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David Atlas
,
Toshio Iguchi
, and
Harold F. Pierce

The authors discuss the origin of a unique footprint on the sea induced by storm winds and rainfall as seen by synthetic aperture radar (SAR) from space. Two hypotheses are presented to explain the origin of an apparent wind shadow downwind of a storm cell. The first suggests that the cool air pool from the storm acts as an obstacle to divert the low-level easterly ambient winds and leaves a “wind shadow” on its downwind side. This theory is discarded because of the excessive storm lifetime needed to cause the long downstream “shadow.” The second hypothesis invokes the cool outflows from two preexisting storm cells such that their boundaries intersect obliquely leaving a triangular wedge of weaker winds and radar cross section (i.e., the shadow). A new precipitation cell is initiated at the point of intersection of the boundaries at the apex of the shadow, giving the illusion that this cell is the cause of the shadow. While the authors lack corroborative observations, this theory is consistent with prior evidence of the triggering of convective clouds and precipitation by intersecting cool air boundaries. The regular observation of such persistent cool air storm outflow boundaries both in satellite observations, and more recently in SAR imagery, suggests that such discontinuities are ubiquitous and serve to trigger new convection in the absence of large-scale forcing.

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Arthur Y. Hou
,
Ramesh K. Kakar
,
Steven Neeck
,
Ardeshir A. Azarbarzin
,
Christian D. Kummerow
,
Masahiro Kojima
,
Riko Oki
,
Kenji Nakamura
, and
Toshio Iguchi

Precipitation affects many aspects of our everyday life. It is the primary source of freshwater and has significant socioeconomic impacts resulting from natural hazards such as hurricanes, floods, droughts, and landslides. Fundamentally, precipitation is a critical component of the global water and energy cycle that governs the weather, climate, and ecological systems. Accurate and timely knowledge of when, where, and how much it rains or snows is essential for understanding how the Earth system functions and for improving the prediction of weather, climate, freshwater resources, and natural hazard events.

The Global Precipitation Measurement (GPM) mission is an international satellite mission specifically designed to set a new standard for the measurement of precipitation from space and to provide a new generation of global rainfall and snowfall observations in all parts of the world every 3 h. The National Aeronautics and Space Administration (NASA) and the Japan Aerospace and Exploration Agency (JAXA) successfully launched the Core Observatory satellite on 28 February 2014 carrying advanced radar and radiometer systems to serve as a precipitation physics observatory. This will serve as a transfer standard for improving the accuracy and consistency of precipitation measurements from a constellation of research and operational satellites provided by a consortium of international partners. GPM will provide key measurements for understanding the global water and energy cycle in a changing climate as well as timely information useful for a range of regional and global societal applications such as numerical weather prediction, natural hazard monitoring, freshwater resource management, and crop forecasting.

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Gail Skofronick-Jackson
,
Walter A. Petersen
,
Wesley Berg
,
Chris Kidd
,
Erich F. Stocker
,
Dalia B. Kirschbaum
,
Ramesh Kakar
,
Scott A. Braun
,
George J. Huffman
,
Toshio Iguchi
,
Pierre E. Kirstetter
,
Christian Kummerow
,
Robert Meneghini
,
Riko Oki
,
William S. Olson
,
Yukari N. Takayabu
,
Kinji Furukawa
, and
Thomas Wilheit

Abstract

Precipitation is a key source of freshwater; therefore, observing global patterns of precipitation and its intensity is important for science, society, and understanding our planet in a changing climate. In 2014, the National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration Agency (JAXA) launched the Global Precipitation Measurement (GPM) Core Observatory (CO) spacecraft. The GPM CO carries the most advanced precipitation sensors currently in space including a dual-frequency precipitation radar provided by JAXA for measuring the three-dimensional structures of precipitation and a well-calibrated, multifrequency passive microwave radiometer that provides wide-swath precipitation data. The GPM CO was designed to measure rain rates from 0.2 to 110.0 mm h−1 and to detect moderate to intense snow events. The GPM CO serves as a reference for unifying the data from a constellation of partner satellites to provide next-generation, merged precipitation estimates globally and with high spatial and temporal resolutions. Through improved measurements of rain and snow, precipitation data from GPM provides new information such as details on precipitation structure and intensity; observations of hurricanes and typhoons as they transition from the tropics to the midlatitudes; data to advance near-real-time hazard assessment for floods, landslides, and droughts; inputs to improve weather and climate models; and insights into agricultural productivity, famine, and public health. Since launch, GPM teams have calibrated satellite instruments, refined precipitation retrieval algorithms, expanded science investigations, and processed and disseminated precipitation data for a range of applications. The current status of GPM, its ongoing science, and its future plans are presented.

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