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A Comparison of Saturation Pressure Differences and GOES VAS Estimates to Surface Observations of Cloudiness

Randall J. AllissDepartment of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina

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Sethu RamanDepartment of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina

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Abstract

Saturation pressure differences, a measure of parcel saturation, are calculated from upper-air soundings and compared to manual surface observations of cloudiness. The saturation pressure level p* (more commonly referred to as the lifted condensation level, LCL), can be calculated for each level in a sounding using the temperature and dewpoint temperatures. Thus, p* of an unsaturated air parcel is found by dry-adiabatic ascent to the pressure level where the parcel is just saturated. The difference between air parcel pressure and saturation pressure level defines the parcel saturation pressure difference. The mean saturation pressure difference between 1000 and 700, 700 and 400, and 400 and 300 mb is calculated and compared to the observed composite cloudiness for those layers. Results indicate that as the absolute value of saturation pressure difference decreases toward zero, the resulting ground observed composite cloud amount increases. However, the mean saturation pressure difference for high clouds ranges from 64 mb under clear skies to 16 mb for overcast conditions. This corresponds to relative humidities between 25% and 76%. Most previous studies do not indicate such large cloud amounts at these humidities. Three empirical relationships that define low, middle, and high clouds are developed based on one year of comparisons. These relationships are then tested on an independent dataset that include a wide variety of cloud cover conditions. Qualitative comparisons are made to manual observations of cloudiness and indicate that the relationships overall slightly overestimate the frequency of cloudiness. Cloudiness derived from the Visible-Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) onboard the Geostationary Environmental Operational Satellite (GOES) 7 using the CO2 slicing technique is also compared to surface observations. Results indicate that the satellite-derived cloudiness overestimates cloudiness compared with surface observations but is also very similar to the saturation pressure difference estimates.

Abstract

Saturation pressure differences, a measure of parcel saturation, are calculated from upper-air soundings and compared to manual surface observations of cloudiness. The saturation pressure level p* (more commonly referred to as the lifted condensation level, LCL), can be calculated for each level in a sounding using the temperature and dewpoint temperatures. Thus, p* of an unsaturated air parcel is found by dry-adiabatic ascent to the pressure level where the parcel is just saturated. The difference between air parcel pressure and saturation pressure level defines the parcel saturation pressure difference. The mean saturation pressure difference between 1000 and 700, 700 and 400, and 400 and 300 mb is calculated and compared to the observed composite cloudiness for those layers. Results indicate that as the absolute value of saturation pressure difference decreases toward zero, the resulting ground observed composite cloud amount increases. However, the mean saturation pressure difference for high clouds ranges from 64 mb under clear skies to 16 mb for overcast conditions. This corresponds to relative humidities between 25% and 76%. Most previous studies do not indicate such large cloud amounts at these humidities. Three empirical relationships that define low, middle, and high clouds are developed based on one year of comparisons. These relationships are then tested on an independent dataset that include a wide variety of cloud cover conditions. Qualitative comparisons are made to manual observations of cloudiness and indicate that the relationships overall slightly overestimate the frequency of cloudiness. Cloudiness derived from the Visible-Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) onboard the Geostationary Environmental Operational Satellite (GOES) 7 using the CO2 slicing technique is also compared to surface observations. Results indicate that the satellite-derived cloudiness overestimates cloudiness compared with surface observations but is also very similar to the saturation pressure difference estimates.

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