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Neil F. Laird

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Neil F. Laird

Abstract

A large dataset of aircraft cloud traverses from the Small Cumulus Microphysics Study (SCMS) was used to add to the existing knowledge of humidity halo characteristics for small cumulus clouds in a tropical environment. The findings from this investigation show a larger frequency of observed humidity halos than earlier studies. Regardless of the radial direction with respect to shear, humidity halos were observed with a frequency of 77%–90%. The difference in frequency of halo occurrences between upshear and downshear regions was much smaller than previously reported observations. These findings likely resulted from the absence of a strong vertical wind shear environment.

SCMS cumuli had a mean cloud diameter (i.e., in-cloud traverse distance) of 1.1 km and mean halo lengths of about 0.6, 0.7, and 0.8 cloud radii for upshear, cross-shear, and downshear regions, respectively. Humidity halos of less than one cloud radius were observed during about 70% of SCMS aircraft traverses. Approximately 98% of humidity halos had radial lengths of less than four cloud radii. Although considerable differences were not observed between upshear and downshear halo lengths for clouds of similar age, large increases in the frequency and length of halos occurred with an increase in cloud age.

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Jason M. Cordeira
and
Neil F. Laird

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It is generally understood that extensive regions of significant lake ice cover impact lake-effect (LE) snow storms by decreasing the upward heat and moisture fluxes from the lake surface; however, it is only recently that studies have been conducted to more thoroughly examine this relationship. This study provides the first examination of Great Lakes LE snow storms that developed in association with an extensively ice-covered lake. The LE snow events that occurred downwind of Lake Erie on 12–14 February 2003 and 28–31 January 2004 produced maximum snowfall totals of 43 and 64 cm in western New York state, respectively. The presence of widespread ice cover led these snows to be less anticipated than snowfalls from Lake Ontario, which had limited ice cover. For both events, a variety of ice-cover conditions and meso- and synoptic-scale factors (i) helped support LE snow storm development, (ii) lead to the transitions in LE convective type, and (iii) resulted in noteworthy snowfalls near Lake Erie. Thinner ice cover along with favorable fetch directions during the 2004 event likely aided the development of more significant snowband time periods and the resulting greater snowfall. Although Lake Erie had regions with lower ice concentration during the 2003 event, thicker ice cover was present across a greater area of the lake, fetch directions during lake-effect time periods were positioned over higher ice concentration regions, and snowbands had a shorter duration and impacted the same region to a lesser degree than the 2004 case.

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Neil F. Laird
and
Nicholas D. Metz
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Macy E. Howarth
and
Neil F. Laird

Abstract

Wind chill temperature (WCT) is a measure of the human sensation of cold and also is a parameter used to represent the severity of winter weather. This study provides a unique investigation to quantify the spatial patterns of monthly mean, extreme, and severe WCTs across Canada and the United States. WCT was examined across 45 winters (December–February) spanning 1969/70–2013/14 using 156 surface locations reporting hourly meteorological conditions. Intraseasonal analyses of WCT showed that January had 1) the coldest mean WCTs, 2) the most extreme WCTs as statistically represented by the coldest 1% of the monthly WCT frequency distribution at each surface location, and 3) the greatest frequency of severe WCT hours that were ≤ −32°C. The most extreme WCTs were most often located in the Hudson Bay region of Canada, and north-central and northeastern North America experienced the largest monthly changes in WCT during the winter season. Results suggest that intraseasonal changes of air temperature are the primary influence on variations of WCT and that changes of wind speed are a secondary factor.

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Neil F. Laird
and
Nicholas D. Metz
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Melissa Payer
,
Jared Desrochers
, and
Neil F. Laird
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Neil F. Laird
and
David A. R. Kristovich

Abstract

Forecasters in the Great Lakes region have for several decades recognized a general relationship of wind speed and overlake fetch to lake-effect snowstorm morphology. A recent study using idealized mesoscale model simulations of lake-effect conditions over circular and elliptical lakes showed the ratio of wind speed to maximum fetch distance (U/L) may be used to effectively predict lake-effect snowstorm morphology. The current investigation provides an assessment of the U/L criteria using observational datasets. Previously published Great Lakes lake-effect snowstorm observational studies were used to identify events of known mesoscale morphology. Hindcasts of nearly 640 lake-effect events were performed using historical observations with U/L as the predictor.

Results show that the quantity U/L contains important information on the different mesoscale lake-effect morphologies; however, it provides only a limited benefit when being used to predict mesoscale morphology in real lake-effect situations. The U/L criteria exhibited the greatest probability of detecting lake-effect shoreline band events, often the most intense, but also experienced a relatively large number of false hindcasts. For Lakes Erie and Ontario the false hindcasts and biases were reduced and shoreline band events that occurred under higher wind speed conditions were better identified.

In addition, the Great Lakes Environmental Research Laboratory ice cover digital dataset was used in combination with observations from past events to assess the impact of ice cover on the use of U/L as a predictor of lake-effect morphology. Results show that hindcasts using the U/L criteria were slightly improved when the reduction of open-water areas due to lake ice cover was taken into account.

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David A. R. Kristovich
and
Neil F. Laird

Abstract

Large spatial and temporal variations were observed in the location of the upwind cloud edge over Lake Michigan during five westerly wind lake-effect events in November 1995 through January 1996. This study examines the impacts of variations of Lake Michigan surface water temperatures (and corresponding surface fluxes) and upwind static stability on the location of the upwind edge of lake-effect clouds, which develop as cold air crosses the lake during the winter. Data used in this study were collected during the 1995/96 National Weather Service Lake-Effect Snow study. Spatial variations in the location of the upwind lake-effect cloud edge are shown to be related to spatial variations in surface heat and moisture fluxes between the lake surface and overlying air. Surface fluxes are influenced by both the distribution of lake surface water temperatures and variations of surface wind speed, air temperature, and relative humidity. Temporal variations of heat and moisture fluxes from the lake surface and low-level static stability upwind of the lake correlate well with changes in locations of the upwind lake-effect cloud edge. In general, increases in total flux over a particular period tended to correspond with westward change in the position of the upwind cloud edge, whereas decreases in total flux corresponded to eastward shifts of the upwind cloud edge. Atmospheric static stability below the upwind inversion was found to be more important than the inversion height in controlling the location of the upwind cloud edge over the lake, with increases in stability corresponding to eastward shifts in its location.

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Neil F. Laird
,
Jared Desrochers
, and
Melissa Payer

Abstract

This study provides the first long-term climatological analysis of lake-effect precipitation events that developed in relation to a small lake (having a surface area of ≤1500 km2). The frequency and environmental conditions favorable for Lake Champlain lake-effect precipitation were examined for the nine winters (October–March) from 1997/98 through 2005/06. Weather Surveillance Radar-1988 Doppler (WSR-88D) data from Burlington, Vermont, were used to identify 67 lake-effect events. Events occurred as 1) well-defined, isolated lake-effect bands over and downwind of the lake, independent of larger-scale precipitating systems (LC events), 2) quasi-stationary lake-effect bands over the lake embedded within extensive regional precipitation from a synoptic weather system (SYNOP events), or 3) a transition from SYNOP and LC lake-effect precipitation. The LC events were found to occur under either a northerly or a southerly wind regime. An examination of the characteristics of these lake-effect events provides several unique findings that are useful for comparison with known lake-effect environments for larger lakes. January was the most active month with an average of nearly four lake-effect events per winter, and approximately one of every four LC events occurred with southerly winds. Event initiation and dissipation occurred on a diurnal time scale with an average duration of 12.1 h. In general, Lake Champlain lake-effect events 1) typically yielded snowfall, with surface air temperatures rarely above 0°C, 2) frequently had an overlake mesolow present with a sea level pressure departure of 3–5 hPa, 3) occurred in a very stable environment with a surface inversion frequently present outside the Lake Champlain Valley, and 4) averaged a surface lake–air temperature difference of 14.4°C and a lake–850-hPa temperature difference of 18.2°C. Lake Champlain lake-effect events occur within a limited range of wind and temperature conditions, thus providing events that are more sensitive to small changes in environmental conditions than are large-lake lake-effect events and offering a more responsive system for subsequent investigation of connections between mesoscale processes and climate variability.

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