Tuesday, 24 January 2012: 11:00 AM
Air Flow Through Winter Weather Systems Moving Over Mountains
La Nouvelle A (New Orleans Convention Center )
Air flow through mid-latitude cyclones was examined to improve forecasting of winter weather over the rugged mountains of British Columbia. The Snow-V10 research team provided the data set used in this study. A Doppler C-band radar, several atmospheric profilers and surface observations were used to probe major cyclones as they crossed the mountains during the 2010 Olympics. The observations indicated wind shifts, reflectivity and moisture layering, (upper, mid-level and surface) fronts and multi-scale oscillations. Surface data on precipitation (type and amount), temperature, humidity and winds were also used. Two major storms are compared. The first is a mature occlusion observed on February 13-14 and the second is a developing occlusion observed on March 12. Air quality data (ozone, CO and humidity) from Whistler Mountain Peak (2182 m asl) allowed further testing of airflow models. The repeatable ozone-CO-humidity pulsations observed for each storm is best explained by dry ozone enriched tropospheric folding (downwards ahead of the warm front and behind the cold front). This drier air is sliced and overridden by the ozone deprived (and surface derived) warm moist conveyor (WCB) belt. The trowal (Trough of Warm Air Aloft) is the manifestation of the WCB above about 850 mb. Both are enhanced by diabatic heating aloft caused by huge convective cells lifting upwards and northwards ahead of the surface cold front.. In contrast, diabatic cooling due to snowfall melting caused weak down valley winds, but flow speeds from the models were too slow. Pressure rises over the mountains due diabatic cooling, adiabatic lift (orgraphic and gravity wave), and frontal motion modified the timing and strength of boundary layer winds. The strong outflow of dry air through coastal valleys enhanced diabatic cooling caused by snow melting, evaporation and/or sublimation until the fronts swept inland. A feedback between storm intensification, diabatic cooling and heavy precipitation is suggested. The precipitation scheme in the experimental Olympic GEM 2.5 and 1 km models reproduced diabatic effects. This paper shows how conceptual models of air flow can be used to improve the understanding of major wintertime precipitation events caused by mid-latitude cyclones interacting with mountains.