Space Grant Consortium - University of Wisconsin - Green Bay

modeling efforts that the highest possible initial-value datasets are provided. This challenge is
already underway in terms of land-surface modeling, where there are dozens of land types which
influence the surface heat and moisture flux, sometimes significantly depending on the soil
moisture and land use. As LaCasse (2008) showed in her investigation of WRF simulations over
the Florida peninsula, coastal mesoscale gradients in SST can lead to distinct changes in cloud,
precipitation, and temperature forecasts for the immediate onshore and offshore areas. The
weather and water conditions near the shore have a significant impact on the near-term forecast
for the shoreline communities. More precision is needed for accurate forecasts in assessing the
movement of lake breezes.
Furthermore, per Minnett (2003), diurnal differences in SST are typically much less than those
over land, but under calm conditions, can be notable. Increases in SST have been realized over
the Great Lakes under full sun, high pressure, and calm winds. For example, on 23 July 2007,
MODIS SST data collaborated with midlake automated buoy observations over Lake Michigan,
found skin water temperature increases can exceed 1 degree Fahrenheit per hour. Such large
changes can have significant impacts on NWP accuracy which models are not currently
parameterized to correct. Water temperatures generally remain fixed in models applying landsurface
schemes. This alters the differential flux which could be important for the (de)generation
of a lake breeze. Tijm et al. (1999) found that the pressure field responds as the surface flux
changes at the land-sea interface upon the inception of the sea breeze.
Changes in WSTs lead to a difference in land-sea surface flux comparison and thus signal a
response within the MBL. The sea-breeze circulation model is similar to the pressure forcing
model, in which changes in the pressure gradient influence the surface wind field. The WRF
boundary layer height is an approximation based on a bulk Richardson formulation (Troen and
Mahrt 1986), which depends on the conditions in the near-surface layer. As a consequence,
changes in the WRF planetary boundary layer (PBL) height are a byproduct of changes in the
sensible heat flux (LaCasse 2008). This allows for the PBL height to be used as a surrogate for
locating marine-modified air masses in the WRF during the day, regardless of the mechanism,
mesoscale or synoptic scale, for advecting marine air onshore.
Case Study: 26 May 2008
During the afternoon and evening hours of 26 May 2008, a strong convergence boundary
descended south out of Lake Superior and accelerated down the western coast of Lake Michigan.
There was no precipitation associated with this boundary, but the temperature and wind shift was
significant, with coastal temperature declines of 20 degrees Fahrenheit and larger within an hour
period. Reporting stations well inland did not experience significant temperature falls, though
winds statewide become increasingly northeasterly during the late afternoon and evening. The
National Weather Service in Milwaukee, Wisconsin, described this boundary as a pneumonia
front consistent with both the working definition and Behnke’s definition. As shown in figures
later, the boundary was first recognized on base reflectivity data from the Marquette, Michigan,
(KMQT) Weather Surveillance Radar (WSR) as a hybrid of convective outflow and a lake
breeze. The WSRs at Green Bay, Wisconsin, (KGRB) and Sullivan, Wisconsin, (KMKX) also
captured the convergence boundary as it progressed into their scan area (124 nautical miles from
the radar site). Since the boundary was relatively shallow, it was well captured using the halfdegree
base reflectivity slice near radar sites, but difficult to detect beyond 80 nautical miles
from the site, especially when it was not well-established over northern Wisconsin.
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