Climate Science Branch, Science Directorate NASA Langley Research Center

Shashi K. Gupta, Paul W. Stackhouse Jr.*,Taiping Zhang, and J. Colleen Mikovitz

Climate Science Branch, Science DirectorateNASA Langley Research Center (LaRC)

Hampton, Virginia

*PI: NASA/GEWEX Surface Radiation Budget (SRB) Project

Computation of Surface Longwave Fluxes

Effect of Skin-Air Temperature Differences

LANDFLUX Assessment and Organization Workshop

Toulouse, France28-31 May 2007

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The NASA/GEWEX SRB Dataset

•A 22-year (Jul1983-Jun2005) dataset produced at LaRC

•Monthly, Daily, Monthly/3-Hourly, and 3-Hourly at 1°x1°

•Algorithms: Primary and Quality-Check - SW and LW

•Input Data: Cloud Properties Derived from ISCCP-DXMeteorological Profiles from GEOS-4Column Ozone: TOMS and TOVS Archives

•Validation Data: BSRN, WRDC, and GEBA Archives

•Submitted to GEWEX Radiative Flux Assessment

•Dataset and ancillary information will be available at: http://eosweb.larc.nasa.gov/PRODOCS/srb/table_srb.html

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22-Year Average Surface Radiative Fluxes

PR

QC

SW LW

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Multi-Year Global Averages

* Normalized to S0 = 1367 W m-2 ; (F0 = S0/4)

Parameter

Ohmura & Gilgen (1993)

GEBA Surf. Obs.

Kiehl and Trenberth (1997)ERBE/CCM3

Zhang & Rossow (2004)

21-Year Mean (1984-2004)

NASA/GEWEX SRBRelease 2.5/2.81*

(NASA LaRC)22-Year Mean

(July 1983 - June 2005)

SW, LW SW, LW QC

Flux % F0 Flux % F0 Flux % F0 Flux % F0 Flux % F0

SW Down169.0

49.4 198 57.9 189.2 55.4 186.4 54.5 183.7 53.8

SW Net142.0

41.6 168 49.2 165.9 48.5 165.6 48.5 161.0 47.1

LW Down 345 100.9 324 94.8 343.8 100.6 343.1 100.4 348.7102.0

LW Net-

40.0-11.7 -66

-19.3

-49.6 -14.5 -53.0 -15.5 -50.0-

14.6

Total Net102.0

29.8 102 29.8 116.3 34.0 112.6 32.9 111.0 32.5

SW CRF -- -- -- -- -53.0 -15.5 -56.2 -16.4 -59.2-

17.3

LW CRF -- -- 46 13.5 29.5 8.6 35.3 10.3 34.3 10.0

Total CRF -- -- -- -- -23.5 -6.9 -20.9 -6.1 -24.9 -7.3

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Introduction

• SRB Longwave Algorithms:1. GEWEX LW (Fu/Stackhouse): CERES 2/4-Stream LW model

(Fu et al., 1997), maximum-random cloud overlap, CERES spectral surface emissivity. Results: TOA and Surface, 3-Hourly, Daily, Monthly/3-Hourly, and Monthly data products.

2. GEWEX LW QC (Gupta et al., 1989, 1992): RT based parameterizations for clear/cloudy downward LW fluxes, CERES broadband surface emissivity. Results: Surface 3-Hourly, Daily, Monthly/3-Hourly, and Monthly data products.

• Both models overestimate DLF for some regions at the high end of DLF range.

• Determine the cause, assess the magnitude, anddevelop a remedy for this overestimation.

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Comparison of 3-Hourly Fluxes With BSRN Data

Primary Quality-Check

Investigate the cause focussing on the quality-check model

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WHAT IS CAUSING IT AND WHERE?

• Effective Emitting Temperature:

Teff = 0.60 Ts + 0.35 T1 + 0.05 T2

Ts - Surface skin temperatureT1 - Average temperature for Sfc. - 800 mb layerT2 - Average temperature for 800 - 680 mb layer

• Works well when Ts, T1, and T2 conform to nominal lapse rate requirements.

• When Ts >> T1 (and T2), it results in overestimation of DLF.

• Mostly over dry/arid regions during hot times of day.

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Weighting Function for DLF Reaching the Surface

(U.S. Standard Atmosphere - 50 mb Layers)

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Methodology

• Selected two sites for study:

Alice Springs, Australia - Dry/Arid Goodwin Creek, MS (USA) -

Moderate/Humid

• Performed flux computations using a stand-alone version of the quality-check model on a 3-hourly time resolution for all months of 2000.

• Compared model-derived DLF with ground-based observations for the above sites obtained from BSRN.

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Analysis of Alice Springs Overestimation

• Points with overestimation of > 100 Wm-2

32 points during the year (16 in Jan; 20 in DJF)

Mean = 120 Wm-2; Range: 100-160 Wm-2

Ts: Mean = 324.3 K; Range: 302-336 K

Ps: Mean = 939 mb; Range: 935-946 mb

T800: Mean = 292.1 K; Range: 284.7-297.7 K

Ts - T800: Mean = 32.2 K; Range: 17.0-40.3 K

• Ts - T800 should be about 10 K, but no more than 15 K

• Decided that lapse rates > 10K/100mb in the lower layer

are too steep, and need to be adjusted.

• Adjusted skin temperature to not exceed 10K/100mb limit.

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Results From the Modified Computation

Alice Springs Goodwin Creek

Bias for Alice Springs - reduced greatly; Change for Goodwin Creek - minimal

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Monthly Temperature Adjustment and Frequency

Temperature Adjustment Adjustment Frequency

Jan

Jul

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Model-Derived vs. Ground-Measured BSRN DLF

(Primary Model)Alice Springs Goodwin Creek

Alice Springs: Slight overestimation at the high end; small negative bias overall.

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Model-Derived vs. Ground-Measured DLF(Primary Model)Alice Springs

Some overestimation at the high end during DJF; underestimation during JJA

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Summary and Future Work

• Determined the cause of DLF overestimation: Occurs when skin temperature (Ts) is much higher than atmospheric temperatures.

• Instantaneous overestimation can be 100 Wm-2 or even more.

• Developed a method for detecting this condition and constraining Ts prior to using it in DLF computation, largely eliminating the bias.

• Constrained Ts is not used for upward flux computation.

• Tested method with quality-check model for all months of 2000.

• Similar analysis is underway for the primary LW algorithm.

• Results of analysis to be applied in the next round of processing.

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Back-up Slides

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Model-Derived vs. Ground-Measured BSRN DLF

(Quality-Check Model)Alice Springs Goodwin Creek

Significant overestimation over Alice Springs; almost none over Goodwin Creek

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Model-Derived vs. Ground-Measured DLF

(Quality-Check Model)Alice Springs

Severe overestimation during DJF; slight underestimation during JJA

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Comparison of Daily Averages With BSRN Data

Primary Quality-Check

Overestimation not so apparent after diurnal averaging

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Monthly Temperature Adjustment and Frequency

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Surface and Atmospheric Temperatures Over

Sea of Japan - January 2000

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Surface Skin Temperature From Different Datasets

Jan2000 Jul2000

GEOS-4

ISCCP

PrimaryLW

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Surface Skin Temperatures From Different Datasets