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Robert W. Fett and Ronald G. Isaacs

Abstract

Visible, Very High Resolution (VHR) and Light Fine (LF) data from the Defense Meteorological Satellite Program (DMSP) frequently exhibit areas of light-toned gray shades termed “anomalous gray shades” which appear most notable over the ocean but also over land areas, where contrast is suitable. These gray shades are distinctly different in appearance from visible, highly reflective cloud formations, and they are often particularly well delineated in DMSP VHR or LF data and either appear weakly or do not appear at all in near-simultaneous data from other satellite systems. This paper identifies low-level aerosols, such as haze, diffuse cloud particles or light fog, as one of the major causes of anomalous gray shades and shows examples with available documentation.

Since the DMSP VHR and LF sensor response curves are drastically different from those of other U.S. operational meteorological satellite systems, it was assumed that wavelength dependence on absorption and scattering effects was involved in the enhanced capability of the DMSP sensors to render anomalous gray shades visible.

A simple radiative transfer model is adopted to test the effect of haze on emergent radiation intensity as a function of wavelength and as measured by a meteorological satellite. These results are compared to previously obtained experimental measurements of particulate matter optical thickness as a function of wavelength under varying atmospheric conditions ranging from foggy or hazy to essentially clear. The results confirm that near-infrared wavelengths, except in narrow band regions where appreciable water vapor absorption occurs, provide much greater contrast between hazy and clear conditions that do shorter wavelengths in the visible portion of the spectrum. Since the DMSP VHR and LF sensors have their peak response in the near infrared, this appears to be an important factor in accounting for the sensitivity of these sensors in delineating hazy areas more adequately than other sensors from other systems which yield a peak response at shorter wavelengths.

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Thomas Nehrkorn, Ross N. Hoffman, Jean-François Louis, Ronald G. Isaacs, and Jean-Luc Moncet

Abstract

The potential improvements of analyses and forecasts from the use of satellite-observed rainfall and water vapor measurements from the Defense Meteorological Satellite Program Special Sensor Microwave (SSM) T-1 and T-2 instruments are investigated in a series of observing system simulation experiments using the Air Force Phillips Laboratory (formerly Air Force Geophysics Laboratory) data assimilation system. Simulated SSM radiances are used directly in a radiance retrieval step following the conventional optimum interpolation analysis. Simulated rainfall rates in the tropics are used in a moist initialization procedure to improve the initial specification of divergence, moisture, and temperature.

Results show improved analyses and forecasts of relative humidity and winds compared to the control experiment in the tropics and the Southern Hemisphere. Forecast improvements are generally restricted to the first 1–3 days of the forecast.

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Ross N. Hoffman, Christopher Grassotti, Ronald G. Isaacs, and Thomas J. Kleespies

Abstract

Satellite emission computed tomography retrieves the temperature of the atmosphere from radiances observed at multiple viewing angles and frequencies. To the extent that it provides independent information, the use of multiple viewing angles should improve the accuracy of the retrieval. Additionally, the tomographic retrievals should be more horizontally consistent since the fields of view overlap. The present study assesses these capabilities by performing a series of simulation experiments in which two-dimensional temperature fields (XZ plane) are retrieved. Several limitations cited in previous work (by H. Fleming) are addressed by realistically treating the geometry of the sensor instantaneous field of view and by using appropriate instrumental noise levels. We have used observed atmospheric cross sections and the sensor geometry and simulation codes appropriate for the HIRS2 sensor. It is found that the tomographic approach is superior to the single angle approach in the cases studied when observational noise is 1.5 brightness temperature degrees (K) in each channel. For smaller noise levels (0.75 K) the two approaches are found to be comparable.

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Ross N. Hoffman, Christopher Grassotti, Ronald G. Isaacs, Jean-Francois Louis, Thomas Nehrkorn, and Donald C. Norquist

Abstract

A series of observing system simulation experiments (0SSEs) was conducted to assess the impacts on the Air Force Geophysics Laboratory (GL) global data assimilation system (GDAS) of a satellite Doppler wind lidar sounding system (WINDSAT) and of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave (SSM) T-1 and T-2 temperature and moisture retrievals. (The SSM/T-2 is expected to be launched in the early 1990s.) In simulating the SSM data, some horizontal correlations were induced because the simulated errors had different biases in different geophysical regimes. As an interpretative aid we calibrated our results to a series of real data experiments.

In an experiment in which the WINDSAT data is added to the observational database, the analyses and forecasts are improved relative to the control experiment. These improvements are large in the Southern Hemisphere extratropics. The addition of the SSM data improves the analysis of moisture particularly in the tropics and Southern Hemisphere extratropics.

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