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Chuntao Liu

Abstract

The rainfall contributions from precipitation features (PFs) with full spectra of different sizes and convective intensities over the tropics and subtropics are summarized using 12 yr of version 6 Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and Microwave Imager (TMI) observations. Regional, seasonal, and diurnal variations of the rainfall contributions from various PFs are shown, with the global distribution of the sizes, PR echo tops, maximum heights of 30 dBZ, and minimum TMI 85-GHz brightness temperatures of PFs above which contribute half of the rainfall in each 2° × 2° region. Though the results from radar and microwave observations generally agree with each other, some large differences exist over land. Seasonal variations of sizes and intensities of precipitation systems are found over the northeast Pacific, northern SPCZ, and some land areas in addition to the well-known monsoon regions. The diurnal cycles of rainfall over land and ocean are interpreted with the combinations of life cycles of various precipitation systems, using the diurnal variations of rainfall contributions from precipitation systems with different sizes and intensities. The long-duration rainfall events with more than four consecutive 3-h periods with rain at a grid point are identified from 11 yr of TRMM 3B42 products. These “12-h rain events” contribute a larger proportion of the total rainfall over ocean than over land. They are mostly correlated with precipitation systems with large sizes and intense convection. However, they can also be caused by some shallow persistent precipitation systems, such as those over the northeast slope of the Andes in Peru in spring and fall and over the west coast of India in summer.

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Chuntao Liu
and
Edward Zipser

Abstract

With 15 yr of the Tropical Rainfall Measuring Mission (TRMM) observations, the passive microwave radiometers [TRMM Microwave Imager (TMI)] and the precipitation radar (PR) report a close geographical distribution of annual precipitation between 36°S and 36°N. However, large discrepancies between PR and TMI precipitation retrievals are also found over several specific regions, such as central Africa, the Amazon, the tropical east Pacific, and north Indian Ocean. To understand these discrepancies, the PR near-surface and the TMI surface precipitation retrievals are compared at both pixel and precipitation system levels using collocated pixels and a precipitation feature database from 1998 to 2012. Over land, the TMI overestimates precipitation in deep and intense convective systems, but misses significant amounts of warm rainfall in shallow systems. Over the ocean, because of the partial beam filling of large footprints of the lower-frequency sensors, the TMI reports a larger precipitation area than the PR and underestimates the precipitation rate in the convective precipitation region. The TMI tends to overestimate precipitation compared to the PR in a large proportion of shallow systems over the tropical east Pacific and trade wind regions with large-scale descent. The PR tends to overestimate precipitation compared to the TMI in a large proportion of shallow systems over rainy oceans, such as the west Pacific and the Atlantic ITCZ. All these findings imply that there are still large uncertainties in the precipitation climatology over some regions. Further ground validation campaigns are still needed, especially over the ocean.

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Lindsey Hayden
and
Chuntao Liu

Abstract

Satellite-based instruments are essential to the observation of precipitation at a global scale, especially over remote regions. Each instrument has its own strengths and limitations in accurately determining the rate of precipitation at the surface. By using the complementary strengths of two instruments, a more complete analysis of global precipitation can be performed. The Global Precipitation Measurement (GPM) Core Observatory’s Dual-Frequency Precipitation Radar (DPR) is capable of measuring precipitation at high and medium precipitation rates by using Ku-band (13.6 GHz) radiation. The CloudSat satellite’s Cloud Profiling Radar (CPR) uses higher-frequency W-band (94 GHz) radiation and is therefore capable of measuring precipitation at low rates not detected by the GPM DPR. CloudSat observations from January 2007 to December 2016 and DPR observations from March 2014 to February 2018 are combined and the results examined. Since these datasets are not completely coincident, this study is conducted as a multiyear analysis. Observed precipitation from CloudSat is used starting at the lowest precipitation rates and increasing rates until the occurrence observed by GPM surpasses that of CloudSat, at which point data from GPM are used. The precipitation rate at which this change occurs contains important information on the amount of precipitation missed by each instrument and implications as to the size of the hydrometeors present. Liquid precipitation retrieval from CloudSat is not performed over land; analysis over land is produced here using the information available. By combining the two datasets, a more complete picture of precipitation occurring globally is obtained.

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Abishek Adhikari
,
Chuntao Liu
, and
Lindsey Hayden

Abstract

The uncertainties in the version 5 Global Precipitation Measurement (GPM) Microwave Imager (GMI) precipitation retrievals are evaluated via comparison with the radar–radiometer (so-called “Combined”) retrievals between 40°S and 40°N. Results show the precipitation estimates are close (~7% GMI overestimation) globally. However, some specific regions, such as central Africa, the Amazon, the Himalayan region, and the tropical eastern Pacific, show a large overestimation (up to 50%) in GMI retrievals when compared to Combined retrievals. The uncertainties are further evaluated based on precipitation system properties, such as size and intensity of the system. GMI tends to underestimate precipitation volume when the system is relatively warm (>250 K) and small (<200 km2) due to the lack of ice scattering signatures. However, for large systems (>2000 km2), GMI-derived precipitation is typically higher than Combined over all surfaces. Based on the system properties, a simple bias correction methodology is proposed to implement in the Goddard Profiling Algorithm (GPROF) to reduce GMI biases. GMI precipitation volume is adjusted in each precipitation system based on the size and minimum 89 GHz polarization-corrected temperature (PCT) over land and ocean separately. The overall GMI bias is reduced to 3%, with significant improvement over land. The GMI biases (up to 50%) over the previously mentioned regions are significantly or partially removed, becoming less than 20%. This method also shows effectiveness in removing zonal and seasonal biases from GMI estimates. These results suggest the importance of utilizing the information of whole precipitation systems instead of individual pixels in the precipitation retrieval.

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Christina L. Wall
,
Edward J. Zipser
, and
Chuntao Liu

Abstract

Using 13 yr of data from the Tropical Rainfall Measuring Mission (TRMM) satellite, a regional climatology of monsoonal precipitation is created for portions of the southwest United States. The climatology created using precipitation features defined from the TRMM precipitation radar (PR) shows that the population of features includes a large number of small, weak features that do not produce much rain and are very shallow. A lesser percentage of large, stronger features contributes most of the region’s rainfall. Dividing the features into categories based on the median values of volumetric rainfall and maximum height of the 30-dBZ echo is a useful way to visualize the population of features, and the categories selected reflect the life cycle of monsoonal convection. An examination of the top rain-producing features at different elevations reveals that extreme features tend to occur at lower elevations later in the day. A comparison with the region studied in the North American Monsoon Experiment (NAME) shows that similar diurnal patterns occur in the Sierra Madre Occidental region of Mexico. The population of precipitation features in both regions is similar, with the NAME region producing slightly larger precipitation systems on average than the southwest United States. Both regions on occasion demonstrate the pattern of convection initiating at high elevations and moving downslope while growing upscale through the afternoon and evening; however, there are also days on which convection remains over the high terrain.

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