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Thomas F. Lee

Abstract

Seasonal and interannual variations in Sierra Nevada winter storms are discussed with reference to precipitation augmentation. Seasonal variations occur with respect to freezing level, storm type, vertical cloud distribution, mesoscale precipitation systems, snowmelt and runoff. Statistical results from a previous operational program by Mooney and Lunn suggest that “cold westerly” storms yield increased precipitation from cloud seeding. Case studies from the Sierra Cooperative Pilot Project have shown that postfrontal conditions, which closely correspond to cold westerly storms, are characterized by high supercooled liquid water contents. Eight years of data from the American River Basin have been analyzed here which show that cold westerly storms 1) are more frequent in late winter and spring than earlier in the precipitation season; 2) contribute greater precipitation in seasons of normal and below-normal precipitation than in above-normal seasons. Hydrological factors make these storms attractive targets for precipitation augmentation.

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Thomas F. Lee

Abstract

The Special Sensor Microwave Water Vapor Sounder (SSM/T-2) is a five-channel passive microwave instrument aboard recently launched spacecraft of the Defense Meteorological Satellite Program (DMSP). Rather than address the primary purpose of the SSM/T-2, which is to retrieve atmospheric moisture, this paper examines its ability to sense precipitation as shown by images of a frontal system off the west coast of the United States. Images from the three SSM/T-2 183-GHz channels depict large regions of upper-level water vapor as evidenced by depressed brightness temperatures. Within the moist regions, even lower brightness temperatures at 183 GHz mark embedded precipitation due to volume scattering by precipitation-sized ice particles. Images of the SSM/T-2 channels at 150 and 92 GHz show ice-phase precipitation marked by low brightness temperatures and, over the ocean, low-level clouds and water vapor, both marked by warming with respect to the radiometrically cold background.

This paper compares images of precipitation from the SSM/T-2 with a coincident visible image from the DMSP Operational Line Scanner (OLS) sensor and passive microwave images from the DMSP Special Sensor Microwave/Imager (SSM/I). The discussion emphasizes potential applications to operational workstation users who are increasingly able to produce real-time SSM/T-2 images by processing direct readout telemetry. The ability to produce useful images from the 92-, 150-, and 183-GHz microwave frequencies will increase substantially when the new DMSP Special Sensor Microwave/Imager Sounder (SSM/IS) replaces the SSM/I, the SSM/T-1 (for temperature sounding), and SSM/T-2 sensors later this decade.

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Thomas F. Lee

Abstract

Ten-year climatologies from the Sierra Cooperative Pilot Project (SCPP) show diurnal variations of clouds, precipitation, supercooled liquid water, stability and temperature. Diurnal variations were generally more pronounced in March than in January and February. March, in particular, developed a pronounced maximum of afternoon convection due to strong modulation of spring air masses by surface heating. Supercooled liquid water was most abundant between midnight and dawn and least abundant at midday; this trend grew stronger from January through March.

Two effects help to explain the diurnal trend of supercooled liquid water at Squaw Peak, a mountain top location. First, when the freezing level stayed above Squaw Peak during the day but descended below it at night, water cloud was only supercooled at the site at night. Second, low-level water cloud often dissipated as surface heating raised the temperature of overlying air above its dewpoint; water cloud formed at night through a reverse process.

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Thomas F. Lee

Abstract

Advanced Very High Resolution Radiometer channels 4 (11 μm) and 5 (12 μm) are used together to produce images which greatly enhance contrails. Four steps are required: 1) select coregistered digital data sets from the two channels; 2) convert each raw grayshade to a calibrated brightness temperature; 3) substract corresponding channel 5 temperatures from channel 4 temperatures, creating a field of temperature differences; and 4) display these differences as an image. On the image, the earth's surface and all but thin ice clouds are associated WM small temperature differences (of about −1 to +2 K in the midlatitudes) and appear dark. Newly formed contrails and other thin ice clouds, which are associated with larger temperature differences (of about +2 to +6 K in the midlatitudes), appear bright and stand out well against a dark background.

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Steven D. Miller, Thomas F. Lee, and Robert L. Fennimore

Abstract

This paper presents two multispectral enhancement techniques for distinguishing between regions of cloud and snow cover using optical spectrum passive radiometer satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS). Fundamental to the techniques are the 1.6- and 2.2-μm shortwave infrared bands that are useful in distinguishing between absorbing snow cover (having low reflectance) and less absorbing liquid-phase clouds (higher reflectance). The 1.38-μm band helps to overcome ambiguities that arise in the case of optically thin cirrus. Designed to provide straightforward, stand-alone environmental characterization for operational forecasters (e.g., military weather forecasters in the context of mission planning), these products portray the information that is contained within complex scenes as value-added, readily interpretable imagery at the highest available spatial resolution. Their utility in scene characterization and quality control of digital snow maps is demonstrated.

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