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Calibration Plans for the Global Precipitation Measurement (GPM) S. W. Bidwell, J. Turk*, G. M. Flaming, C. R. Mendelsohn, D. F. Everett, W. J. Adams, and E. A. Smith NASA Goddard Space Flight Center Greenbelt, MD 20771 U.S.A *Naval Research Laboratory, Monterey, CA 93943 U.S.A

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Calibration Plans for the Global

Precipitation Measurement (GPM)

S. W. Bidwell, J. Turk*, G. M. Flaming,

C. R. Mendelsohn, D. F. Everett,

W. J. Adams, and E. A. Smith

NASA Goddard Space Flight Center

Greenbelt, MD 20771 U.S.A

*Naval Research Laboratory,

Monterey, CA 93943 U.S.A

Steven W. Bidwell

bidwell@agnes.gsfc.nasa.gov

(301) 614-5639

µCal 2002 Workshop,

Barcelona, October 9 – 11, 2002


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GPM Calibration Plans Outline

Outline

  • GPM Overview

  • GMI Instrument

  • GPM Calibration

  • IV.Conclusions


I gpm overview l.jpg

I. GPM Overview

GPM Overview


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  • OBJECTIVES

  • Understand horizontal & vertical structure of rainfall, its macro- & micro-physical nature, & its associated latent heating

  • Transfer Standard for constellation radiometers

  • OBJECTIVES

  • Provide sufficient global sampling to significantly reduce uncertainties in short-term rainfall accumulations

  • Extend scientific and societal applications

GPM Concept

Constellation

Core

  • Core Satellite

  • TRMM-like spacecraft (NASA)

  • H2-A rocket launch (NASDA)

  • Non-sun-synchronous orbit

  • ~ 65° inclination

  • ~400 km altitude

  • Dual frequency radar (NASDA)

  • Ku-Ka Bands (14-35 GHz)

  • ~ 5 km horizontal resolution

  • ~250 m vertical resolution

  • Multifrequency radiometer (NASA)

  • 10, 19, 23, 36, 89, (150/183 ?) GHz V&H

  • Constellation Satellites

  • Pre-existing operational-experimental & dedicated satellites with microwave / millimeter-wave radiometers

  • Revisit time

  • 3-hour goal at ~90% of time

  • Sun-synch & non-sun- synch orbits

  • 600-900 km altitudes

  • Precipitation Validation Sites for Error Characterization

  • Select/globally distributed ground validation “Supersites” (research quality radar, up looking radiometer-radar-profiler system, raingage-disdrometer network, & T-q soundings)

  • Dense & frequently reporting regional raingage networks

  • Precipitation Processing System

  • Produces global precipitation products

  • Products defined by GPM partners


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GPM Core Satellite Instruments

35.55 GHz radar

(phased array)

GMI Microwave Radiometer

13.6 GHz radar

(Similar to TRMM PR phased array)

+ Possible “Instrument of Opportunity”


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Core Satellite Observations

Flight Direction

Surface Track

Speed = 7.2 km/s

Altitude = 400 km

DPR

Dual-frequency

Precipitation Radar

Ku and Ka Bands

GMI

Microwave

Radiometer

PR-U Swath Width= 245 km

PR-A Swath

Width = 100 km

GMI Swath

Width = 890 km

5 km


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GPM Sampling Goal

Percentage of 3-Hour Intervals Sampled

in 7-Day Period (Simulation)

EOS Era

2002-2007

GPM Era

2007-2010

GPM Core, GPM Constellation, DMSP F18,

DMSP F19, GCOM-B1, Megha-Tropiques,

Euro-GPM, and Additional Partner

TRMM, DMSP F13, F14, F15,

AQUA, and ADEOS-II

Constant Area Pixels


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II. GMI Instrument

GMI Instrument


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Conical Scanning Radiometers

?


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GMI View Geometry


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GMI Parameters

Highlights represent some changes from TRMM (TMI Requirements in parentheses)


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GMI Performance Enhancements

GMI Enhancements in Order of Preference:

  • 1)Improved surface spatial resolution on low frequency channels (10.65 GHz, 18.7 GHz, 23.8 GHz)

  • Implies: an antenna diameter increase to 1.2 meters

  • Improved capabilities for the measurement of light rain and snow

  • Specifically:150 GHz or 166 GHz (V and H)

  • 183 GHz ±1, ±3, ±6, ±9 GHz (V or H)

  • Complete surface scene sampling on all channels

  • Implies: multiple 89 GHz feedhorns to ensure swath contiguity


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High Frequency Channels

Snowfall Over Ocean

Courtesy of:

Prof. Guosheng Liu/FSU

Suggested primary channels for snow detection are 163 GHz at V-pol & H-pol.

Dual polarization allows use of PCT which is more sensitive to snowfall than any

single polarization.

If only one polarization is possible, use V-pol instead of H-pol, because dynamic

range is much greater at V-pol (i.e., background is much warmer).


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High Frequency Channels

Snowfall Over Land

Courtesy of:

Prof. Guosheng Liu/FSU

Suggested primary channels for snow detection are 183±3 & 183±9, both at V-pol.

First channel is for low water vapor densities while second is for high water vapor densities.


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Radiometer Altitude / Resolution Comparisons

SSMI (0.61 m)

CMIS

(2.2 m)

AMSR-E

(1.6 m)

GMI Const.

(1.0 m)

Orbital Altitude (km)

GMI Core

(1.0 m)

TMI (0.61 m)

30 km

17 km

10.65 GHz

Resolution (km)

18.7 GHz / 19.35 GHz


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GMI Down-Track (Cross-Scan) Sampling

  • GMI will have increased resolution than TRMM at 10, 18, and 21 GHz

  • GMI will have similar resolution to TRMM at 36 and 89 GHz

  • Contiguity is required at 36 GHz

  • Rotational rates greater than 40 rpm viewed with caution


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GMI Along-Scan Sampling


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III. GPM Calibration

GPM Calibration


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GPM Transfer Standard

  • What is meant by a Transfer Standard?

  • Two distinct things:

  • GMI as the Radiometric Calibration Standard

  • Core Satellite Retrievals as the Precipitation Standard

  • ___________________________________

  • (1) GMI as the Radiometric Calibration Standard:

  • For GPM purposes, the GMI aboard the Core satellite will act as a calibration reference for the conically scanning member radiometers of GPM, e.g.:

  • SSMI, AMSR, CMIS, MADRAS, GPM Const. GMI,

  • E-GPM, etc.

  • As a routine GPM procedure, prior to processing and ‘merging’ of data, calibrations of other sensors will be adjusted for consistency with the GMI.


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GMI Radiometric Calibration Standard

  • Why the Core Satellite GMI as the Calibration Reference?

  • The Core GMI is NASA-managed throughout mission life

  • (i.e. its calibration will be well-understood by GPM)

  • The Core GMI will have an excellent on-board calibration system

  • The Core GMI will have the highest spatial resolution amongst

  • sensors

  • One instrument needs to be designated as the standard


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Precipitation Standard

(2) Core Satellite Retrievals as the Precipitation Standard:

What is the Precipitation Transfer Standard?

The Core Satellite combined radar / radiometer retrievals will create a data base ‘standard’ of observed brightness temperatures, precipitation systems, and precipitation structures

Retrievals using GPM Constellation Radiometers will access the data base standard to choose optimal precipitation parameters consistent with its brightness temperature observations (similar to Goddard Profiling Algorithm GPROF currently in use)

Why the Core Satellite as the Precipitation Transfer Standard?

The Core Satellite is a unique platform with the combined Dual-frequency Precipitation Radar (DPR) and GMI Radiometer


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External Calibration

  • External Calibration Methods

  • (1)Radiometer Calibrations with an Absolute Reference at the Ground Validation (GV) Sites:

    • Tropical Oceanic Site

    • Tropical Continental Site (uniform vegetation)

  • Under clear sky conditions, with GV measurement of:

    • water vapor and temperature profiles

    • surface state conditions

  • Inter-Sensor Comparison Events:

    • Orbital intersections (Core with Constellation Members) with swath overlap within specified time delay and spatial separation


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    GPM Ground Validation

    Potential Supersites and Regional Raingauge Networks

    North Europe BALTEX

    Canada

    England

    EC

    NASA Land

    South Korea

    ARM/UOK

    Japan

    NASA KSC

    Taiwan

    NASA Ocean

    Brazil

    Regional Raingauge

    Network

    Supersite & Regional

    Raingauge Network

    Supersite


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    GPM Ground Validation

    Supersite Template

    Data Analysis Facility

    Multi-Parameter Radar

    {

    Matched Dual Freq. Radar &

    Multi-Channel Radiometer

    S-/X- Band Radars / Profilers

    Cloud Radar Profiler

    Meteorological Tower &

    Sounding System

    Site Scientist (3)

    DELIVERY

    Technician (3)

    150 km

    • GV Products

    • (1) Error Biases & Bias Uncertainties

    • (2) Error Structures / Error Covariances

    • GV Product Customers

    • (1) Data Assimilation Specialists

    • (2) Climate Diagnosticians

    • (3) Algorithm Specialists

    Low Resolution Domain

    100-Gauge Site, Centered on Multi-Parameter Radar

    150 km

    5 km

    Triple Gauge Site

    (3 Economy Scientific Gauges)

    • High Resolution Domain

    • 50-Gauge Site, Center-Displaced with

    • Matched Radar / Radiometer [ 14, 35 / 10, 19, 22, 37, 85 GHz ]

    • S-/X-band Doppler Radar Profilers

    • Cloud Radar Profiler

    Single Disdrometer/

    Triple Gauge Site

    (1 High Quality-Large Aperture/

    2 Economy Scientific Gauges)


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    Azimuth Viewing Differences

    Example: An Aqua (AMSR-E swath in red) overpass occurs, then a GPM core (GMI swath in blue, simulated by the TMI instrument) follows Δt minutes later.

    The Δt offset is complicated by the fact that although the satellite zenith angles are designed to be the same, their azimuth angles are different.

    AMSR on-Earth

    beam projection

    GMI on-Earth

    beam projection


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    Inter-Sensor Comparisons / No Rain

    Narrower beamwidth 85 GHz channels (red) sense primarily along-path water vapor and cloud, and some of the surface (more so for drier atmospheres)

    Wider beamwidth, lower frequency channels (green) sense along-path cloud and azimuthal variations of surface wind speed and direction (+/-3 K)

    Over land, variations arise from variable soil conditions

    (not drawn to any scale)


    Inter sensor comparisons no rain27 l.jpg

    Inter-Sensor Comparisons / No Rain

    Baseline No-Rain Conditions, Coincident TMI-SSMI

    land

    water

    T=3 months Δt=1 minute Δd=10 km TMI=no-rain, 3x3 ave


    Inter sensor comparisons no rain28 l.jpg

    Inter–Sensor Comparisons / No Rain

    Baseline No-Rain Conditions, Coincident TMI-SSMI (binned)

    land

    water

    T=3 months Δt=1 minute Δd=10 km TMI=no-rain, 3x3 ave, then binned to 1-degree


    Inter sensor comparisons raining conditions l.jpg

    Inter-Sensor Comparisons / Raining Conditions

    Narrower beamwidth 85 GHz channels (red) sense higher in the cloud, but horizontal asymmetries give rise to large TB variations depending upon azimuth view

    Wider beamwidth, lower frequency channels (green) sense lower in the cloud where horizontal asymmetries are smaller

    The 3-D effects should average-out with many events and with spatial averaging

    (not drawn to any scale)


    Inter sensor comparisons raining conditions30 l.jpg

    Inter-Sensor Comparisons / Raining Conditions

    Raining Conditions, Coincident TMI-SSMI

    land

    water

    3-D effects

    Δt=1 minute Δd=10 km TMI rainflag=rain, 3x3 average Nocean=13433 Nland=1320


    Inter sensor comparisons raining conditions31 l.jpg

    Inter-Sensor Comparisons / Raining Conditions

    Rainy Conditions, Coincident TMI-SSMI

    TMI-SSMI Difference vs. TMI 2A12 Rain Rate

    Δt=1 minute Δd=10 km TMI rainflag=rain, 3x3 average Nocean=13433 Nland=1320


    Inter sensor comparisons raining conditions32 l.jpg

    Inter-Sensor Comparisons / Raining Conditions

    Rainy Conditions, Coincident TMI-SSMI

    TMI-SSMI Difference vs. Relative Azimuth

    Δt=1 minute Δd=10 km TMI rainflag=rain, 3x3 ave N=5336


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    IV. Conclusions

    Conclusions

    • NASA will procure two radiometers, GPM Microwave Imagers (GMI), for the GPM.

    • Core GMI Calibration will be used as a Reference Standard for the Constellation Member Radiometers.

    • GPM plans External Calibration Techniques for Calibrating the Core GMI:

      • (1) Ground Validation System and

      • (2) Inter-Sensor Comparisons from Swath Intersection Events.


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