Influence of topography on a snowfall event in Iqaluit, NU with comparisons to GEM-LAM

This presentation evaluates cloud and precipitation features during a snowfall event in Iqaluit, Nunavut located on Baffin Island. The event was small in accumulation (2.2 mm), but was not captured well by the regional forecast. Better understanding of atmospheric-topographic interactions is critical to improved precipitation forecasting and modeling in the Arctic. Due to a lack of field observations in this region historically, cloud and precipitation characteristics associated with topographic interactions and the seasonal variation of heterogeneous surface conditions (sea ice versus open water) have been identified as significant knowledge gaps. Using data collected as part of the Storm Studies in the Arctic (STAR) research network in November 2007, the first to use both aircraft and ground based radar systems in the eastern Canadian Arctic; observations of high-latitude orographic cloud and precipitation during a fall snowfall event are presented.

Persistent onshore flow, associated a quasi stationary low-pressure system northwest of Baffin Island, and low-level convective instability, initiated by the sharp contrast between near-surface air and ocean temperatures resulted in moist unstable air to be lifted over the Meta Incognita mountain range (west of Iqaluit). Using a research aircraft, upstream atmospheric conditions on the windward slopes of the topography were investigated. A cloud profiling radar (W-band) mapped the vertical structure (reflectivity and doppler velocity) and drop sonde measurements evaluated the dynamic and thermodynamic conditions. Upslope precipitation was characterized by strong reflectivity values (15-18 dBZ) and high IWC measurements during this event. Vertical velocities were estimated between 1-3 m/s, increasing towards the surface, indicating an increase in particle mass as they fell. Updrafts of 1 m/s in the upper 500 m of the cloud where measured, identifying regions of small-scale convection at cloud top. A ground based X-band radar (located in Iqaluit) was used to analyze the evolution of the system as it transitioned to the lee side of the topographical barrier. Snow fell in Iqaluit for a period 5-hours, reducing the visibility to less than 300 m at times. Vertical cross sections, perpendicular to the terrain, were created by combining Range Height Indicator (RHI) scans from the 135° and 315° azimuths. The profiles revealed that the vertical structure of precipitation was associated with streamers and high reflectivity (20-24 dBZ). Doppler velocity measurements show that there was an increase in the vertical velocity of the hydrometeors as they fell towards the surface. Surface macrophotography indicate that precipitation fell mainly as heavily rimed ice particles, identifying accretion as an important growth mechanism in this case. Radar profiles from the research aircraft indicate that upslope precipitation remained strong on windward slope of the topography for > 4-hours after precipitation stopped falling in Iqaluit. This presentation concludes with a comparison of data collected during this event with the Canadian regional (1-km resolution) forecast model GEM-LAM (Global Environmental Multi-Scale - Limited Area Model). Insights into model strengths and weaknesses in this case are discussed.