Latest Documents

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    A Global Precipitation Mission (GPM) Validation Network Prototype
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    A Validation Network (VN) prototype is currently underway that compares data from the Tropical Rainfall Measuring Mission (TRMM) satellite Precipitation Radar (PR) to similar measurements from the U.S. national network of operational weather radars. This prototype is being conducted as part of the ground validation activities of the Global Precipitation Measurement (GPM) mission. The purpose of the VN is to provide a means for the precipitation community to identify and resolve significant discrepancies between the U.S. national network of ground radar observations and satellite observations. The ultimate goal of such comparisons is to understand and resolve the first order variability and bias of precipitation retrievals in different meteorological/hydrological regimes at large scales. The VN prototype is based on research results and computer code described by Anagnostou et al. (2001), Bolen and Chandrasekar (2000), and Liao et al. (2001).

  • The Vertical Cross Section Display Program for GPM Validation Network Geometry-Matched PR and GV Data Sets
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    The IDL procedure pr_and_geo_match_x_sections.pro provides the capability to interactively select locations for, and display, vertical cross sections of PR and GV reflectivity from geometry-matched data produced by the GPM Validation Network prototype. These data are contained in a set of netCDF data files, one per “rainy” site overpass (a TRMM PR overpass of a GV radar site, with precipitation echoes present). By default, the procedure also displays a vertical cross section of the difference (PR-GV) between the PR and GV reflectivity from the geo-matched data. By default, the cross sections are along the PR cross-track scan line through the point selected by the user (i.e., perpendicular to the orbit track). Random cross section alignments are not supported.


    The procedure has a feature allowing a calibration offset to be applied to the GV reflectivity data. If a GV site or GV data for a particular event is known to have an error in calibration relative to the PR, the calibration of the GV reflectivity data may be adjusted up or down in 1 dBZ increments on the currently displayed cross sections, so that the relative vertical structures of the PR and GV reflectivity fields can be evaluated with the calibration bias removed.

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    GPM Ground Validation System Level 3 Operations Concept
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    This specification defines the Level 3 functional and performance requirements for NASA’s Global Precipitation Measurement (GPM) mission Ground Validation System (GVS). Overall, the GPM mission has defined a series of scientific objectives which include improvement in predicting terrestrial weather, climate, and hydrometeorology through a better observational understanding of the global water cycle. The purpose of the GPM GVS is rooted in the need for independent and objective evaluation of the precipitation products generated by the GPM mission. For its part, the GVS provides an independent means of evaluation, diagnosis, and ultimately improvement of the GPM spaceborne measurements and precipitation retrievals. These goals are more completely defined as follows:

    • Evaluation—Quantify the uncertainties in GPM standard precipitation retrieval algorithms
    • Diagnosis—Understand the time and space error characteristics of GPM precipitation products generated by these algorithms, and
    • Improvement—Contribute to the improvement of GPM precipitation retrieval algorithms throughout the mission.

    Achieving these goals is seen as a necessary step for improved GPM data products and for increased utilization of these products in Global Climate Models (GCMs), Numerical Weather Prediction (NWP) models, and hydrometeorological models for climate and weather forecasting.

  • The Statistical Analysis and Display Program for GPM Validation Network Geometry-Matched PR and GV Data Sets
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    The IDL procedure geo_match_z_pdf_profile_ppi_bb_prox_sca_ps.pro provides the capability to compute statistics and generate displays of PR and GV reflectivity from geometry-matched data produced by the GPM Validation Network prototype. These data are contained in a set of netCDF data files, one per “rainy” site overpass: a TRMM PR overpass of a ground radar (GV) site, with precipitation echoes present, referred to below as an “event”. The procedure computes and displays tables of mean differences (PR-GV) between the PR and GV reflectivity from the geo-matched data for a selected event, with the data stratified into vertical layers in two manners: (1) by height above the surface, in 1.5-km-deep layers, for 15 levels centered from 1.5 to 19.5 km, and (2) into three layers defined by proximity to the bright band (freezing level): above, within, and below the bright band. For purposes of the latter, match-up samples are categorized as above (below) the bright band if their base (top) is 500 m or more above (750 m or more below) the mean bright band height. The remaining points are assigned as within the bright band. The mean bright band height is computed from the bright band analysis in the TRMM PR 2A-25 product. Only the attenuation-corrected PR reflectivity is used in the program, even though the “raw” PR reflectivity also is present in the netCDF data files.

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    C3VP Access Information
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    A summary of C3VP data access.

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    Disdrometer Derived Z-S Relations in South Central Ontario, Canada
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    During the winter of 2006-2007, a number of in-situ and remote sensing precipitation measuring devices were operated at the Center of Atmospheric Research Experiment (CARE) site located near Egbert, Ontario about 30 km to the NW of the King City C-band operational dual-polarized radar. While the experiment was originally designed to measure winter precipitation for the Canadian Cloudsat/CALIPSO validation program (C3VP), the NASA’s Global Precipitation Measurement (GPM) ground validation program joined the efforts (cf. Petersen et al., 2007; this conference) bringing optical disdrometers (2D-video and two Parsivel disdrometers) and a multi-frequency radar. The CARE is a well- instrumented facility including Vaisala FD12P visibility sensor, Precipitation Occurrence Sensor System (POSS), the McGill University’s vertically-pointing X- band Doppler, and Hydrometeor Velocity and Shape Detector (HVSD).

    In this paper we focus on two case studies, (a) the 6 December 2006 and (b) the 22 January 2007 snow events. Our objectives are six-fold, (a) to determine the characteriscs of snow size spectra, (b) to determine the bulk density of snow by comparing measurements of Parsivel and FD12P, (c) to estimate a density (ρ) versus ‘size’ relation for snow by comparing the 2D-video derived Zh measurements with the well-calibrated King City Zh data, (d) to compare the Zh between 2D-video, POSS and King City radars, (e) to estimate snowfall rate (SR) and equivalent melt water (MWR) rate, including comparison of melt water accumulations from 2D-video, POSS, and other ground-based instruments at the CARE site, and (f) to derive the Zh-SR and Zh-MWR power law relations from 2D-video and Parsivel data.

     

  • Light Precipitation Validation Experiment (LPVEx) Science Plan
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    The Light Precipitation Evaluation Experiment (LPVEx) planned for the Gulf of Finland in September and October, 2010 will seek to address this shortcoming by collecting microphysical properties, associated remote sensing observations, and coordinated model simulations of high latitude precipitation systems to drive the evaluation and development of precipitation algorithms for current and future satellite platforms. Specifically, LPVEx seeks to characterize the ability of CloudSat, the Global Precipitation Measurement mission (GPM) Dual-frequency Precipitation Radar (DPR), and existing/planned passive microwave (PMW) sensors such as the GPM microwave imager (GMI) to detect light rain and evaluate their estimates of rainfall intensity in high latitude, shallow freezing level environments. Through the collection of additional microphysical and environmental parameters, the campaign will also seek to better understand the process of light rainfall formation and augment the currently limited database of light rainfall microphysical properties that form the critical assumptions at the root of satellite retrieval algorithms. Specific science questions include: identify source of spread in current satellite estimates of rainfall- minimum detectable rain rates, phase discrimination, physical assumptions in algorithms, spatial variability,

    • What are the minimum rainrates that can be detected by current satellite precipitation sensors in environments with shallow freezing levels (lower than 2 km)?
    • How will rainfall detection be improved by proposed future platforms?
    • How well can these sensors discriminate rain from falling snow?
    • Are the microphysical assumptions, such as raindrop size distribution, cloud water contents, and properties of the melting layer and precipitating ice aloft, currently employed in global satellite precipitation algorithms representative of high latitude precipitation in a statistical sense?
    • What is the impact of variability in these microphysical assumptions and those related to vertical structure and spatial inhomogeneity on random errors in retrieved rainfall rate?
    • Collectively, are the above inter-sensor differences large enough to explain the wide spread in current satellite estimates of high-latitude rainfall?
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    Profiler Data Sets for the NASA PMM Community
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    In support of NASA PMM, observations collected near Darwin, Australia, have been processed and data sets are being provided to the NASA PMM community to help validate and improved satellite retrieval algorithms and cloud resolving models.

    The data sets are derived from surface rain gauges, a disdrometer, and vertical pointing profilers deployed at the Australian Bureau of Meteorology Research Centre (BMRC) wind profiler site near Darwin, Australia during the Tropical Western Pacific – International Cloud Experiment (TWP-ICE).

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    D3R Technical Specifications
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    Detailed specifications document for the Dual-Frequency, Dual-Polar, Doppler Radar (D3R)

  • Scientific and Engineering Overview of the NASA Dual-Frequency Dual Polarized Doppler Radar (D3R) System for GPM Ground Validation
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    ABSTRACT:

    As an integral part of Global Precipitation Measurement (GPM) mission, Ground Validation (GV) program proposes to establish an independent global cross-validation process to characterize errors and quantify uncertainties in the precipitation measurements of the GPM program. A ground- based Dual-Frequency Dual-Polarized Doppler Radar (D3R) that will provide measurements at the two broadly separated frequencies (Ku- and Ka-band) is currently being developed to enable GPM ground validation, enhance understanding of the microphysical interpretation of precipitation and facilitate improvement of retrieval algorithms. The first generation D3R design will comprise of two separate co- aligned single-frequency antenna units mounted on a common pedestal with dual-frequency dual-polarized solid- state transmitter. This paper describes the salient features of this radar, the system concept and its engineering design challenges.

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