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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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slide1

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

gpm calibration plans outline
GPM Calibration Plans Outline

Outline

  • GPM Overview
  • GMI Instrument
  • GPM Calibration
  • IV. Conclusions
i gpm overview
I. GPM Overview

GPM Overview

gpm concept

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
gpm core satellite instruments
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”

core satellite observations
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

gpm sampling goal
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

ii gmi instrument
II. GMI Instrument

GMI Instrument

gmi parameters
GMI Parameters

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

gmi performance enhancements
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
high frequency channels
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).

high frequency channels14
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.

radiometer altitude resolution comparisons
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

gmi down track cross scan sampling
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
iii gpm calibration
III. GPM Calibration

GPM Calibration

gpm transfer standard
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.
gmi radiometric calibration standard
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
precipitation standard
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

external calibration
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
gpm ground validation
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

gpm ground validation24
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)

azimuth viewing differences
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

inter sensor comparisons no rain
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
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
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
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
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
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
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

iv conclusions
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.