| Description |
This dissertation examines the influence of environmental conditions and orographic barriers on the frequency, occurrence, and morphology of Great Salt Lake Effect (GSLE) precipitation. The analysis consists of the development of an updated event climatology, statistical examination of the factors necessary for events, and two case studies that employ numerical modeling to investigate orographic influences. For the climatology, events were identified using cool season (16 Sep- 15 May) WSR-88D radar imagery, radiosonde soundings, and MesoWest surface observations from 1997-98 - 2009/10. During this period, the frequency of GSLE events features considerable interannual variability that is more strongly correlated to large-scale circulation changes than lake-area variations. Events are most frequent in fall and spring, with a minimum in January when the climatological lake-surface temperature is lowest. Although forecasters commonly use a 16°C lake-700-hPa temperature difference (∆T) as a threshold for GSLE occurrence, a seasonally varying threshold based on a quadratic fit to the monthly minimum ∆T values during GSLE events is more appropriate than a single value. A probabilistic forecast method based on the difference between ∆T and this seasonally varying threshold, 850-700-hPa relative humidity, and 700-hPa wind direction offers substantial improvement over existing methods. An important consideration for forecasting because of their higher precipitation rates, banded features- with a horizontal aspect ratio of 6:1 or greater-dominate only 20% of the time the GSLE is occurring, while widespread, nonbanded precipitation is much more common. The two events examined in the second part of the study (27 Oct 2010 and 5 Nov 2011) produce synergistic interactions between lake-effect and orographic processes. A dramatic decrease in precipitation intensity and coverage occurs in numerical simulations when either the lake or terrain forcings are removed. A foehn-like flow over upstream orography reduces the relative humidity of the incipient low-level airmass and limits the intensity of both events. A convergence zone in the lee of isolated upstream topography in positioned over the north arm of the GSL, and may play a role in orgainizing the 27 Oct 2010 lake-effect band. Downstream orographic influences are large in both events, and include (1) overlake convergence due to flow stagnation along and/or blocking by the Wasatch Mountains, (2) enhancement of blocking effects due to a horizontal moisture gradient, (3) flow deflection around the Oquirrh Mountains into an orographic concavity, and (4) hydrometeor transport into high terrain. These influences are not unique to the GSL region, and our results suggest applicability to other areas where lake-effect occurs in close proximity to mountain barriers, particularly in the case of small water bodies. |
