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Peter J. Sousounis

Abstract

The structure of a meso-α-scale vortex that developed over the Great Lakes in late autumn is described. Understanding the structure of this particular vortex is important because 1) it altered local- and regional-scale precipitation, 2) it developed in a complicated fashion, and 3) it represents a class of vortices that develop frequently during cold-air outbreaks over the Great Lakes and whose development is not yet understood.

Detailed perturbation analyses of the synoptic-scale and mesoscale conditions over the Great Lakes that led to the vortex development are presented using model output from previous numerical simulations that included all of the Great Lakes (WL) and none of the Great Lakes (NL). The initial thermal perturbation was characterized by an elongated warm plume near the surface that advanced southeastward toward the mid-Atlantic coast as cold air overspread the entire lakes region. The plume then became more circular and deepened as it rotated toward the northeast in response to the changing synoptic-scale flow. The perturbation winds revealed several small meso-β-scale circulations that developed during the first 36 h within the warm plume. By 48 h, a 3-km-deep meso-α-scale vortex developed several hundred kilometers to the northeast of Lake Huron with cyclonic flow in the lower half and anticyclonic flow in the upper half. Its size, location, and evolution indicate that it was likely generated by aggregate-lake as opposed to individual-lake heating and moistening. It is therefore referred to as a mesoscale aggregate vortex (MAV).

The MAV that developed represents a class of vortices that can be described as inertially stable, meso-α-scale warm-core vortices approximately 500–1000 km wide and 2–4-km deep that develop from aggregate heating and moistening over the Great Lakes. They are usually identifiable on standard surface weather charts as a weak low over the Great Lakes region with 1–3 closed isobars at 2-mb intervals. The outermost closed isobar typically encloses an area approximately as large as that spanned by the upper Great Lakes (e.g., Lakes Superior, Huron, and Michigan).

Because the MAV in this study developed nearly 36 h after the coldest air left the region, it is not clear whether other physical mechanisms besides sensible and latent heating were involved, to what extent they were important, and at what stages they occurred. Additionally, the MAV developed within a large elliptical region of lake-aggregate heated air, which suggests the importance of geostrophic adjustment, and from smaller individual-lake scale circulations, which suggests the importance of vortex merger. Development mechanisms will be discussed in a follow-up study.

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Peter J. Sousounis

Abstract

Many studies have noted that cyclone development in the Great Lakes region during winter is the result of strong diabatic heating and low-level destabilization from the lakes. The exact mechanisms, however, by which this heating and moistening lead to sea level pressure falls, and to weak cyclones over the lakes (e.g., mesoscale aggregate vortices), have not been investigated previously.

In this study, model output that includes all of the Great Lakes and none of the Great Lakes is analyzed to understand more completely the importance of synoptic-scale forcing, diabatic heating, and perturbation–synoptic-scale processes for the development of a mesoscale aggregate vortex over the region during a 48-h period between 0000 UTC 13 and 0000 UTC 15 November 1982. The analysis indicates that the sea level pressure falls and vortex development were not simply the hydrostatic result of heat from the Great Lakes “spreading” over a large region. Rather, the synoptic-scale flow contributed to vortex development during the first 24 h by providing strong cold northwesterly flow, which generated significant surface heat fluxes; and during the second 24 h by providing low-level warm advection and midlevel positive vorticity advection from southwesterly flow, which enhanced large-scale ascent and horizontal perturbation heat flux convergence near the surface. The eventual collocation of strong cyclonic perturbation southerly winds at 900 hPa, strong anticyclonic perturbation southerly winds at 700 hPa, and east–west-oriented isotherms in between greatly enhanced the warm advection and vortex development in the region. Finally, the intensifying cyclonic perturbation flow contributed significantly to surface sensible and latent heat fluxes and to further vortex development when it phased with the synoptic-scale flow at the surface.

The one case that has been examined does not likely serve as an explanation for all mesoscale aggregate vortices. More studies are needed to determine the climatology of these vortices that develop over the Great Lakes region in winter.

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Peter J. Sousounis
and
J. Michael Fritsch

A high-resolution numerical model is employed to examine effects of the Great Lakes aggregate, defined to be the five major Great Lakes, on regional and synoptic-scale weather. Simulations wherein the effects of the lakes are included and then excluded are performed on a selected cold air outbreak episode during late autumn when the lakes are still ice-free.

Examination of the differences between the model simulations reveals that several dynamical effects result from heating and moistening by the lake aggregate. These effects are manifested primarily in the form of a 4-km-deep, 2000-km-wide, lake-aggregate mesoscaie disturbance (circulation) that develops slowly over the region. The simulated lake-aggregate circulation splits a synoptic-scale high into two distinct centers and redirects and intensifies a weak synoptic-scale low, as verified by existing observations. These modifications of the synoptic-scale environment result in additional precipitation over, downstream, and upwind from the lakes.

The model simulations also reveal that the developing lake-aggregate circulation influences significantly the lake shore surface winds. In some locations, the surface winds switch from onshore to offshore or vice versa. Because it is well known from observations that the location and orientation of lake-induced snow bands are very sensitive to the low-level wind direction over the lakes, it is concluded that the exact locations of heavy snowfall are the result of a complex multiscale interaction among circulations on three different scales: synoptic, individual lake, and lake aggregate.

In addition to the developing primary lake-aggregate circulation, a secondary dynamic response appears at a distant location, adjacent to the eastern seaboard. The organization of this secondary circulation suggests that the lakes may play a direct role in some cases of East Coast cyclogenesis.

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Christopher P. J. Scott
and
Peter J. Sousounis

Abstract

The impact of initializing a mesoscale model with additional sounding data over the Great Lakes region is investigated. As part of the Lake-Induced Convection Experiment (Lake-ICE) field study during the winter of 1997/98, six supplementary Cross-chain Loran Atmospheric Sounding System (CLASS) units and three Integrated Sounding System (ISS) units were used in addition to those from the standard synoptic upper-air network. The three ISS units were in the vicinity of Lake Michigan, and the six CLASS units were in the data-sparse region of central and northeastern Ontario and western Quebec.

The Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model running on a doubly nested grid is used to simulate the lake-effect snow event of 4–5 December 1997. This model output from a 30-km horizontal resolution grid shows that the six CLASS soundings capture a warm layer below 850 hPa that appears to be the result of diabatic heating from the Great Lakes. This leads to an improved simulation of the surface pressure fields over the course of the simulation. A nested 10-km horizontal resolution grid shows that the initialization data from the CLASS sites seemed to have a greater influence on the propagation of a mesoalpha-scale trough that caused significant snowfall to the lee of Lake Michigan than data from the ISS sites. The inclusion of the CLASS sounding data changes the track of the precipitation maximum by approximately 25 km and agrees better with reflectivity data from the Weather Surveillance Radar-1988 Doppler. Implications for forecasters in the Great Lakes region are discussed.

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Peter J. Sousounis
and
Hampton N. Shirer

Abstract

The steady boundary-layer responses that occur over the Great Lakes region during wintertime cold air outbreaks are examined using a two-dimensional, linear, analytic model. The planetary boundary layer (PBL) is modeled as an idealized, constantly stratified, viscous, rotating Boussinesq fluid that moves uniformly between two horizontally infinite, rigid, stress-free plates. The heat from the lakes is parameterized in terms of a specified diabatic forcing function.

Solution of the governing differential equation yields an integral expression for the vertical motion of the general response. Further assessment of the response is gained by examining closed-form analytic solutions to several limiting cases. Four response types are identified that depend upon the values of the Froude number Fr, the mechanical Ekman number E x , the thermal Ekman number E x , and the eddy Prandtl number Pr.

Four different flow regimes are found. When 0 ≤ Fr < 1 and Pr ≥ 1, there is a purely exponentially damped response that exists over and on both sides of the heating. A flow characterized approximately by 1 ≤ Fr2 < 1 + E r 2 + E x and Pr ≥ 1 yields a purely exponentially damped response that exists only over and downstream of the heating, while a flow characterized approximately by Fr2 > 1 + E r 2 + E x 2 and Pr ≥ 1 yields a mixed oscillating-exponentially damped response that exists only over and downstream of the heating. When Pr < 1 and Fr > 1, either of the previous two response types can occur, while when 0 < Fr < 1, a fourth type of response can occur that is mixed oscillating-exponentially damped and exists over and on both sides of the heating.

The model is used to demonstrate the effects that rotation, stability, mean flow speed, and mechanical and thermal dissipation have on the PBL responses that occur over the Great Lakes during wintertime cold air outbreaks. The simulation of heating by the lakes of strong flow within a moderately cold, shallow PBL produces a model response with ascent and implied clouds and precipitation extending well downstream of the lakes, as are typically observed soon after such a response develops. The simulation of heating by the lakes of weak flow within a very cold, deep PBL produces a model response with ascent and implied clouds and precipitation that are collocated with the lakes, as are typically observed just before such a response decays.

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Christopher C. Weiss
and
Peter J. Sousounis

Abstract

This study examines the frequency and intensity with which collective lake disturbances (COLDs) develop. These disturbances develop when cold air overspreads the Great Lakes region in winter. The heat and moisture that is transferred from the Great Lakes aggregate into the lower atmosphere, and that spreads across a large region, allows eventually for the development of a meso-α-scale pressure perturbation and circulation.

Cases from the period 1980–90 were identified based on the existence of a surface trough or closed low over the Great Lakes region in the presence of cold air. Output from the Limited-Area Fine Mesh (LFM) model was used rather than performing numerous with-lake and no-lake numerical simulations to determine whether the feature was indeed the result of aggregate heating by the lakes. The LFM did not include the lakes in its simulations, so the 24-h forecast served as an optimal no-lakes simulation. Subtracting the initialization sea level pressure (SLP) field valid at the same time allowed for an assessment of the COLD events in terms of the SLP perturbation.

An average of 33 events per year with an average SLP perturbation of 3–4 hPa was found for the 10-yr period. The synoptic-scale conditions for weak events with SLP perturbations less than 3 hPa differed significantly from those for strong events with SLP perturbations greater than 9 hPa. The weak scenario was characterized by a weak trough over the Great Lakes with high static stability and weak cold advection below 500 hPa and weak vorticity advection at 500 hPa. The strong scenario was characterized by a nearly closed low over the Great Lakes with low static stability and strong cold advection below 500 hPa and strong positive vorticity advection at 500 hPa.

The current study is the first attempt to measure the frequency and intensity with which the Great Lakes collectively generate meso-α-scale disturbances in winter. The LFM-based technique provides a result that cannot likely be obtained without a herculean effort from a numerical modeling standpoint. Future numerical studies using the identified scenarios, however, will be extremely useful to better understand the sensitivities of COLD events to the large-scale conditions.

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Hui-Ya Chuang
and
Peter J. Sousounis

Abstract

A new idealized initialization technique has been developed for the Mesoscale Model version 5 modeling system. The technique allows the specification of baroclinic disturbances that feature vertical variations of the height, temperature, and wind fields in terms of phase lag, wavelength, and phase speed. The technique involves specifying a sounding profile at some reference point, generating the desired height fields using an analytic formulation, constructing the wind fields to be in geostrophic balance, and generating temperature fields using the hydrostatic relationship.

A distinct advantage of this technique over existing ones is that the boundary conditions are not restricted to being specified as periodic. The flexibility means that 1) users do not have to specify a domain whose size is equal to an integer number of wavelengths of the specified flow; 2) users can specify a flow that consists of different wavelengths at different heights, as is typically observed; and 3) any responses that are generated orographically or thermally in the domain and which leave the eastern boundary will not reenter the western boundary. This last item is particularly advantageous because it allows users to study the effects of a preconditioned environment on subsequent development of a featured disturbance rather than studying the repetitive effects of the same forcing mechanism on the same disturbance.

Examples of simulations using initial conditions that are generated with this technique are shown for 1) zonal flow and 2) continuous sinusoidal waves. Flat terrain was adopted for both examples. In example 1, the boundary layer parameterization scheme and surface fluxes were turned off for a simplified zonal flow situation to demonstrate the stability of this technique. During the simulations, the flow remained zonal, exactly as specified, even after 48 h. In example 2, a situation consisting of continuous sinusoidal waves moving across an array of four warm circular lakes was created to demonstrate the utility of the technique for examining how disturbances may be affected by the Great Lakes. Realistic-looking highs, lows, and fronts, along with individual and lake-aggregate enhancements developed by 48 h. Good stability and lack of distortion throughout the domain in both examples add credibility to the technique.

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Emily K. Grover
and
Peter J. Sousounis

Abstract

A synoptic climatology is presented of the precipitation mechanisms that affect the Great Lakes Basin. The focus is on fall because increasing precipitation in this season has contributed to record high lake levels since the 1960s and because the causes can be synoptically evaluated. Precipitation events were identified for the period 1935–95 from NOAA Daily Weather Maps. Precipitation days were classified as one of nine types. Trends in the precipitation classifications, 24-h precipitation totals, and the frequency and intensity of precipitation days and events were analyzed.

It was found that the precipitation increased 15% over the basin and 35% at Grand Rapids, Michigan, from 1935–65 to 1966–95. The increased precipitation was driven by an increase in the amount of precipitation per day (from low pressure systems and warm, stationary, and occluded fronts) and an increase in the frequency of precipitation days (from troughs and cold, warm, stationary, and occluded fronts). All classifications except for isolated convection contributed to the increase. Increases from warm, stationary, and occluded fronts contributed the most.

Analysis of precipitation mechanisms and large-scale circulation features for two 10-yr periods from 1950 to 1959 and from 1980 to 1989 revealed that higher precipitation amounts were associated with a more zonal flow pattern that existed over the United States during 1980–89. This pattern was accompanied by more baroclinicity and moisture over the Rockies, a stronger upper-troposphere subtropical jet, and stronger low-level flow from the Gulf of Mexico. These features allowed a greater number of southern systems with more moisture to influence the region. Specifically, the increased frequency of low pressure systems approaching from the south(west) and their associated more rapid deepening rates allowed more precipitation from warm, stationary, and occluded fronts. The similarities in the synoptic precipitation classifications and precipitation amounts between the two 10-yr periods and the two 30-yr periods examined suggest that more meridional flow was present for much of the 1935–65 period and that more zonal flow was present for much of the 1966–95 period.

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Peter J. Sousounis
and
Greg E. Mann

Abstract

It is known that lake-effect snowstorms in the Great Lakes region depend on the synoptic-scale flow conditions. These conditions are determined in part by the synoptic-scale features that traverse the area. Forecasting the development of these storms has improved dramatically in the last decade. A remaining complicating aspect, however, is that the heating and moistening from all the Great Lakes (e.g., the aggregate) affects the large-scale winds, temperature, moisture, and stability near the individual lakes, which in turn affect the characteristics of the lake-effect storms that develop.

The effects of the Great Lakes aggregate on lake-effect precipitation is examined for a particular case in November 1982 that was characterized by a cold air outbreak followed by the approach of a weak trough into the region. Existing model output from numerical simulations that 1) included all of the lakes and that 2) excluded all of the lakes is used in conjunction with output from two additional numerical simulations that were performed and that include 3) only Lake Michigan and 4) only Lakes Erie and Ontario.

The intercomparison of output from these simulations indicates that the lake aggregate enhanced lake-effect precipitation in northern lower Michigan and in southern Ontario but diminished lake-effect precipitation in regions south and east of Lakes Erie and Ontario. The effects were the result of combined changes in wind, temperature, moisture, and stability, which likely altered the morphology, intensity, locations, and orientations of the convective bands.

These results indicate that understanding more completely how and when lake aggregate-scale circulations develop can enhance 1–2-day lake-effect and regional-scale precipitation forecasts. More importantly, these results suggest that aggregate-scale circulations that develop from clusters of heat sources or sinks can impact significantly the local precipitation distribution adjacent to a particular heat source or sink.

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Hui-Ya Chuang
and
Peter J. Sousounis

Abstract

The effects of a group (aggregate) of relatively warm circular meso-β-scale lakes on different flow regimes were investigated by conducting a series of idealized numerical experiments. This investigation was motivated by the observed behavior of synoptic-scale cyclones moving through the Great Lakes region during winter. Three with-lake (WL) and three corresponding no-lake (NL) simulations were initialized with 1) zonal flow, 2) a solitary trough, and 3) continuous sinusoidal waves, respectively. The WL experiments were intercompared to examine the importance of a preexisting disturbance and preconditioning. The NL simulations were compared to the corresponding WL simulations to study the contributions of the lake aggregate. The simulation results suggest that the lake aggregate induced or enhanced warm fronts when there were preexisting disturbances. They also suggest that a perturbation mesoscale aggregate vortex was generated in each of the three different flow scenarios even though the lake aggregate alone could only generate a weak meso-α-scale trough.

To identify the physical processes that were altered by the lake aggregate to enhance cyclone development, surface pressure tendency diagnosis using the extended Zwack–Okossi (ZO) equation was applied to the simulation results. The results of the ZO surface pressure (PSFC) tendency diagnosis indicated that the preconditioning from the preceding ridge contributed to the further development of the lake-aggregate–enhanced cyclones. The results also indicated that the lake aggregate not only reduced the PSFC locally through surface sensible heating but also and, more importantly, contributed to large-scale surface pressure deepening by enhancing the surface warm front.

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