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Robert M. Rauber
,
Michael Garstang
, and
David A. R. Kristovich
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David A. R. Kristovich
,
Luke Bard
,
Leslie Stoecker
, and
Bart Geerts

Abstract

Annual lake-effect snowstorms, which develop through surface buoyant instability and upward moisture transport from the Laurentian Great Lakes, lead to important local increases in snowfall to the south and east. Surface wind patterns during cold-air outbreaks often result in areas where the air is modified by more than one Great Lake. While it is known that boundary layer air that has crossed multiple lakes can produce particularly intense snow, few observations are available on the process by which this occurs. This study examines unique observations taken during the Ontario Winter Lake-effect Systems (OWLeS) field project to document the process by which Lake Erie influenced snowfall that was produced over Lake Ontario on 28 January 2014. During the event, lake-effect clouds and snow that developed over Lake Erie extended northeastward toward Lake Ontario. OWLeS and operational observations showed that the clouds from Lake Erie disappeared (and snow greatly decreased) as they approached the Lake Ontario shoreline. This clear-air zone was due to mesoscale subsidence, apparently due to the divergence of winds moving from land to the smoother lake surface. However, the influence of Lake Erie in producing a deeper lake-effect boundary layer, thicker clouds, increased turbulence magnitudes, and heavier snow was identified farther downwind over Lake Ontario. It is hypothesized that the combination of a low-stability, high-moisture boundary layer as well as convective eddies and limited snow particles crossing the mesoscale subsidence region locally enhanced the lake-effect system over Lake Ontario within the plume of air originating over Lake Erie.

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Bart Geerts
,
Binod Pokharel
, and
David A. R. Kristovich

Abstract

Winter storms are often accompanied by strong winds, especially over complex terrain. Under such conditions freshly fallen snow can be readily suspended. Most of that snow will be redistributed across the landscape (e.g., behind obstacles), but some may be lofted into the turbulent boundary layer, and even into the free atmosphere in areas of boundary layer separation near terrain crests, or in hydraulic jumps. Blowing snow ice crystals, mostly small fractured particles, thus may enhance snow growth in clouds. This may explain why shallow orographic clouds, with cloud-top temperatures too high for significant ice initiation, may produce (usually light) snowfall with remarkable persistence. While drifting snow has been studied extensively, the impact of blowing snow on precipitation on snowfall itself has not.

Airborne radar and lidar data are presented to demonstrate the presence of blowing snow, boundary layer separation, and the glaciation of shallow supercooled orographic clouds. Further evidence for the presence of blowing snow comes from a comparison between snow size distributions measured at Storm Peak Laboratory (SPL) on Mount Werner (Colorado) versus those measured aboard an aircraft while passing overhead, and from an examination of snow size distributions at SPL under diverse weather conditions. Ice splintering following the collision of supercooled droplets on rimed surfaces such as trees does not appear to explain the large concentrations of small ice crystals sometimes observed at SPL.

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Neil F. Laird
,
John E. Walsh
, and
David A. R. Kristovich

Abstract

Idealized model simulations with an isolated elliptical lake and prescribed winter lake-effect environmental conditions were used to examine the influences of lake shape, wind speed, and wind direction on the mesoscale morphology. This study presents the first systematic examination of variations in lake shape and the interplay between these three parameters. The array of 21 model simulations produced cases containing each of the three classic lake-effect morphologies (i.e., vortices, shoreline bands, and widespread coverage), and, in some instances, the mesoscale circulations were composed of coexisting morphologies located over the lake, near the downwind shoreline, or inland from the downwind shore.

As with lake-effect circulations simulated over circular lakes, the ratio of wind speed (U) to maximum fetch distance (L) was found to be a valuable parameter for determining the morphology of a lake-effect circulation when variations of lake shape, wind speed, and wind direction were introduced. For a given elliptical lake and strong winds, a morphological transform from shoreline band toward widespread coverage accompanied changes in ambient flow direction from along to across the major lake axis. For simulations with weak winds over a lake with a large axis ratio, the morphology of the lake-effect circulation changed from vortex toward shoreline band with a change in wind direction from along to across the major lake axis. Weak winds across lakes with smaller axis ratios (i.e., 1:1 or 3:1) produced mesoscale vortices for each wind direction. Across the array of simulations, a shift in mesoscale lake-effect morphology from vortices to bands and bands toward widespread coverage was attended by an increase in U/L. Last, the elliptical-lake results suggest that the widths of the lake-effect morphological transition zones in U/L parameter space, conditions favorable for the coexistence of multiple morphologies, were greater than for circular lakes.

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Sudheer R. Bhimireddy
,
Junming Wang
,
April L. Hiscox
, and
David A. R. Kristovich

Abstract

Harvesting of crops in a weakly sloping Midwestern field during the Stable Atmospheric Variability and Transport (SAVANT) observation campaign allowed for a systematic investigation of the influence of surface roughness and static stability magnitude on the applicability of the Monin–Obukhov similarity (MOST) and hockey-stick transition (HOST) theories during stable boundary layer periods. We analyze momentum flux and turbulent velocity scale V TKE in three regimes, defined using the gradient Richardson number Ri and flux Richardson number Ri f as regime 1 (0 < Ri ≤ 0.1 and 0 < Ri f ≤ 0.1), regime 2 (0.1 < Ri ≤ 0.23 and 0.1 < Ri f ≤ 0.23), and regime 3 (both Ri and Ri f > 0.23). After harvest, in regime 1, stability varied from near-neutral to weakly stable and both MOST and HOST were applicable to estimate the momentum fluxes and V TKE as a function of mean wind speed. In regime 2, the momentum flux deviated from the MOST linear relationship as stability increased. In regimes 1 and 2, a HOST-defined threshold wind speed Vs was identified beyond which V TKE increased linearly with wind speed at a rate of 0.26 for all observation heights. Below this threshold wind speed, V TKE behaved independent of mean wind and observation heights. Alternatively, for preharvest periods, MOST was applicable in regimes 1 and 2 for all heights and HOST was applicable with reduced Vs for heights above the crop layer. Regime 3 during pre- and postharvest consisted of strongly stable periods and very weak to weak winds, where MOST was found to be invalid and V TKE remained low and independent of wind speed. The results suggest that roughness due to crops enhances the turbulence generation at lower wind speeds.

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ROLLS, STREETS, WAVES, AND MORE

A Review of Quasi-Two-Dimensional Structures in the Atmospheric Boundary Layer

George S. Young
,
David A. R. Kristovich
,
Mark R. Hjelmfelt
, and
Ralph C. Foster

The atmospheric boundary layer is home to a number of horizontally elongated quasi-two-dimensional phenomena including cloud streets, roll vortices, thermal waves, and surface layer streaks. These phenomena, their dynamics, and their interactions are explored via a review of the literature. Making a clear distinction between the various quasi-two-dimensional phenomena allows improved synthesis of previous results and a better understanding of the interrelationships between phenomena.

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David A. R. Kristovich
,
Eugene Takle
,
George S. Young
, and
Ashish Sharma

Abstract

This chapter outlines the development of our understanding of several examples of mesoscale atmospheric circulations that are tied directly to surface forcings, starting from thermally driven variations over the ocean and progressing inland to man-made variations in temperature and roughness, and ending with forced boundary layer circulations. Examples include atmospheric responses to 1) overocean temperature variations, 2) coastlines (sea breezes), 3) mesoscale regions of inland water (lake-effect storms), and 4) variations in land-based surface usage (urban land cover). This chapter provides brief summaries of the historical evolution of, and tools for, understanding such mesoscale atmospheric circulations and their importance to the field, as well as physical processes responsible for initiating and determining their evolution. Some avenues of future research we see as critical are provided. The American Meteorological Society (AMS) has played a direct and important role in fostering the development of understanding mesoscale surface-forced circulations. The significance of AMS journal publications and conferences on this and interrelated atmospheric, oceanic, and hydrological fields, as well as those by sister scientific organizations, are demonstrated through extensive relevant citations.

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Neil F. Laird
,
David A. R. Kristovich
,
Xin-Zhong Liang
,
Raymond W. Arritt
, and
Kenneth Labas

Abstract

A method was developed to identify the occurrence of lake-breeze events along the eastern, western, and both shores of Lake Michigan during a 15-yr period (1982–96). Comparison with detailed observations from May through September of 1996–97 showed that the method reasonably identified Lake Michigan lake-breeze events. The method also demonstrated the important ability to distinguish non-lake-breeze events; a problem experienced by previously developed lake-breeze criteria. Analyses of the 15-yr climatological data indicated that lake breezes tended to occur more frequently along the eastern shore of Lake Michigan than along the western shore. On average, a maximum number of lake-breeze events occurred during August at each location. This maximum is most closely associated with weaker monthly average wind speeds. Even though the air–lake temperature difference ΔT provides the local forcing for the development of the lake-breeze circulation, large temperature differences are not required. Nearly 70% of all events occurred with a daytime maximum ΔT ⩽ 12°C. The evaluation of a lake-breeze index ε used in past studies and many forecasting applications showed indices computed using offshore or shore-perpendicular wind speeds (U or |U|, respectively) at inland sites resolved ≥95% of identified events based on critical ε values of 2–6. When wind speed, irrespective of wind direction, was used to calculate ε, the success of the critical indices decreased by as much as 26%. Results also showed that the lake-breeze index has a considerable tendency to overestimate the number of events. Although the possibility was suggested by previous investigations, the critical value of ε may not be appreciably affected by changes in location along the shoreline. In addition, noteworthy differences in the position of synoptic-scale sea level pressure and wind fields with respect to Lake Michigan were found to occur during eastern, western, and both-shore lake-breeze events.

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Thomas E. Workoff
,
David A. R. Kristovich
,
Neil F. Laird
,
Robert LaPlante
, and
Daniel Leins

Abstract

The influence that the overlake boundary layer has on storm intensity and structure is not well understood. To improve scientists’ understanding of the evolution of storms crossing Lake Erie, 111 events during 2001–09 were examined using observations from Weather Surveillance Radar-1988 Doppler (WSR-88D), surface, buoy, and rawinsonde sites. It was found that on average, all storm modes tended to weaken over the lake; however, considerable variability in changes of storm intensity existed, with some storms exhibiting steady-state or increasing intensity in specific environments. Noteworthy changes in the storm maximum reflectivity generally occurred within 60 min after storms crossed the upwind shoreline. Isolated and cluster storm modes exhibited much greater weakening than those storms organized into lines or convective complexes. The atmospheric parameters having the greatest influence on storm intensity over Lake Erie varied by mode. Isolated and cluster storms generally weakened more rapidly with increasingly cold overlake surface air temperatures. Linear and complex systems, on the other hand, tended to exhibit constant or increasing maximum reflectivity with cooler overlake surface air temperatures. It is suggested that strongly stable conditions near the lake surface limit the amount of boundary layer air ingested into storms in these cases.

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Kenneth E. Kunkel
,
David R. Easterling
,
David A. R. Kristovich
,
Byron Gleason
,
Leslie Stoecker
, and
Rebecca Smith

Abstract

Daily extreme precipitation events, exceeding a threshold for a 1-in-5-yr occurrence, were identified from a network of 935 Cooperative Observer stations for the period of 1908–2009. Each event was assigned a meteorological cause, categorized as extratropical cyclone near a front (FRT), extratropical cyclone near center of low (ETC), tropical cyclone (TC), mesoscale convective system (MCS), air mass (isolated) convection (AMC), North American monsoon (NAM), and upslope flow (USF). The percentage of events ascribed to each cause were 54% for FRT, 24% for ETC, 13% for TC, 5% for MCS, 3% for NAM, 1% for AMC, and 0.1% for USF. On a national scale, there are upward trends in events associated with fronts and tropical cyclones, but no trends for other meteorological causes. On a regional scale, statistically significant upward trends in the frontal category are found in five of the nine regions. For ETCs, there are statistically significant upward trends in the Northeast and east north central. For the NAM category, the trend in the West is upward. The central region has seen an upward trend in events caused by TCs.

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