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Larry M. McMillin

Abstract

The retrieval of vertical temperature profiles from radiances measured from a satellite is a difficult inversion problem that has been solved to varying degrees of accuracy by several approaches. A method that uses a classification approach for the initial estimate is described. The initial estimate is modified by the application of a single set of regression coefficients that is valid for all latitudes and seasons. Retrievals from this method are compared to the operational retrievals obtained by the National Environmental Satellite, Data, and Information Service (NESDIS). The comparison is based on the carefully matched radiosonde-retrieval pairs that are used in the operational system to update retrieval coefficients. About 10% of the data covering the period July 1987–July 1988 was used to generate the coefficients for both the classification and the retrieval that are used in the classification method. The classification method was then used to process data covering the period July 1987–July 1989. Results from about 100 000 soundings covering the two-year period show the classification method to be consistently (0.2–0.3 K rms) better than the operational method in the lower atmosphere, the region for which the classification was optimized, and slightly worse near 100 hPa. A significant feature of the evaluation is that all the coefficients used in the classification method were constant for the two years. Most retrieval systems are periodically tuned to agree with radiosondes. Examination of the mean from the classification retrievals produced with fixed coefficients show a slight drift over the two years in the lower atmosphere, a sudden change in the upper atmosphere, (probably due to a change in one or more channels), and seasonal changes in bias at some levels. These effects are diminished in the operational results because the operational coefficients are adjusted to agree with radiosonde reports once a week. The increase in accuracy of the classification retrievals over the operational ones could be enhanced by updating the class means using a procedure similar to the one currently employed by the operational system. The evaluation period happened to include the time when a major change was made to the operational method and shows a slight improvement in the operational results after the change.

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Larry M. McMillin

Abstract

To obtain temperature profiles from radiances measured from satellites, the radiances are first corrected for cloud contamination in the field of view. Many current techniques assume identical cloud height in adjacent fields of view and employ a comparison of radiances measured in adjacent scan spots. When multiple cloud levels occur, the resulting radiances are in error. Until now, there has been no reliable means of identifying these areas; such a means is presented here. Use of this technique resulted in increased coverage and accuracy of the derived clear radiances when compared with the current operational method.

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Larry M. McMillin and Kamesh Govindaraju

Abstract

Now that satellite retrieval are being used in global forecasts, there is increased interest in using satellite retrievals for use such as mesoscale forecasts and inputs to crop models. These uses require more accurate retrievals near the ground where satellite retrievals tend to be affected by the large temperature difference between the lower atmosphere and the earth's surface. Standard regression techniques tend to produce a normal difference and low level atmospheric retrievals tend to be too warm over a hot surface and too cold over a cold surface. Because radiances are measured at several wavelength regions with very different relationships between temperature and radiance, these differences can be detected and used to stratify the data for which a given set of coefficients is applied. Data were stratified into three groups based an differences between radiances in the 4.3 and 15 μm regions. The stratification increased the accuracy of retrievals in the lower atmosphere by up to 0.3 K depending on the level and the error. This is an improvement of about 10% of the existing error.

As part of the study, retrievals based on hourly surface observations were compared with retrievals based on radiosondes for the surface temperature. Retrievals based on hourly surface observations and compared with hourly surface observations produced ret6cvals with rms differences less than 2.0 K while retrievals based on radiosondes produced differences over 3.0 K. This indicates that satellite soundings are more accurate near the surface than the values indicated by comparisons with radiosondes.

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Larry M. McMillin and Charles A. Dean

Abstract

Several recent papers have reported that temperatures from satellites lack the variance found in radiosonde temperatures. Because of these results, we performed an experiment to distinguish between limitations of the satellite processing system and characteristics of the procedures used to produce temperatures. In the experiment, we calculated radiances, the quantities observed by the satellite, for a set of radiosondes for which we also had observed radiances. We then obtained regression coefficients for both the observed and calculated radiances and applied them to an independent set. We discovered two significant features: One is that the variances produced by the current satellite retrieval system are significantly closer to variances obtained from radiosondes than was reported in the earlier papers. We attributed the change to modifications in the retrieval system which wore designed to increase the accuracy of satellite retrievals. The second is that with the present system, a large portion of the remaining variance loss is recovered through using more accurate radiances.

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Larry M. McMillin and Henry E. Fleming

Abstract

In the design of satellite sounding instruments there are many factors that determine the accuracies of the retrieved temperature and moisture profiles. However, the three major factors are: instrument noise, number of channels and weighting function half-widths. The effect of these three factors on retrieved temperatures are examined through simulation studies to determine trade-offs among them. We conclude that the trade-offs among the three factors suggest that year different instrument designs can yield similar accuracies. Consequently, the instrument design that provides optimum performance can be recognized only after a trade-off analysis is made. If the design with the best performance is to be selected, it is particularly important that the designs be given equal benefit of factors which are not intrinsic design differences.

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Larry M. McMillin and David S. Crosby

Abstract

Measurements of infrared radiation from the National Oceanic and Atmospheric Administration series of satellites are used to retrieve atmospheric temperature, moisture, and ozone. It is well known that the measurements from the 4.3-μm channels of the High-Resolution Infrared Radiation Sounder (HIRS) are affected by solar radiation. These effects can have an important effect on retrieved parameters and have caused difficulties in the use of the higher-peaking shortwave channels for many applications. In the channels that respond to the upper atmosphere, this effect can reach 2 K for high solar angles. In this paper, a regression procedure is used on nighttime data to determine a relationship between these channels and the other channels of the HIRS and the Microwave Sounding Unit of the Television and Infrared Operation Satellite Operational Vertical Sounder. This regression then is applied to daytime data to study the effect of solar radiation on these channels. This paper provides a method for estimating and partially removing these effects.

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Mitchell D. Goldberg and Larry M. McMillin

Abstract

Deep-layer mean temperatures from Microwave Sounding Unit (MSU) observations have been used by scientists to study trends and interannual variations of tropospheric and lower-stratospheric temperature. The spatial resolution of MSU deep-layer mean temperatures is rather poor for studying trends in localized regions. A method is developed in which infrared observations from the High-resolution InfraRed Sounder (HIRS) is used in combination with MSU to derive deep-layer mean temperatures with improved vertical and horizontal resolution. Even though the relationship between infrared radiance and temperature is not linear, the layer associated with the mean temperature is shown to be well defined with a small airmass dependency that is similar to MSU’s airmass dependency. Preliminary validation of HIRS–MSU-derived layer mean temperatures with radiosonde layer mean temperatures show similar precision when compared to MSU-only derived temperatures.

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Larry M. McMillin and Murty G. Divakarla

Abstract

For future satellite instruments, two scan geometries have been proposed: cross-track and conical scanning. With the mixtures of possible instruments, a future sounding suite may consist of all cross track, all conical, or a combination with temperature channels on one and moisture channels on the other. This paper evaluates the effect of scan angle on the accuracy of moisture soundings. It is found that, as the scan angle increases from nadir, the accuracy of the moisture soundings near the surface decreases because less of the surface signal reaches the satellite. At the same time, the accuracy of the upper-level sounding increases because the weighting functions become sharper as the angle increases. If a mixed system is required where the temperature and moisture channels have different scan geometries, the best accuracy is obtained if the temperature channels are on a cross-track scanner. The scan angles also affect the number of measurements that are too cloud contaminated to provide accurate meteorological information. A second study shows that the number of radiances that exceed a given error level increases with scan angle. This results in a decrease in coverage for a conical scanning instrument.

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Thomas J. Kleespies and Larry M. McMillin

Abstract

The split window technique makes use of two differentially absorbing channels in the 11 μm region to remove the attenuating effects of atmospheric absorption so as to achieve a better estimate of the underlying skin temperature than could be produced by a single channel measurement. Since the primary absorber in this region is water vapor, it follows that split window measurements should be able to produce bulk water vapor retrievals as well. When observations are made with split window channels under conditions where the surface contribution to measured radiance changes, but the atmospheric contribution does not, it is possible to estimate the ratio of the transmittance of the two split window channels. This transmittance ratio is inversely related to precipitable water. This paper applies this technique to observations from the Advanced Very High Resolution Radiometer, and the VISSR Atmospheric Sounder, and demonstrates the capability of both instruments to determine precipitable water under two different operational scenarios.

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Larry M. McMillin, Melvyn E. Gelman, A. Sanyal, and Mojgan Sylva

Abstract

This paper presents the results of a feasibility study to evaluate a method for the use of satellite measurements as a transfer standard to determine temperature biases between radiosonde types. The method was evaluated on a sample of satellite observations that were paired with nighttime radiosonde observations. Only nighttime cases were used in this study in order to avoid the additional complication of heating of the radiosonde by solar radiation; however, the method should be equally valid in daylight. Radiances were calculated from radiosonde temperature profiles and compared to radiances for the same location as measured from the satellite. With the use of radiosonde–satellite pairs for two different radiosonde types, the satellite radiances were used to remove radiance differences due to atmospheric temperature differences from the radiosonde radiances. This step allowed radiosondes at different locations to be compared. Biases between the U.S. civilian instrument and the Väisälä instrument were derived and compared with published values; the results of the new method were found to be consistent with results obtained from direct comparisons of radiosonde instruments when measurements were made under similar atmospheric conditions. However, the direct measurements were made in a limited range of atmospheric conditions, whereas the satellite measurements were made under a wide range of atmospheric conditions. Results based on the indirect satellite comparisons showed substantial variation in the biases obtained under different atmospheric conditions. This variation is consistent with differences in temperature errors that are the result of differences in the radiation balance between the instrument and its surroundings. The results demonstrated the ability of the method to provide estimates of radiosonde biases. They also show that radiosondes are subject to substantial errors owing to longwave radiation and other sources. These errors are not only large (0−2 K), but also highly variable.

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