COMPARISON OF TRMM PR V6 AND V7 FOCUSING HEAVY RAINFALL 1 Shinta Seto Institute of Industrial Science, the University of Tokyo, Japan Toshio Iguchi National Institute of Information and Communications Technology (NICT), Japan Robert Meneghini Goddard Space Flight Center, NASA 1 This study is partially supported by KAKENHI(22760365), JSPS, Japan ABSTRACT The Tropical Rainfall Measuring Mission (TRMM) / Precipitation Radar (PR) has been working for more than 12 years, and the latest version (Version 7; V7 in short) of the standard product is published in 2011. A test product (called ITE233), which is essentially the same with the final product of V7, is compared with the previous version (Version 6; V6 in short). Generally, rain rate estimates are larger in V7 than in V6 both over land and over ocean. Histogram, the incident angle dependence, and the geographical distribution of heavy rainfall events are shown and the reasons why rain rates are increased over land in going from V6 to V7 are discussed. Index Terms— TRMM, PR, SRT, heavy rainfall 1. INTRODUCTION The standard algorithm for the Tropical Rainfall Measuring Mission (TRMM) / Precipitation Radar (PR) is updated to Version 7 (V7 in short) in 2011. A test product (called ITE233), which is mostly the same with the final product of V7, is compared with the previous version (Version 6; V6 in short). 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 reference technique (SRT). However, only H-B method is used for lighter rainfall, for which SRT is unreliable to estimate path integrated attenuation (PIA). The SRT can employ three references for over ocean; along-track spatial reference (ATSR), temporal reference (TR), and hybrid spatial reference (HSR). If all the pixels in a swath are over-ocean pixels, the HSR is selected. In V6, the angle bin dependence of surface backscattering cross section (σ 0 ) is assumed to follow a quadratic function, but this assumption is not very appropriate and causes unnatural angle bin dependence of rain rate estimates [1]. In V7, two quadratic functions are used separately for inner swath and outer swath to fit σ 0 more accurately. For land pixels, HSR is never selected. Between ATSR and TR, one with higher reliability is selected in V6. ATSR generally refers neighboring pixels, which are expected to have similar land surface conditions with the current pixel. However, ATSR in V6 sometimes refers pixels far from the current pixel and large biases in PIA and rain rates are resulted. Heavy rain rate estimates (more than 50 mm/h, hereafter in this paper) are found mostly in the center of the swath, where σ 0 is unstable, and a lot of heavy rain rate estimates 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 is applied 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 moisture effects [1] are not well explained in TR and the underestimation in rain rates is partly caused. In V7, to explain soil moisture effects, an artificial increment of 0.5 dB is given for all over-land pixels. Moreover, the database is reproduced by 10-year observations and for higher spatial resolutions of 0.1 by 0.1 degrees in V7.