Rapid Development of Dry Intrusion Filaments in an Intensifying Trowal Interacting with Mountains

Rapid intensification of an occluded cyclone is observed when distinctive dry intrusion filaments develop ahead of the Trowal (meaning a “trough of warm air aloft” as described by Shultz and Vaughn, 2011 and references therein). In this study we will examine several storms moving over the BC Coast Mountains and will investigate why precipitation intensified with the appearance of these filament structures. The filament structures in the upper level storm clouds have been described by Browning and Reynolds (1994) and may be identified on satellite imagery as dark lines in the 6.7 micron wavelength “water vapour” imagery. These filament related structures were further examined for this study using ground based radar, surface weather data and vertical profilers (MRR, MW Radiometer, UHF Wind and RASS) using the Olympic SNOW-V10 data set which includes high resolution numerical modeling. The distinctive dark lines (considered dry intrusion filaments or dry streaks) were observed to suddenly connect along tremendous distances when an intense Trowal arrived nearly parallel to the Coast Mountains. The most distinctive leading dry streak in the Trowal appeared to synchronize with a large mountain wave emanating from the surface and then link to form a large subsidence gap east of the Coast Ranges. These cloud structures are similar in many respects to the entrainment, convection and gravity wave related features observed in intense thunderstorms (Uccellini and Koch, 1987; Koch and Siedlarz, 1999) and observed in rapidly developing hurricanes (Rozoff et al, 2006; Yuqing, 2007).

The intense Trowals with dry streaks examined in this study appeared to have progressive diabatic cooling aloft, strong diabatic heating within, and variable diabatic cooling below the Warm Conveyor Belt (WCB). The increasing depth of the cooling aloft above a Trowal is shown by the profiler data and mapped as an overriding Cold Front Aloft (CFA) similar to Browning (1977, 1997) and Young (1994) using frontal definitions of Godson (1951). This overriding CFA is consistent with progressive entrainment of dry air folded or intruded slantwise downwards due to intense convection with strong jets aloft. This intense convection produces the latent heat of condensation and strong warm advection necessary to maintain the warm moist core of the Trowal. Internal gravity waves and major critical layers were identified in the profiler data sets, especially the Microwave Radiometer data, and appear to be important for storm intensification and the filament structures. Unique MSC ozone air quality data showed air was folded, mixed and/or intruded down from the stratosphere to Whistler Mountain Peak both ahead and behind these storms, but not within the warmest core of each Trowal.

This study sheds new light on how intense occluded cyclones can play an important role in the global exchange of energy and air between the troposphere and the stratosphere, especially when these cyclones interact strongly with mountains. The processes controlling the rapid development of dry intrusion filaments are important for weather forecasting in mountainous areas because they can help determine precipitation intensity, type and airflow patterns, as well as the life cycle of the storm itself.