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ARNOLD GRUBER, LEROY HERMAN, and ARTHUR F. KRUEGER

Abstract

Resultant winds over the Pacific are derived from an Advanced Technology Satellite (ATS 1) for the month of November 1969. Since the method uses clouds as tracers, these resultant winds are biased toward cloud-producing circulation features. These biases appear to be smallest in the Tropics and, consequently, the winds here can be very useful for studies of the average circulation at low latitudes. Some of the important features of this circulation over the equatorial Pacific are clearly revealed.

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Arnold Gruber and James J. O'Brien

Abstract

An objective technique for adjusting wind data, such that the total mass divergence in a volume of the atmosphere is zero, is developed. The adjustment is obtained by applying a least-squares smoothing with a Lagrangian multiplier to constrain the total mass divergence to a specified amount. The computational details are derived and the method is applied to several examples. Both for theoretical wind profiles and for actual data, a very satisfactory adjustment is achieved without destroying the physical information contained in the data.

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Istvan Laszlo, Arnold Gruber, and Herbert Jacobowitz

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Observations made with the current and proposed narrowband shortwave channels aboard the NOAA series of satellites were simulated for a number of different surfaces (ocean, vegetative land, desert, cloud and snow) using the ATRAD radiation model to study the relative merit of each channel and, in various combinations to predict the broadband albedo. Solar zenith angles were varied over the range from 0 to 60 degrees. The results indicated that for all of the surfaces considered there would be no significant difference in predicting the broadband albedo with either the current (0.58–0.68 μn) or proposed (0.58–0.68 μm) channel 1 of the AVHRR. The proposed narrower channel 2(0.84–0.87 μm), however, would be a better predictor than the current wider channel 2(0.725–1.0 μm). Channel 1 is better than channel 2 for surfaces of low or moderate reflectivity, while over snow, the error in using channel 2 would be less than half of that for channel 1. Combining channels 1 and 2 would reduce the error by about 50% for vegetation, ocean and snow. Adding the proposed channel 3A (1.58–1.64 μm) to channels 1 and 2 would further improve the prediction of the broadband albedo. Channel 20(0.65–0.73 μm) of the HIRS instrument was similarly studied to ascertain how well the broadband albedo would be predicted if the spectral filter was removed to widen the bandpass. Two different detectors (Si and InGaAs) with the current and a modified beamsplitter were considered. The results indicated that the modified beamsplitter was preferred. The use of this beamsplitter with the Si detector (0.46–1.04 μm) gave the best prediction for ocean, vegetative land, and desert scenes, while the InGaAs detector (0.64–1.74 μm) was best for cloud and snow scenes. Although the use of a widened channel 20 was shown to be less successful than the combination of channels 1, 2 and 3A of the AVHRR, flattening the response curve for the InGaAs detector using a compensating filter was comparable to using the AVHRR channels.

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Arnold Gruber and Arthur F. Krueger

National Oceanic and Atmospheric Administration (NOAA) satellites have provided over eight years of observations from which estimates of the earth's total longwave emittance can be derived. Changes in satellite instrumentation, orbit, and algorithms used in obtaining these estimates are briefly summarized. The algorithms used by NOAA in obtaining a longwave radiation data set are provided.

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Xungang Yin, Arnold Gruber, and Phil Arkin

Abstract

The two monthly precipitation products of the Global Precipitation Climatology Project (GPCP) and the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) are compared on a 23-yr period, January 1979–December 2001. For the long-term mean, major precipitation patterns are clearly demonstrated by both products, but there are differences in the pattern magnitudes. In the tropical ocean the CMAP is higher than the GPCP, but this is reversed in the high-latitude ocean. The GPCP–CMAP spatial correlation is generally higher over land than over the ocean. The correlation between the global mean oceanic GPCP and CMAP is significantly low. It is very likely because the input data of the two products have much less in common over the ocean; in particular, the use of atoll data by the CMAP is disputable. The decreasing trend in the CMAP oceanic precipitation is found to be an artifact of input data change and atoll sampling error. In general, overocean precipitation represented by the GPCP is more reasonable; over land the two products are close, but different merging algorithms between the GPCP and the CMAP can sometimes produce substantial discrepancy in sensitive areas such as equatorial West Africa. EOF analysis shows that the GPCP and the CMAP are similar in 6 out of the first 10 modes, and the first 2 leading modes (ENSO patterns) of the GPCP are nearly identical to their counterparts of the CMAP. Input data changes [e.g., January 1986 for Geostationary Operational Environmental Satellite (GOES) precipitation index (GPI), July 1987 for Special Sensor Microwave Imager (SSM/I), May 1994 for Microwave Sounding Unit (MSU), and January 1996 for atolls] have implications in the behavior of the two datasets. Several abrupt changes identified in the statistics of the two datasets including the changes in overocean precipitation, spatial correlation time series, and some of the EOF principal components, can be related to one or more input data changes.

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George Ohring, Arnold Gruber, and Robert Ellingson

Abstract

Nimbus-7 satellite observations are used to determine the relationship between the total longwave radiation flux and the radiance in the 10-12 μm infrared window. The total longwave fluxes are obtained from the earth radiation budget (ERB) narrow-field-of-view (NFOV) observations of total radiance; the IR window radiances are those measured by the Temperature Humidity Infrared Radiometer (THIR). Regression equations are obtained relating the total flux equivalent brightness temperatures to the radiance equivalent brightness temperature of the IR window. These empirical equations are compared to similar regression equations based on radiative transfer calculations for a large sample of atmospheric soundings. The latter theoretical equations are used by NOAA in the processing of IR window observations from operational polar orbiting satellites to obtain total longwave flux estimates. The observational results indicate that there is a very high correlation between the flux equivalent brightness temperature and the IR window radiance equivalent brightness temperature, and that the former can indeed be determined from measurements of the latter, thus validating the general NOAA approach. Tests on independent data suggest that rms flux errors of ∼11 w m−2 are to be expected for single applications of the empirical equations. The theoretical equations used by NOAA have an average positive bias of ∼13 wm−2 or a relative bias of ∼6% with respect to the ERB NFOV observations; the relative bias disappears at high flux values and increases with decreasing flux. A preliminary attempt to determine the cause of the discrepancy between the empirical and theoretical results indicates that a major factor may be the unrepresentativeness of the atmospheric soundings used in developing the theoretical regression coefficients.

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Jeffrey R. McCollum, Arnold Gruber, and Mamoudou B. Ba

Abstract

The Global Precipitation Climatology Project (GPCP) satellite estimates have approximately twice the magnitude of estimates produced from the rain gauges used by the GPCP in central equatorial Africa. Different possible explanations are identified and investigated. The first is that there may not be enough GPCP rain gauges in the area to provide accurate estimates of rainfall for comparisons with satellite estimates. A comparison of the time-averaged GPCP rain gauge estimate with a long-term (over 40 yr) climatology indicates that the GPCP gauge estimates are similar to long-term rainfall averages, suggesting that the GPCP rain gauge analysis is not underestimating rainfall. Two other possible explanations related to the physical properties of the air masses in this region are studied. Evidence from the literature and from estimates of the effective radii of cloud droplets suggests that there may be an abundance of aerosols in central Africa, resulting in an abundance of cloud condensation nuclei, small drops, and inefficient rain processes. The second explanation is that convective clouds forming under dry conditions generally have cloud bases considerably higher than those of clouds forming in moist environments. This leads to an increase in the evaporation rate of the falling rain, resulting in less precipitation reaching the ground. Analysis of the moisture distributions from both the National Centers for Environmental Prediction numerical weather prediction model reanalysis data and the National Aeronautics and Space Administration Water Vapor Project global moisture dataset reveals that the lower troposphere in this region of Africa is relatively dry, which suggests that cloud bases are high.

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Robert G. Ellingson, Hai-Tien Lee, David Yanuk, and Arnold Gruber

Abstract

Simultaneous observations by the Earth Radiation Budget Experiment (ERBE) scanning radiometer and the High-Resolution Infrared Sounder (HIRS) on board the NOAA-9 spacecraft have been used to validate a multispectral technique for estimating the outgoing longwave radiation (OLR) from the earth-atmosphere system. Results farm approximately 100 000 collocated observations show that the HIRS technique provides instantaneous OLR estimates that agree with the ERBE observations just as well as different ERBE scanners agree with each other—about 5 W m−2 rms. Although there are differences between the HIRS and ERBE estimates that depend upon the scene type and time of day, the HIRS technique explained more than 99% of the variance of the ERBE observations for both day and night observations. The results suggest that the HIRS OLR technique is a suitable replacement for the Advanced Very High Resolution Radiometer technique now used by the National Oceanic and Atmospheric Administration for operational estimates of the OLR.

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Noah Wolfson, Albert Thomasell, Arnold Gruber, and George Ohring

Abstract

The impact of satellite sounding data on the systematic errors of the numerical weather prediction model of the Israel Meteorological Service has been investigated. In general, satellite data have been shown to reduce systematic error, and in particular, the greatest impact is near where the data have been introduced in the vicinity of low pressure systems.

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Jerry Sullivan, Lev Gandin, Arnold Gruber, and Wayman Baker

Abstract

In 1988, the algorithm for retrieving temperature soundings from radiances measured by NOAA's polar-orbiting satellites was changed from a statistical to a “physical” retrieval system. Because of this change, the National Meteorological Center (NMC) wanted to update the satellite temperature error statistics used in the NMC analyses. The authors, therefore, updated the estimates of observational error variances, horizontal covariances, and vertical correlations for layer mean temperatures retrieved from NOAA-10 satellite radiance data. The temperature error statistics have also been used to estimate analogous error statistics for isobaric height.

The computations used radiosonde data as a substitute for true temperatures. Each “matchup” in the dataset consisted of a satellite retrieval close in space and time to a radiosonde sounding. The matchups were stratified into clear, partly cloudy, and cloudy cases, depending on the amount of cloud contamination in the satellite radiance data. In each of the nine mandatory pressure layers considered, from 1000–850 to 50–100 mb, the clear and partly cloudy matchup cases have nearly equal temperature error variances, while the variances for cloudy cases are substantially larger. Vertical error correlations for all three stratifications are similar. Root-mean-square height errors computed from satellite temperature errors are comparable to those computed from radiosonde errors in the clear and partly cloudy matchup cases, but larger in cloudy cases.

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