Contains reports on one research project.National Aeronautics and Space Administration (Contract NAS5-21980

Ledsham, William H.

Pettyjohn, Ronald L.

Rosenkranz, Philip W.

Staelin, David H.

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VIII. RADIO ASTRONOMYAcademic and Research StaffProf. Alan H. Barrett Prof. David H. Staelin John D. KiersteadProf. Bernard F. Burke Dr. Alan Parrish D. Cosmo PapaProf. Philip C. Myers Dr. Philip W. Rosenkranz Steven J. SavatskyJohn W. BarrettGraduate StudentsPatrick C. Crane William H. Ledsham Ronald L. PettyjohnArthur D. Fisher Robert N. Martin Bruce M. SchechterThomas S. Giuffrida Joseph W. Orenstein Matthew H. SchnepsAubrey D. Haschick Robert J. Parr Michael ShaoPaul T. Ho Robert C. WalkerA. RETRIEVAL OF ATMOSPHERIC TEMPERATURE PROFILESBY A SCANNING MICROWAVE SPECTROMETERNational Aeronautics and Space Administration (Contract NAS5-21980)Philip W. Rosenkranz, David H. Staelin, Ronald L. Pettyjohn,William H. LedshamEarly data from the Scanning Microwave Spectrometer (SCAMS) on the Nimbus-6satellite were presented in RLE Progress Report No. 117 (pp. 31-33). The accuracyof remotely sensed temperature profiles achieved by using this instrument has now beenevaluated, by using conventional (radiosonde-based) measurements for comparison.The SCAMS is an improvement of the Nimbus-5 Microwave Spectrometer.1,2 It viewsthe Earth at 13 equally spaced angular positions extending to 430 on either side of nadirat the satellite. The antennas step at 1-s intervals. The incidence angle at the Earththus ranges from 0 (vertical) to 53' Horizontal resolution near the suborbital trackis ~150 km (see Fig. VIII-1). The spectrometer has 5 channels, centered at 22.23 GHzSCAMS SPOTS-3dB CONTOURSFLIGHT99 km146 km-1200 -800 -400 0 400 800 1200DISTANCE FROM SATELLITE SUBPOINT (km)Fig. VIII-1. Contours of the antenna half-power points on the Earth. Allthree antennas have approximately the same pattern.PR No. 118(VIII. RADIO ASTRONOM\IY)(water-vapor line), 31.65 GHz (window), and 52.85, 53.85, 55.45 GHz (oxygen band). Theweighting functions of the three oxygen-band channels (Fig. VIII-2) peak at heights of0, 5, and 13 km (pressure levels 1000, 540, and 180 mb) in order of frequency, for ver-tical incidence. The altitudes sensed by the weighting functions (Fig. VIII-3) increasegradually with angle of incidence, reaching ~2 km higher at the extremes of the scan.66I161412t: 015 084O0 002 0.04 006 0.08 0.10WEIGHTING FUNCTION (km-')Fig. VIII-2.Weighting functions for the oxygen-bandchannels. Shaded areas represent thereflected contribution when the surfaceis a smooth ocean (t = transmittance).The U. S. Standard Atmosphere with2 cm precipitable water vapor was usedfor the calculations.55 45 GHz53 85 GHzLAND OCEAN52.85 GHz200 400 600 80DISTANCE FROM SATELLITE SU0 1000 1200BPOINT (km)Fig. VIII-3.Miean heights of the weighting functionsas a function of the scan position.The spectrometer is calibrated by viewing cold space and ambient temperature tar-gets in two additional positions at the end of each scan. The three oxygen-band channelsshare the same antenna and calibration target. Difference mode errors among thesethree channels are thereby minimized.Residual system errors were investigated by comparing brightness temperatures(corrected for antenna sidelobes) measured October 22-25, 1975, with brightness tem-peratures computed from the National Meteorological Center objective analysis (NIMCgrid) for the regions of North America, Europe, and Japan, excluding locations wherethe surface was higher than 1 km (Fig. VIII-4). The oxygen absorption coefficient wascomputed from expressions given by Rosenkranz 3 using recent measurements of theresonant linewidths by Liebe et al. 4 These comparisons revealed mean errors that werePR No. 118(VIII. RADIO ASTRONOMY)800 20 40 60 8 20 0ONORTHA MERICA 0 W150120W 60W90WFig. VIII-4. Regions used to compare SCAMS measurements withNational Meteorological Center data.removed by assuming that the radiometric temperature of the calibration target was1.2 K ± 0.6 K colder than the physical temperature measured by imbedded thermistors.Root-mean-square errors about the mean were typically ~0. 8 K at 53.85 and 55.45 GHz,of which 0. 5 K is accounted for by instrument noise. The 52. 85 GHz channel is stronglyinfluenced by the terrestrial surface; consequently, errors on this channel were greater,approximately 1. 5 K rms.Atmospheric temperature profiles were retrieved from the SCAMS measurementsby a linear statistical method 5 that optimizes the retrieval on a set of a priori sta-tistics. These statistics comprised 354 radiosonde records from stations at WeatherShip Papa, Peoria, Illinois, Pt. Mugu, California, San Juan, Puerto Rico, and PointBarrow, Alaska, in the period 1966-1970. Matrices of coefficients for the retrievalwere determined separately from these statistics for land and ocean surfaces. Thesematrices were applied globally.Again, the NMC grid was used to verify the temperature profile retrieval in the threeregions. Problems involved in verification of remotely sensed temperature profilesare discussed elsewhere. Retrieval errors are typically ~2 K rms in the mid tropo-sphere, including the mean error, if any (see Figs. VIII-5, VIII-6, and VIII-7). Nearthe surface and around the tropopause the errors increase to >4 K, then decrease againPR No. 118Fig. VIII-5.Retrieval errors for August 7-9,1975. Broken lines indicate errorsfor mean temperatures of layers.Fig. VIII-6.Retrieval errors for February 3-6,1975.987605cra-150 mb2500 mbSCAMS-NMC0AUG SEPT OCT NOV DEC JAN1975 1976Fig. VIII-7.Retrieval errors at two levelsas a function of time.FEB MARPR No. 118150E200cr 250cu 30040050070085010005070100150E200n 250C 300RMS ERROR (K)RMS ERROR (K)a-(VIII. RADIO ASTRONOMY)in the stratosphere. These errors result from temperature profile structure that isfiner than the vertical resolution of the weighting functions. This is illustrated byretrievals of the mean temperature of atmospheric layers, denoted by broken lines inFigs. VIII-5 and VIII-6. Errors for these retrievals are more uniform with altitude.References1. J. W. Waters, K. F. Kunzi, R. L. Pettyjohn, R. K. L. Poon, and D. H. Staelin,"Remote Sensing of Atmospheric Temperature Profiles with the Nimbus-5 MicrowaveSpectrometer," J. Atmos. Sci. 32, 1953-1969 (1975).2. D. H. Staelin, W. H. Ledsham, R. L. Pettyjohn, P. W. Rosenkranz, R. K. L. Poon,and J. W. Waters, "Microwave Sensing of Atmospheric Temperature and Humidityfrom Satellites," in Space Research XVI (Akademie-Verlag, Berlin, 1975), pp. 143-147.3. P. W. Rosenkranz, "Shape of the 5 mm Oxygen Band in the Atmosphere," IEEE Trans.on Antennas and Propagation, Vol. AP-23, No. 4, pp. 498-506, July 1975.4. H. J. Liebe, G. Gimmestad, and J. Hopponen, "Atmospheric Oxygen MicrowaveSpectrum - Experiment versus Theory" (to appear in IEEE Trans. on Antennas andPropagation).5. C. D. Rodgers, "Some Theoretical Aspects of Remote Sounding in the Earth'sAtmosphere," J. Quant. Spec. Rad. Trans. 11, 767-777 (1971).PR No. 118

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