Sills and King ELBOW 97 paper from SLS 19

1. INTRODUCTION

The Effects of Lake Breezes On Weather (ELBOW) project investigates the role of lake breezes in the occurrence of severe weather in southern Ontario, Canada, using data collected during the summer of 1997. One goal of the project is to examine the nature of lake-induced cloud lines that tend to occur in association with moderate surface winds, especially those from the southwest that blow along parallel Lake Huron and Lake Erie shoreline segments. These lines have been implicated in the initiation of severe weather (King, 1996). However, their nature has been difficult to characterize due to sparsity of observational data in the region, ambiguous signatures in satellite and surface data, and suspected frictional and topographical effects. For this study, GOES-8 visible satellite imagery, ELBOW field observations, and mesoscale numerical modelling results are used to elucidate the origins and evolutions of lake-induced cloud lines that preceded severe weather on July 14, 1997.

2. OBSERVATIONAL DATA AND ANALYSIS

On July 14, 1997, a moderate southwesterly surface wind was accompanied by inland temperatures up to 34°C and unusually high dew point temperatures up to 27°C. Lake surface temperatures ranged from a minimum of 16°C on Lake Huron to a maximum of 24°C on western Lake Erie. Though synoptic-scale dynamics were favourable for the formation of strong thunderstorms in areas to the southwest and northeast of the Great Lakes region, dynamics over the study area were quite weak.

Two lines of cumulus clouds developed between Lake Erie and Lake Huron aligned with the southwesterly surface wind after 1300 LST (Local Standard Time). The lines appeared to originate from headlands along the north shore of Lake Erie. These cloud lines remained nearly stationary with respect to the shore while gradually extending farther inland to the northeast. A line of cumulus clouds extending inland from the south shore of Lake Huron toward the northeast became apparent at 1500 LST. Skies over nearby lake areas were cloudless. Though observed surface winds showed no strong deviations from southwesterly flow near the shorelines, differences in air temperatures over land and lake surfaces exceeded 10°C at times.

Figure 1 shows the satellite image valid at 1545 LST. The positions of cloud lines inferred from animated satellite images are indicated using dashed lines. Two distinct lines are still evident north of Lake Erie. Enhanced convection is apparent where Lake Huron and Lake Erie lines converge. Showers and thunderstorms were also forming at lines northwest of Lake St. Clair and near western Lake Huron. Convective debris from thunderstorms initiated at a cloud line along Lake Erie's south shore is evident over Lake Erie. Convective debris from morning showers and thunderstorms is also visible east of Lake Huron (outflow boundaries from these storms did not appear to influence afternoon convection). The ELBOW base station at Exeter (labelled 'A') had a temperature of 33°C and a dew point of 25°C at 1545 LST. The temperature at 'B' was 32°C while the temperature and dew point were 31°C and 26°C at 'C'. Winds at these stations suggest mesoscale surface convergence. A radiosonde launched from the Exeter base station near this time gave a surface-based CAPE exceeding 6000 J kg^-1, storm motion from 300° at 7 m s^-1, and a 0-3 km storm-relative helicity value near 160 m² s^-2. Unusually high dew points were confined to the near-surface layer of the sounding and were likely the result of enhanced evapotranspiration following morning showers and thunderstorms.

After 1545 LST, a few thunderstorm cells rapidly developed at the apparent point of merger of Lake Huron and Lake Erie cloud lines. These cells evolved into a quasi-stationary multicell cluster that appeared to be anchored to the point of merger. This storm produced approximately 200 mm of rain in five hours at locations about 50 km east of Exeter. Localized flooding, wind gusts to 115 km h^-1 and hail were reported. Storms initiated at convergence lines along the shores of eastern Michigan intensified and moved east into southwestern Ontario after 1900 LST. King and Sills (1998) provide additional details regarding the ELBOW field campaign and analysis of events on this day.

3. THE MESOSCALE NUMERICAL MODEL

To investigate the three-dimensional structure and evolution of these cloud / convergence lines, we used a fully compressible, non-hydrostatic, mesoscale model developed by the University of Québec at Montréal and Environment Canada. This model, called the Mesoscale Compressible Community (MC2) Model (Benoit et al., 1997), is linked to a comprehensive and well-tested physics package. An attractive feature of the model is its self-nesting capability.

To obtain results on a fine grid mesh, the self-nesting strategy shown in Table 1 was used. Initial and boundary conditions for the coarse run were provided by analyses at six hour intervals from an Environment Canada global model. For each run, we used 28 vertical levels with 11 levels in the lowest 1.5 km and a lid near 24 km. Initial geophysical fields for the various model runs were based on climate data if case-specific data could not be found. High resolution topography, land-sea mask, surface roughness, and lake surface temperatures were obtained for the 5 km grid run. Since no convective parameterization scheme was available for a simulation with 5 km grid spacing, the focus of the modelling experiment was on identifying mesoscale structures in the pre-storm environment rather than the dynamics associated with storm initiation.

TABLE 1

Model Run

Grid Size (km)

Time Step

(min)

Coverage

Area

coarse

100

Canada/US

medium

Great Lakes

fine

SW Ontario

4. MODEL RESULTS AND ANALYSIS

The fine-scale (5km) model run for July 14,1997, was initialized at 0700 LST and was carried out to 1900 LST. The development of convergence lines was studied using cross-sections and animations of various fields such as the horizontal and vertical wind fields. Weak lake breezes began to form by 0800 LST but were limited to about 250 m in depth. As the circulations increased in strength and depth, they were shifted downwind by the southwesterly flow causing lake breeze fronts to stretch toward the northeast. By 1130 LST, lake breeze circulations had grown to about 750 m in depth and lake breeze fronts along the shores of eastern Michigan began to link together. Lake breeze circulations were near their maximum depth at 1600 LST with updraft regions reaching up to 2000 m in height.

Figure 2 shows positive vertical motion at 625 m valid at 1600 LST with the surface wind field superimposed. Several convergence lines are apparent. The strongest is the line associated with the linked lake breeze fronts along the eastern shores of Michigan from western Lake Erie to western Lake Huron. Other strong convergence lines are associated with lake breeze fronts along the north and south shores of Lake Erie with the convergence line north of Lake Erie extending inland to the northeast. A much weaker convergence line associated with a lake breeze front south of Lake Huron also extends inland to the northeast. Another weak line is located over inland southeastern Michigan northwest of Lake St. Clair.

The general pattern of lines is similar to that indicated in the Figure 1 though the observed lines north of Lake Erie appear to be located farther inland. A cross-section through both the Lake Huron and Lake Erie convergence lines is shown in Figure 3 valid at 1600 LST. The Lake Huron lake breeze circulation has its western front over the western side of the lake and its descending branch nearly over the southern shore. Its southern front is much weaker and is located near the southern shore. The Lake Erie front is located well inland. Air temperatures over land exceed 32°C while much cooler temperatures between 22°C and 24°C are evident over the lakes. A nearly dry adiabatic lapse rate near 9°C km^-1 exists in the lowest kilometre over land while strong temperature inversions are evident over the lakes.

Based on these and other cross-sections, and animations of the development of these lines, it is believed that most of the cloud lines indicated in Figure 1 are indeed lake breeze fronts though the front south of Lake Erie is likely enhanced by the steep topography there and a topographic feature is likely the cause of the convergence line over inland southeastern Michigan. Clearly, lake breeze circulations and their associated fronts are highly perturbed by the large-scale wind field.

Weak, modelled convergence lines also appear along the eastern shore of Lake Huron and the northern shore of central Lake Erie (Figure 2). Animations show that these lines are nearly stationary over the shorelines. We believe that they are caused by frictional drag as fast-moving air over water encounters the rough and quickly rising terrain. Potential temperature profiles over these areas (not shown) give boundary layer heights of less than 100 m. Thus, clouds would not be expected to form with these convergence lines and are not apparent at those locations in the satellite imagery (Figure 1).

Finally, Figure 4 summarizes the suggested causes of convergence and cloud lines at 1545 LST based on satellite and surface observations and the MC2 model output.

5. CONCLUSIONS

Based on GOES-8 visible satellite imagery, observations collected during the ELBOW field campaign, and simulations with the MC2 mesoscale model, we find that:

1) the pattern of convergence lines that developed on July 14, 1997, was a complex combination of highly-perturbed lake breeze fronts, topographical effects and frictional effects with observed cloud lines being associated mainly with lake breeze fronts, and

2) thunderstorms were initiated along many of these convergence lines including a severe thunderstorm that appeared to be initiated at and anchored to the merger point of Lake Huron and Lake Erie lake breeze fronts.