Upwind Coastline-induced Downdrafts and Influence on Growth of the Lake-effect Convective Boundary Layer

Numerous studies have indicated the importance of upwind atmospheric conditions (e.g., low-level stability, temperature, moisture content, wind velocity profile, etc.) on the development and intensity of convection over a relatively warm lake or ocean surface. For example, cloud and snow development in Great Lakes lake-effect snow storms has often been observed to be rapid when planetary boundary layer air reaching the coastline exhibits low static stability, positive lake-air temperature differences and high moisture content. For this reason, airmass modification by upwind lakes would be anticipated to be ideal for increasing lake-effect snowfall produced over the eastern Great Lakes. Indeed, several studies have shown that lake-effect convective bands of clouds and snow often stretch from one Great Lake to another, with increasing snow intensity near the downwind lake. Complicating this scenario, however, are rapid changes in the thermodynamic characteristics of the boundary layer as the air moves from upwind land surfaces to over the lake surface. This presentation will discuss unique observations available during the Ontario Winter Lake-effect Systems (OWLeS) field project to describe how coastal flow changes impact the influence of upwind lakes on snowfall produced over Lake Ontario.

One of the objectives of the Ontario Winter Lake-effect Systems (OWLeS, https://www.eol.ucar.edu/field_projects/owles) field project, conducted during December 2013 and January 2014 in the vicinity of Lake Ontario, was to determine the processes by which upwind lakes modify boundary layer and snow growth rates over Lake Ontario. This presentation will focus on the 28 January 2014 case, when lake-effect clouds and snow extended northeastward from Lake Erie toward Lake Ontario. Observations from several university-owned radiosonde systems, University of Wyoming King Air in situ instrumentation, Wyoming Cloud Radar (WCR), and NOAA operational sites are used to examine the details of changes in the PBL as the air moves from snow-covered land surfaces to Lake Ontario waters.

A mesoscale stationary region of decreased cloudiness and snowfall rates, and lower PBL tops was evident from about 5 km upwind of the Lake Ontario shoreline to about 10 km downwind. Superimposed on this were intense local downdrafts resulting in local sharp decreases in the depth of the PBL, identifiable by downward penetration of stable above-boundary layer air. These local downdrafts are inferred, based on WCR observations, to be consistently present, but not spatially stationary. Implications of these mesoscale and smaller-scale downdrafts on convective growth over Lake Ontario will be discussed.