ENSO Precipitation Variations Using Passive Microwave and Radar Observations from TRMM and GPM

The Global Precipitation Measurement(GPM) Core Observatory, built on the notable successes of Tropical Rainfall Measuring Mission (TRMM, 1997-2014), was launched in February 2014. With their unique combination of instruments to measure precipitation,TRMM and GPM have become the foremost satellite platforms for diagnosing tropical (and global) precipitation. Now, the availability of 16-plus years of TRMM and 4-plus years of GPM data allow us to define a number of aspects of the climatology of rainfall and the inter-annual variations compared to that climatology.

The length of the TRMM and GPM record (over 20 years) allows research into inter-annual variations and even short-term “trends” or changes, essentially something on an inter-decadal time scale with both passive microwave and radar-based precipitation. While the longer record of the passive microwave—back to 1988—is typically used for the climatological calculations, this kind of independent look with radar-based precipitation information is important to advance our understanding of precipitation variations, especially related to variations in surface temperature (e.g., related to ENSO). In addition, the passive microwave retrievals utilize a freezing level height and are tied in some way to temperature, even if the effect may be subtle and difficult to detect. However, the radar information is independent of temperature. Therefore, on the inter-annual scale rainfall information from TRMM and GPM enables an independent view at some basic precipitation-temperature relations being derived from another precipitation data set, e.g., GPCP. These precipitation-temperature relations are important to understand climate variations on the inter-annual to inter-decadal scale, especially in this era of global warming.

Our previous studies with TRMM V7 data found that the active microwave (PR) surface precipitation-temperature relations do not confirm the relations based on passive microwave (TMI) observations. Over tropical ocean, TMI and GPCP rainfall data show clear positive slopes, but the PR rainfall fitted slope is near zero (even negative), except at the 6 km level. Our more recent analysis with GPM V05 data indicated that the active microwave (DPR) observations at all levels exhibit a positive surface precipitation-temperature relations in the tropical ocean and ocean-land combined, although the rates are much less than those from passive microwave (GMI) observations.

In this study, utilizing the recently available improved TRMM V8 and GPM V06 data using the same algorithms, we re-examineinter-annual and other variations of surface precipitation derived from passive microwave and radar in relation to variations in surface temperature. These results are also compared to the results from other widely used data set (e.g. GPCP).