2002 Annual

Monday, 14 January 2002: 3:45 PM
Propagation and lifetime characteristics of thunderstorm anvil clouds over Florida
David A. Short, ENSCO, Inc., Cocoa Beach, FL; and J. E. Sardonia, W. C. Lambert, and M. M. Wheeler
Poster PDF (28.3 kB)
The propagation and lifetime characteristics of thunderstorm anvil clouds over Florida have been determined by utilizing observations of reflected visible radiation from band 1 (0.55-0.75 micrometers) on the Geostationary Operation Environmental Satellite 8, (GOES-8) and upper tropospheric wind observations from nearby rawinsonde stations. A high correlation is found between the length of the optically non-transparent portion of mature anvils and the average wind speed in the layer between 300 and 150 mb. A higher correlation is found between the direction of anvil propagation and wind direction in the anvil layer, as expected. These observations suggest that a successful, empirically-based anvil forecast model might be formulated for the benefit of space launch activities on the Eastern Range (ER: Kennedy Space Center (KSC), and Cape Canaveral Air Force Station (CCAFS)).

The 45th Weather Squadron (45 WS) Launch Weather Officers (LWOs) have identified anvil forecasting as one of their most challenging tasks when attempting to predict the probability of a triggered lightning Launch Commit Criteria (LCC) violation. It is the optically non-transparent portions of the clouds that are to be avoided. Anvil clouds present a significant hazard to space launch activities because of the potential for lightning to be triggered by a launch vehicle penetrating and passing through the cloud layer. Naturally occurring lightning in anvil clouds also presents a significant hazard. Electrified anvil clouds have been observed over the KSC/CCAFS area, emanating from thunderstorms located more than 200 km away. Anvil clouds can serve as conduits for electrical discharges originating in their parent thunderstorms and detached anvils can remain electrically active for hours after separation. A comprehensive set of LCC for launches and Flight Rules (FR) for the Space Transportation System (STS: the “Space Shuttle”) are used by the LWOs to assure that flight vehicles remain well clear of such potentially hazardous clouds.

The extent of the optically non-transparent portion of more than 150 thunderstorm anvil clouds on 50 separate days has been estimated qualitatively from GOES-8 data over Florida during the past two years. The data show that the anvil length can be estimated by multiplying the wind speed in the anvil layer by a time constant of two hours, an effective transport lifetime. The standard deviation of the time constant is 30 minutes. The average direction of anvil propagation is given by the average wind direction in the anvil layer. The standard deviation of differences between anvil and wind directions is 15 degrees, when the wind speed in the anvil layer is greater than 15 knots (7.7 m/s). The standard deviation is higher when the wind speed is lower.

These empirical observations suggest that now-casting and short-term forecasting of anvil cloud characteristics can be successfully accomplished by using the latest observations and short-term forecasts of upper winds to define a threat corridor upstream of the region of interest. The probability of thunderstorm formation within the threat corridor would be obtained from model guidance products and local forecaster experience.

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