Building on the TRMM Legacy

The GPM Core Observatory satellite is the successor to TRMM. With the help of the other constellation members, GPM aims to provide 3-hour global precipitation maps.

Global Precipitation Measurement (GPM) is a follow-on mission to TRMM whose goal is to provide global rain and snow maps every 3 hours. This can only be accomplished through a constellation of satellites that provide similar observation capabilities in near real time. The focus of the GPM mission is the GPM Core Observatory satellite (referred to simply as the GPM satellite). The GPM satellite was launched into a 407 km, 65-degree inclination orbit in early 2014, and carries next-generation versions of the TRMM precipitation mapping instruments. GPM's Microwave Imager (GMI) observes 13 microwave channels from 10 GHz to 183 GHz to measure all types of precipitation - from light rain to heavy snow and hail. Its conical scanning geometry was engineered to emulate the TRMM TMI and ensure continuity from TRMM to GPM. The GPM satellite also has a Dual-Frequency Precipitation Radar (DPR) that provides 3D measurements of storm structure. The inclusion of both Ku-band and Ka-band radars allows the GPM DPR to infer the sizes of hydrometeors in the storm. As with the TRMM PR, the GPM DPR swath is narrower than the other instruments. The Ka-band radar has a 120 km swath, while the Ku-band radar has a 245 km swath.

The GPM Core Observatory satellite is one member of the broader GPM constellation. In total, 10 different types of satellites from international partners have contributed passive microwave measurements to the GPM constellation. Data from this diverse collection of passive microwave imagers are intercalibrated and aggregated to accomplish GPM's goal of global precipitation maps with a 3-hour cadence.

The GPM constellation lacks instrumentation for detecting lightning, but it is still useful for atmospheric electricity research. Charge separation in electrified clouds generates conduction currents (Wilson currents) that flow up to the Ionosphere. I developed a passive microwave retrieval algorithm that estimates the total Wilson current for a given storm based on the spatial distribution of its vertically-integrated ice content. I have applied this algorithm to the microwave observations taken by TRMM, the GPM Core Observatory, and the GPM Constellation to quantify the total source current for the Direct-Current branch of the Global Electric Circuit (GEC). The retrieved source currents have been shown to agree with the fair-weather load on the GEC that is measured by the Carnegie curve. The satellite retrieval also confirms land / ocean differences in Wilson currents from thunderstorms observed by high-altitude aircraft. These results also suggest that the trend for oceanic storms to generate stronger currents despite producing less lightning is applicable generally across the tropical belt.