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

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.