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Thomas F. Lee

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Thomas F. Lee

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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

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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

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This note discusses a Geostationary Operational Environmental Satellite product that can reveal the low clouds associated with tropical cyclones at night. It is based on a pixel-by-pixel difference of the longwave (10.8 μm) and shortwave (3.9 μm) brightness temperature fields. This product is compared with daytime visible images from the same satellite and passive microwave images from the Tropical Rainfall Measuring Mission Microwave Imager and the Defense Meteorological Satellite Program Special Sensor Microwave/Imager. Implications for weather analysis and forecasting are discussed for weak and shearing systems.

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Thomas F. Lee

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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

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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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Thomas F. Lee

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Visible and infrared (IR) images from the National Oceanic and Atmospheric Administration's (NOAA's) Advanced Very High Resolution Radiometer are composited to improve the depiction of airborne dust over coastlines. On IR images, wind-raised dust stands out well against heated land surfaces. Once advected over ocean, however, dust on IR images can not be easily distinguished from the cool surface. The situation is reversed on visible images: dust contrasts well with dark ocean backgrounds but poorly with bright land surfaces. To illustrate the optimal use of both data types, a composite image juxtaposes visible data over water with IR data over land. Accompanying synoptic data provide additional information about the distribution of dust in the vertical.

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Thomas F. Lee, Susan Atwater, and Charles Samuels

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The authors develop and discuss satellite image enhancements of sea ice boundaries to the north and west of Alaska. Using data from the NOAA Advanced Very High Resolution Radiometer (AVHRR), the authors illustrate how a pixel-by-pixel difference image of the two AVHRR solar channels can suppress the effects of optically thin cloud cover, revealing underlying detail. These difference images are compared to satellite ice products derived from the DMSP Special Sensor Microwave Imager (SSM/I).

AVHRR enhancements have improved daily operational support to fishing, crabbing, scientific research, and especially oil exploration in the coastal waters of Alaska. In particular, the AVHRR solar difference image has greatly facilitated the preparation of daily maps depicting the major ice edge. During a two-week period in 1991 characterized by frequent cloudiness, analysts were able to prepare information locating the major ice edge 90 percent of the time using this product. The potential of SSM/I ice products, new in nonmilitary ice edge mapping support, is also discussed.

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Gregory Thompson, Randy Bullock, and Thomas F. Lee

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Overprediction of the spatial extent of aircraft icing is a major problem in forecaster products based on numerical model output. Dependence on relative humidity fields, which are inherently broad and smooth, is the cause of this difficulty. Using multispectral satellite analysis based on NOAA Advanced Very High Resolution Radiometer data, this paper shows how the spatial extent of icing potential based on model output can be reduced where there are no subfreezing cloud tops and, therefore, where icing is unlikely. Fifty-one cases were analyzed using two scenarios: 1) model output only and 2) model output screened by a satellite cloud analysis. Average area efficiency, a statistical validation measure of icing potential using coincident pilot reports of icing, improved substantially when satellite screening was applied.

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Thomas F. Lee and James S. Boyle

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Near-surface wind speed analyses from the Special Microwave/Imager (SSM/I) are compared with those from an operational numerical forecast system. Substantial agreement exists, especially over open ocean. Nevertheless, the use of both fields together should lead to a better understanding of the synoptic situation than the use of either field alone. The SSM/I winds give a superior representation of spatial variations of wind speeds. Also, the SSM/I information can he used to examine continental influences on marine wind speeds. Such mesoscale effects are not analyzed well by global numerical models. On the other hand, the output from the numerical forecast system provides wind direction, a parameter not available from the SSM/I. In addition, forecast system winds fill in information between satellite passes and in regions contaminated by precipitation.

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