1. Introduction*

*TRMM satellite and the Precipitation Radar (PR) has been collecting 3-Dimensional global rainfall data for more than 10 years without any major troubles.** *
*The rainfall amount estimated by PR shows a decrease after the satellite altitude was changed from 350 km to 402.5 km in August 2001 to extend its lifetime. It is an issue in  trend analysis of global precipitation.*
*The objective of this study is to quantitatively evaluate the effect of the orbit boost of the TRMM satellite on PR rain estimates.*

## 2. Time Series of global precipitation from PR standard products

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*2. Time Series of global precipitation from PR standard products*

- Global Monthly Rainfall (3A25)

|   | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 1998-2000 | 2002-2005 | Rate of Change |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| nSurf | 77.68 | 78.13 | 77.38 | 75.88 | 73.08 | 73.43 | 74.1 | 74.41 | 77.73 | 73.75 | -5.1% |
| eSurf | 73.92 | 74.29 | 73.6 | 72.02 | 69.09 | 69.42 | 70.06 | 70.34 | 73.94 | 69.73 | -5.7% |
| 2km | 71.89 | 73.02 | 72.25 | 71.83 | 70.53 | 70.76 | 71.44 | 71.56 | 72.38 | 71.08 | -1.8% |
| 4km | 53.54 | 52.85 | 52.17 | 52.8 | 53.15 | 53.32 | 53.43 | 53.47 | 52.85 | 53.34 | +0.92% |
| 6km | 13.49 | 12.54 | 12.47 | 12.85 | 13.15 | 13.23 | 13.19 | 13.35 | 12.83 | 13.23 | +3.1% |

## 2. Temporal change of global precipitation  from PR standard products

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*2. Temporal change of global precipitation  from PR standard products*

- Global Monthly Rainfall (2A25, Angle bin No. 23-27)

|   | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 1998-2000 | 2002-2005 | Rate of Change |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| nSurf | 76.28 | 76.66 | 76.34 | 75.62 | 75.44 | 75.96 | 76.53 | 77.16 | 76.42 | 76.27 | -0.20% |
| eSurf | 73.93 | 74.27 | 73.98 | 73.3 | 73.09 | 73.6 | 74.15 | 74.76 | 74.06 | 73.9 | -0.22% |
| 2km | 71.25 | 72.55 | 72.11 | 71.59 | 71.29 | 71.44 | 72.07 | 72.48 | 71.97 | 71.82 | -0.21% |
| 4km | 46.69 | 45.79 | 45.31 | 45.69 | 45.48 | 46.13 | 46.2 | 46.31 | 45.93 | 46.03 | +0.21% |
| 6km | 10.86 | 10.17 | 10.18 | 10.31 | 10.18 | 10.29 | 10.29 | 10.47 | 10.4 | 10.31 | -0.92% |

**Notes:**
衛星直下付近のPR2A25データ(アングルビンNo.23-27)を用いて推定した、各月の全球平均降水量の変動(地表面、地表付近、高度2km)
直下付近であれば、高度変更による感度低下以外の影響はないと考えられる。
このグラフで見る限り、高度変更の影響を見ることが出来ない。

## 3. Evaluation of the effect of the orbit boost on the PR rain estimates

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*3. Evaluation of the effect of the orbit boost on the PR rain estimates*

*Major causes of the changes in rain estimates by the orbit boost are:*
*Sensitivity degradation by the increase of satellite altitude*
*Increase of the range of surface clutter by the increase of the footprint size, and*
*Mismatch between the transmission and reception angles for one pulse in every 32 pulses*

**Notes:**
The major causes of the changes in rain estimates by the orbit boost are:
the sensitivity degradation by the increase of satellite altitude
the increase of the range of surface clutter by the increase of the footprint size, and
the mismatch between the transmission and reception angles for one pulse in every 32 pulses
We estimated the effect for each element after this slide. 

## A) Sensitivity degradation by the increase of satellite altitude

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- 402.5km

- 350.0km

**0.7°**

**0.7°**

- IFOV

- 4.3km5.0km

- 52.5km

*A) Sensitivity degradation by the increase of satellite altitude*

*The decrease of the echo signal intensity by the change of the satellite altitude from 350 km to 402.5 km is:*
*	**1.21 (dB) = 20log(402.5/350).*
*The proportion of weak rain echoes that cannot be detected by PR has increased, because the noise power did not change with the altitude change.*

## A) Sensitivity degradation by the increase of satellite altitude- Simulation for 1.2dB degradation (Result) -

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*A)** **Sensitivity degradation by the increase of satellite altitude**- Simulation for 1.2dB degradation (Result) -*

*From the simulation, we estimate a decrease of **0.5%** in the surface rainfall estimates by the sensitivity degradation. *

- -0.50%

- 2A25 Estimated Surface Rain (All, 2001/01)

*Simulation was carried out by subtracting 1.21dB in echo power from original level 0 data to 1B21 but keeping the noise power the same.*
*The corresponding Level 2 data was produced using the simulated 1B21 products for 1 month (2001/02).*
*Version 6 algorithm was used.*

- Rain Rate (mm/h)

- -- Original
- -- Simulated

## B) Increase of the range of surface clutter by the increase of the footprint size

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- a)

- b)

*B) Increase of the range of surface clutter by the increase of the footprint size*

*The increase of the footprint size has raised the heights of clutter free bottom.*
*PR misses shallow rain systems more often after the boost than before the boost.*

## C) Beam mismatch between the transmission and reception angles

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- Received Beam

- Transmitted Beam

*C) Beam mismatch between the transmission and reception angles*

*The mismatch between the transmission and reception angles occurs for one pulse in every 32 pulses.*
*The bias caused by the mismatch is corrected in 1B21.*
*However, the leak of surface echo to the next beam is often over-corrected because of its sharpness.  As a result, rain rate estimates near the surface in the second-half of the PR scan tend to be smaller than those in the first-half on average.*

- Scan

**Notes:**
A beam mismatch correction is executed in the current 1B21 algorithm, but the correction algorithm still has a small problem and rain rate estimates near the surface in the second-half of the PR scan tend to be smaller than those in the first-half on average. 

## Effects of B) the increase of the footprint size and C) beam mismatch correction 

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*Effects of B) the increase of the footprint size and C) beam mismatch correction *

*The changes due to the second and third causes can be estimated by comparing the estimates at off-nadir bins with those at near-nadir bins with the assumption that rain rates at all angle bins should be the same on average.*
*Asymmetrical pattern of precipitation **** Beam Mismatch correction error*
*Precipitation decrease at off-nadir bin **** Increase of the footprint size*
*In order to remove the effect of beam mismatch, symmetrized data for anglebins are made using first-half data of scan.*
*We estimate the effect of beam mismatch by comparison between original data and symmetrized data.*

- Original

- Symmetrized

- Negative biases of rain estimates relative to the nadir bin (%) (2000/2002)

**Notes:**
「欠損率」というのが良くないような気がしますし、「The rate of Precipitation Loss」という英語も良くないと思いますが、とりあえず。

## Effects of B) the increase of footprint size and C) beam mismatch correction (Result)

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*Effects of B) the increase of footprint size and C) beam mismatch correction (Result)*

*Effects of the increase of footprint size*
*Before the boost **	**** **	**2.36 %*
*After the boost**	**** **	**4.85 %*

*Effects of the beam mismatch correction error*
*Before the boost**	****	**2.85 - 2.36 = **0.49%*
*After the boost**	****	**8.18 - 4.85 = **3.33 %*

- Biases in off-nadir rain total from near-nadir rain total (%)

|   | Year | Convective | Stratiform | All | 2002-2000 |
| --- | --- | --- | --- | --- | --- |
| Original | 2000  | -1.15  | 3.78  | 2.85  | 5.33  |
|  | 2002  | 4.57  | 9.03  | 8.18  |  |
| Symmetrized | 2000 | -1.78  | 3.33  | 2.36  | 2.49  |
|  | 2002 | 1.62  | 5.60  | 4.85  |  |

*2.49%*

*2.84%*

**Notes:**
クラッタフリーの高度が上昇することにより、全アングルビンでの降水の欠損率は高度変更前には2.36%、変更後は4.85%。このことからビーム幅の増加による地表面降水量の欠損は2.49%と見積もれる。
高度変更後(2002年)のビームミスマッチに伴う左右非対称の影響は、8.18 - 4.85 = -3.33%。ただし元々高度変更前でもPRの傾向として、左右非対称の傾向があり、2000年のデータでその効果を見積もると2.36 - 2.85 = -0.49%。以上からビームミスマッチによる左右非対称の影響による効果は、-3.33 - (-0.49) = -2.84%と見積もられる

## 4. Summary of the simulations

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*4. Summary of the simulations*

| Causes | Phenomena | Effects in Precipitation Estimation | Decrease in total rain |
| --- | --- | --- | --- |
| A. Sensitivity degradation  | Decrease in S/N | Increase in missing light rain | -0.5% |
| B. Increase of the footprint size | Increase of surface clutter  (Increase of the valid rain echo bottom height) | a) Decrease in detection of shallow rain at off-nadir | -2.5% |
|  |  | b) Taking statistics at higher altitude  |  |
| C. Beam mismatch correction error | Overcorrection of the surface echo to the adjacent beam | Negative bias in reflectivity (Z)
(only in the latter half of scan) | -2.8% |

*The total decrease in e_surface rain is estimated to be **5.8 %** on average in a global scale.*

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## Summary

*Issues*
*The simulation to estimate the sensitivity degradation (A) was carried out using only one month of data. To obtain more accurate statistics, it is necessary to increase data period. *
*The estimation of 2**nd** (B) and 3**rd** (C) effects is based on the assumption that rain rates at all angle bins should be the same on average. *
*Improvement of beam mismatch correction in 1B21 will be implemented in V7.  *