COMPARISON OF TRMM PR V6 AND V7 FOCUSING HEAVY RAINFALL1Shinta Seto
Institute of Industrial Science, the University of Tokyo, JapanToshio Iguchi
National Institute of Information and Communications Technology (NICT), Japan
Robert Meneghini
Goddard Space Flight Center, NASAABSTRACT
The Tropical Rainfall Measuring Mission (TRMM) /Precipitation Radar (PR) has been working for more than 12years, and the latest version (Version 7; V7 in short) of thestandard product is published in 2011. A test product (calledITE233), which is essentially the same with the final productof V7, is compared with the previous version (Version 6; V6in short). Generally, rain rate estimates are larger in V7 thanin V6 both over land and over ocean. Histogram, theincident angle dependence, and the geographical distributionof heavy rainfall events are shown and the reasons why rainrates are increased over land in going from V6 to V7 arediscussed.
Index Terms— TRMM, PR, SRT, heavy rainfall
1. INTRODUCTION
The standard algorithm for the Tropical Rainfall Measuring
Mission (TRMM) / Precipitation Radar (PR) is updated toVersion 7 (V7 in short) in 2011. A test product (calledITE233), which is mostly the same with the final product ofV7, is compared with the previous version (Version 6; V6 inshort).
2. SURFACE REFERENCE TECHNIQUE (SRT)The standard algorithm for the TRMM/PR applies a hybrid
method of the following two methods to retrieve rain rates;
Histchfeld-Bordan (H-B) method and surface referencetechnique (SRT). However, only H-B method is used forlighter rainfall, for which SRT is unreliable to estimate path
integrated attenuation (PIA). The SRT can employ three1

references for over ocean; along-track spatial reference(ATSR), temporal reference (TR), and hybrid spatialreference (HSR). If all the pixels in a swath are over-ocean
pixels, the HSR is selected. In V6, the angle bin dependenceof surface backscattering cross section (σ0) is assumed tofollow a quadratic function, but this assumption is not very
appropriate and causes unnatural angle bin dependence ofrain rate estimates [1]. In V7, two quadratic functions areused separately for inner swath and outer swath to fit σ0
more accurately.
For land pixels, HSR is never selected. Between ATSR andTR, one with higher reliability is selected in V6. ATSR
generally refers neighboring pixels, which are expected tohave similar land surface conditions with the current pixel.
However, ATSR in V6 sometimes refers pixels far from thecurrent pixel and large biases in PIA and rain rates areresulted. Heavy rain rate estimates (more than 50 mm/h,hereafter in this paper) are found mostly in the center of theswath, where σ0 is unstable, and a lot of heavy rain rateestimates are found near the western coast of continents,
where referred pixels can be located in different continents.
In V7, not only forward reference but backward reference isapplied in ATSR and the distance between the current pixel
and referred pixels is limited.
TR refers the database of 1 by 1 degrees in V6, and gives
relatively stable estimates; few suspicious heavy rain rates
are estimated when TR is selected. However, soil moistureeffects [1] are not well explained in TR and theunderestimation in rain rates is partly caused. In V7, toexplain soil moisture effects, an artificial increment of 0.5
dB is given for all over-land pixels. Moreover, the databaseis reproduced by 10-year observations and for higher spatial
resolutions of 0.1 by 0.1 degrees in V7.

This study is partially supported by KAKENHI(22760365), JSPS, Japan

Fig. 1(a) over land
Cumulative histogram of instantaneous rain rates.
3. COMPARISON BETWEEN V6 AND V7

For comparison between V6 and V7(ITE233), all the data inthe year of 2008 are taken.
3.1. Histogram of instantaneous rain rates
Figure 1 shows cumulative histograms of instantaneous rainrates (x mm/h). The vertical axis shows the contribution of
instantaneous rain rates between 0 and x to the total rainamount [mm/month]. Fig. 1(a) is for all over-land pixels.
The total rain amount (accumulation from x=0 to x=300) is
59.4 mm/month in V7 and is larger than in V6 (57.6mm/month). Heavy rainfall events contribute to the increaseof the total rain amount from V6 to V7. The cumulative rainamount from x=0 to x=50 is almost the same between V6and V7. Fig. 1(b) is for all over-ocean pixels. The total
amount is 75.3 mm/month in V7 and is larger than V6 (72.0(a) V6, over land

Fig. 2(c) V6, over ocean
Geographical distribution of heavy rainfall events.

(b) over oceanmm/month). This difference is not mainly caused by heavy
rainfall events.
3.2 Geographical distribution of heavy rainfall events
Figure 2 shows where heavy rainfall events are detected.
Over land, rain rates of 300 mm/h are found both in V6 (Fig.
2(a)) and V7 (Fig. 2(b)). However, they are found only inthe specific orbits and regions in V6, and they are probably
caused by algorithm bugs. In V7, 300 mm/h estimates arefound in various regions, but many are located along steep
mountains and near coast. There are possibilities that SRT
misleads to these ultimately heavy rain estimates in V7 as
ATSR in V6 caused false heavy rain estimates. Over ocean,
the number and geographical distribution of heavy rain rates
are not very different between in V6 (Fig. 2(c)) and V7 (Fig.
2(d)), except for some 300 mm/h estimates in V6, which areprobably caused by bugs.

(b) V7, over land(d) V7, over ocean

(a) V6, over land

Fig. 3(c) V6, over ocean
Incident angle dependence of the total ra(b) V7, over land(d) V7, over oceanin amount with the contribution of heavy rainfall events.

3.3. Incident angle dependence of the total rain amount

may improve the incident angle dependence (Fig. 3(d)); thetotal rain amount tends to be larger at smaller incident angles.

The total rain amount is calculated for each angle bin and isshown in Fig. 3 with the contribution of heavy rainfall events.
4. THE INCREASE OF HEAVY RAINFALL IN V7
3.3.1. Over land
In V6, heavy rain rates are found mostly in the center of theswath and the total rain amount is larger at smaller incident
angles (Fig. 3(a)). In V7, rain rates between 50 and 100mm/h are found even in the edges of the swath, but
extremely heavy rain events (larger than 100 mm/h) is foundmostly at angle bins 24, 25, and 26 (Fig. 3(b)). At angle bin
25, the total rain amount is apparently larger than those atsurrounding angle bins.
3.3.2. Over ocean
In V6, the total rain amount is highest around angle bin 10(Fig. 3(c)). This is probably caused by the inappropriate
fitting in HSR. Heavy rainfall events do not affect the shapeof incident angle dependence. In V7, modification in HSR

As is shown in the previous section, over land, heavy rainfall
events are more frequent in V7 than in V6. In this section, it
is examined if the increase of over-land heavy rainfall iscaused by the modification in SRT.
As is shown in Fig. 1(a), the total rain amount increases ingoing from V6 to V7 by 1.8 mm/month. At some pixels,
only V6 detects rain and the corresponding rain amount is0.6 mm/month. On the contrary, only V7 detects rain atsome other pixels and the corresponding rain amount is 0.1mm/month. In most cases, both V6 and V7 detects rain, andthe difference of the total rain amount at the matched pixels
is 2.3 (=1.8+0.6-0.1) mm/month. Later in this section, rain
rates are compared only at matched pixels.

Fig. 4(a)Comparison of PIA2A21, PIA2A25, and rain rates between V6 and V7.

Fig. 5(a) for heavy rainfall in V7 (> 50mm/h)(b) for heavy rainfall in V6 (> 50mm/h)
Incident angle dependence of PIA2A21, PIA2A25, and rain rates between V6 and V7.

4.1. Heavy rainfall events in V7
Fig. 4(a) compares V6 and V7 for PIA estimates by SRT(PIA2A21), the final PIA estimates (PIA2A25), and rain rateestimates. Pixels are categorized by rain rate estimates in V7
with the step of 1 mm/hour, except that the highest bin is
prepared for rain rates more than 100 mm/hour. Let us focus
heavy rainfall events in V7. PIA2A21 are slightly higher inV6 than V7, but PIA2A25 are higher in V7 than in V6 asrain rates are. In other words, PIA2A25 become closer toPIA2A21 in V7, while PIA2A25 and PIA2A21 are not closein V6. This suggests that SRT is much relied in V7 than V6.
The increase of heavy rainfall in going from V6 to V7 is notcaused by the increase of PIA2A21 but the increase of thereliability of SRT. The above explanation is appliedirrespective of angle bins (Fig. 5(a)).
4.2. Heavy rainfall events in V6
Fig. 4(b) is the same with Fig. 4(a), but the horizontal axis isinstantaneous rain rates in V6. Let us focus on heavy rainfall(b)events in V6. PIA2A21 is much higher in V6 than V7 andPIA2A25 is also higher in V6 than V7. This tendency isapparently seen when the incident angle is small (Fig. 5(b)).
This suggests that suspicious heavy rain estimates in V6 arecaused by overestimated PIA2A21 and they are notsupported by V7 with improved SRT.
5. SUMMARY
TRMM/PR V6 and V7 (ITE233) are compared. Over land,
heavy rainfall events are more frequent in V7 than V6,
which is partly caused by the increase of reliability of SRT.
On the other hand, suspicious heavy rainfall events in V6 are
generally not supported by V7.
[1] Seto, S., T. Iguchi, “Rainfall-induced changes in actual surface
reference backscattering cross sections and effects on rain rateestimates by spaceborne precipitation radar.”, J. Atmos. Oceanic
Technol., Vol. 24, pp1693-1709, 2007.
[2] Seto, S., T. Oki, T. Iguchi, “Evaluation and improvement ofsurface reference techniques for the TRMM PR”, IGRASS, 2008.