10B.3 Modeling study of a winter extreme cold weather episode in the central Alaska basin using the NCAR WRF-RTFDDA system

Wednesday, 3 June 2009: 2:00 PM
Grand Ballroom West (DoubleTree Hotel & EMC - Downtown, Omaha)
Yubao Liu, NCAR, Boulder, CO; and W. Wu, G. Roux, T. Warner, S. Sawerdlin, J. Pace, and E. G. Astling

Strong inversions dominate the central Alaska basin in winter seasons in association of scarce solar radiation, cold snow-covered grounds, and prevailing cold-air pool surrounded by high mountains. Extreme cold weather events typically occur with prolonged cooled air pooling processes and the accompanying heat loss due to outward long-wave radiation. To understand the critical physical mechanisms and explore numerical model capability for forecasting these extreme cold weather processes in the complex terrain region, the NCAR WRF-RTFDDA (Weather Research and Forecasting – Real-Time Four-Dimensional Data Assimilation and forecasting systems) are employed to study the features and driving factors of an extreme cold-air event. WRF is an advanced full-physics mesoscale weather model developed in the United States for supporting weather research and operational forecasting. In this study, the WRF-RTFDDA system is run with continuously analysis and forecasting cycles for a six day period spanning the cold-air event. The system assimilates all available observations during the analysis stages of each cycle. The model results indicate that 1) the extreme cold-air event was associated with initial cold-air invasion and occupation in the basin and then further cooled down due to upward long-wave radiation of the ground, 2) ground snow properties, including snow emissivity, snow cover fractions, and maximum snow albedo, have a profound impact, 3) there exist very rich mesoscale structures in the simulated surface temperatures with large amplitude of variations, and 4) cloud effect on long-wave radiation is extremely crucial for modeling the near-surface cold temperature. Correlations between surface (ground/skin temperatures and 2 meter height temperatures) and column-integrated hydrometeors are computed to investigate the cloud effect. It is shown that even trace amount clouds can dramatically block the ground long-wave radiation heat loss and thus retard the surface temperature decrease. Small variations in cloud amounts can results in large changes of the surface temperature (by up to 2 - 10 C) and the response time of these changes are typically less than 1 hour.
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