Steven W. Bidwell

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) instrument: role, performance, and status

The Global Precipitation Measurement (GPM) microwave imager (GMI) instrument is a multi-channel, conical-scanning, microwave radiometer serving an essential role in the near-global-coverage and frequent-revisit-time requirements of GPM. As a part of its contribution to GPM, NASA will provide a GMI instrument and a spacecraft for the Core observatory and is considering the acquisition of a second GMI instrument for placement aboard a constellation spacecraft with a payload and orbit to be defined. In March 2005, NASA chose Ball Aerospace & Technology Corporation to provide the GMI instrument(s). This paper describes the GMI instrument, the technical performance requirements, its role within the combined passive and active microwave measurements on the Core observatory, and the timeline for GMI development and acquisition.

On the feasibility of a Doppler weather radar for estimates of drop size distribution using two closely spaced frequencies

Dual-frequency weather radar data can be gathered using a single broadband power amplifier and antenna for the purpose of estimating parameters of the hydrometeor size distribution. This is an attractive feature for observation platforms that are limited with respect to mass or available power. Whether useful properties of the scattering medium can be obtained from data of this type is the focus of the paper. Generally, as the center frequency or the bandwidth is decreased, the reflectivity factor difference falls below the level of the inherent signal fluctuations. Even if large numbers of independent samples can be gathered to permit estimates of the differential signals, the question remains as to whether the signal can be related unambiguously to properties of the rain or snow. Center frequencies at or near 35 GHz with bandwidths in excess of 5% give relatively strong differential signals. The signal, moreover, is directly related to the median mass diameter of the size distribution. The differential mean Doppler at frequencies where non-Rayleigh scattering effects are significant is also of use because the quantity depends only on the terminal velocity of the drops and is insensitive to the mean air and platform motion. In principle, the mean and differential mean Doppler velocities from a nadir-viewing radar can be used to estimate the mean vertical air motion and the median drop diameter of the size distribution.

Preparations for Global Precipitation Measurement (GPM) ground validation

The Global Precipitation Measurement (GPM) program is an international partnership led by the National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration Agency (JAXA). GPM will improve climate, weather, and hydrometeorological forecasts through more frequent and more accurate measurement of precipitation across the globe. This paper describes the concept and the preparations for ground validation within the GPM program. Ground validation (GV) plays a critical role in the program by investigating and quantitatively assessing the errors within the satellite retrievals. These quantitative estimates of retrieval errors will assist the scientific community by bounding the errors within their research products. The two fundamental requirements of the GPM ground validation program are: (1) error characterization of the precipitation retrievals and (2) continual improvement of the satellite retrieval algorithms. These two driving requirements determine the measurements, instrumentation, and location for ground observations. This paper describes GV plans for estimating the systematic and random components of retrieval error and for characterizing the spatial and temporal structure of the error. This paper describes the GPM program for algorithm improvement in which error models are developed and experimentally explored to uncover the physical causes of errors within the retrievals. GPM will ensure that information gained through ground validation is applied to future improvements in the space-borne retrieval algorithms. This paper discusses the potential locations for validation measurement and research, the anticipated contributions of GPMs international partners, and the interaction of ground validation with other GPM program elements.

Plans for Global Precipitation Measurement ground validation

This paper introduces plans for ground validation (GV) for the Global Precipitation Measurement. At NASAs request, a Ground Validation Working Group, formed from the meteorological and hydrological communities, is recommending plans to guide the GV program. Ground validation efforts will commence as early as 2003 with the Spring 2003 Pilot Experiment and described herein. The Pilot Experiment is focused on mitigating engineering and scientific risk to the GPM program and, in particular, to the ground validation program.

Application of a differential reflectivity technique to the EDOP radar in ground-based operation

This paper discusses the modification of a single-frequency Doppler radar (9.6 GHz) to accommodate dual-frequency operation for the study of the microphysical character of precipitation. The modification involves the ER-2 Doppler radar (EDOP), an airborne, meteorological research radar of the NASA/Goddard Space Flight Center. Radar operation is modified to provide reflectivity signals at two distinct frequencies at 9% separation. Differential reflectivity results are shown from ground-based measurement. Future work will derive drop size distribution estimates from these measurements.

Path-average drop size distribution estimation from dual-wavelength measurements of attenuation

Microwave path-average measurements of rainfall attenuation at 25.35 GHz (A/sub 25H/), 38.025 GHz (A/sub 38H/), using the upgraded 2.3-km NASA TRMM microwave link, are used to estimate parameters (/spl Lambda/ and N/sub 0/-/spl mu/ fixed) of a path-average gamma drop size distribution N~(D). This dual-wavelength technique requires the computation of the forward scattering amplitude f(D) for oblate raindrops to solve for the drop size distribution parameters numerically. The resulting N~(D) is then used to compute the corresponding instantaneous rain rate, R~, which is compared to rainfall rate estimates based on empirical power law relations, and to path-average rainfall rate measurements from a network of optical and tipping bucket raingauges located beneath the link path. Rain rate estimates using link path-average polarization differential phase-shift measurements (/spl Phi//sub DP/) at 8.45 GHz are also implemented.

A Novel Solid-State, Dual-Polarized, Dual Wavelength Precipitation Doppler Radar/Radiometer

The economic wealth and daily lives of United States citizens and people of all nations are affected by precipitation. Accurate quantitative precipitation measurements locally and on a global scale are needed to improve our ability to forecast and our understanding of these events. Efficient resource mobilization to minimize the impact of devastating precipitation events demands accurate and timely mapping of these event as they occur. To help address these needs, Remote Sensing Solutions, working with NASA, is developing a novel solid-state, dual-polarized, dual-wavelength precipitation Doppler radar/radiometer system. The innovations necessary to realize this unique and powerful system are described and the performance presented.

Capabilities and recent results from the ER-2 Doppler radar (EDOP)

The ER-2 Doppler radar (EDOP) is an X-band (9.60 GHz) Doppler radar flown on the high-altitude (nom. 20 km) NASA ER-2 aircraft. A major objective of EDOP is the study of air motions within convective precipitating regions from the strong updraft regions to the cirrus anvil regions. It is common for EDOP to fly with a complement of passive and active precipitation and water vapor instruments on the ER-2. The purpose of this paper is to describe the EDOP instrument specifications and capabilities and to very briefly describe recent data and research activity.

Global precipitation measurement (GPM) microwave imager (GMI) instrument

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) instrument is a multi-channel, conicalscanning, microwave radiometer serving an essential role in the near-global-coverage and frequent-revisit-time requirements of GPM. As a part of its contribution to GPM, NASA will provide a GMI instrument and a spacecraft for the Core observatory and is considering the acquisition of a second GMI instrument for placement aboard a constellation spacecraft with a payload and orbit to be defined. In March 2005, NASA chose Ball Aerospace & Technology Corporation to provide the GMI instrument(s). This paper describes the GMI instrument, the technical performance requirements, its role within the combined passive and active microwave measurements on the Core observatory, and the timeline for GMI development and acquisition.

Validation and error characterization for the Global Precipitation Measurement

In this paper, we have dealt with the validation and error characterization for the Global Precipitation Measurement (GPM) research initiative. The GPM is a three-year on-orbit duration program with a five-year duration goal. The Core satellite launch is scheduled tentatively for Fall 2008. Presently, GPM is in formulation stage in which the team is developing concepts and requirements prior to design work. As a part of formulation, ground validation is developing its requirements with a top level schedule requirement of commencing GV operations two years prior to the Core satellite launch. The rationale is that GV will benefit from a two-year head start in preparation for the Core observations. The requirements for GV are being developed in collaboration with, and vetted by, the precipitation science community.