Abstract

Intense, small-scale divergent outflows known as microbursts are held responsible for a number of aircraft accidents. This paper describes the morphology of microburst outflows observed in Colorado. Outflows are categorized into morphological types based on analysis of observation by Doppler radars and a surface meteorological network. Outflow life cycle is discussed, and the vertical and horizontal structure is described. Basic characteristics of microburst outflows are summarized from statistics compiled using both single and multiple Doppler analyses.

The microburst outflows are classified into two types: individual microbursts and microburst lines. Examples of observations of each type are shown. Organization of microbursts into microburst lines results in much longer-lasting wind shear than exists with isolated microbursts. The greater lifetime of microburst lines, combined with the much larger area of divergence, can create a much greater potential for hazard to aircraft than is the case for individual microbursts.

Outflow structure was found to resemble many features of the laboratory wall jet. Vertical profiles of horizontal velocity follow curves similar to those of the wall jet. Radial profiles of horizontal velocity through the microburst center also agree with the velocities predicted from wall jet theory out to the velocity maximum. Beyond the velocity maximum, microburst outflow velocities decay more rapidly than wall jet velocities.

Studies of microburst symmetry, as measured across the maximum velocity differential, reveal that the minimum shear is, on the average, only about 60% of the maximum. Implications of outflow structure and symmetry for aviation safety are discussed.

Abstract

A three-dimensional mesoscale computer model is used to assess the importance of urban effects, relative to non-urban effects, on mesoscale boundary-layer vertical air motion and on the height of the boundary layer downwind of St. Louis, Missouri. Simulations are made for south, southwest, west and northwest winds, with urban land uses replaced by rural land uses, both with and without topography. Simulations including urban effects indicated mesoscale upward air motion downwind of the city for all wind directions, strongest for southwest winds and weakest for northwest winds. With urban effects excluded, much weaker upward motion was found downwind for south, southwest and west winds, and downward vertical velocities occurred in the downwind areas for northwest winds.

The results of this study imply that mesoscale boundary-layer upward air motion occurs downwind of St. Louis, primarily as a result of urban effects. Local geographic influences may tend to enhance or suppress this upward air motion, depending on wind direction. Thus, the interaction of urban effects with those resulting from geographic features is important. Comparison of results obtained with and without topography indicates that topography is the primary source of non-urban effects. These simulated effects on boundary layer vertical velocities are reflected in perturbations in the model-predicted boundary-layer height. Comparison of model results with Metropolitan Meteorological Experiment (METROMEX) radar first echo frequencies suggests that the model results are consistent with the hypothesis that cloud and precipitation anomalies are related to perturbations in boundary-layer dynamics caused by the urban heat island and surface roughness.

Abstract

Numerical simulations of lake-effect snowstorms over Lake Michigan show that orography enhances precipitation rates and mesoscale updrafts and strengthens the land breeze. The mild orographic changes east of Lake Michigan as modeled with an 8-km horizontal grid are not sufficient to overcome the dominance of the lake-land temperature difference for any of the cases simulated. However, significant local effects are observed in areas of more prominent topographic features. These local effects are strongly affected by wind direction.

Abstract

Numerical simulations are used to examine the influence of environmental parameters on the morphology of lake effect snowstorms over Lake Michigan. A series of model sensitivity studies are performed using the Colorado State University mesoscale model to examine the effects of lake–land temperature difference, surface roughness, atmospheric boundary layer stability, humidity, and wind speed and direction on the morphology of simulated storms.

Four morphological types of lake effect snowstorms have been identified: (i) Broad area coverage, which may become organized into wind parallel bands or cellular convection; (ii) shoreline bands with a line of convection roughly parallel to the lee shore and a well developed land breeze on the lee shore; (iii) midlake band with low-level convergence centered over the lake; and (iv) mesoscale vortices with a well-developed cyclonic flow pattern in the boundary layer.

The model is able to reproduce all four morphological types. Simulations varying environmental parameters independently define the thermodynamic and wind conditions for the occurrence of each morphological type. In particular, the limiting conditions of lake–land temperature difference, upwind wind speed stability, and humidity for development of a land breeze on the east side of Lake Michigan are defined for lake snow conditions. The effects of wind direction, surface roughness, and latent heat release are also described.

Abstract

This study employs in situ measurements to examine cloud conditions in which hydrometeors develop in mature Oklahoma convective clouds and to develop hypotheses as to how they formed. The measurements were made from penetrations on six days using a T-28 aircraft. Values of the maximum vertical velocity W in cells ranged from 5 to 35 m s⁻¹, and the liquid water content (LWC) up to 6 gm^minus;3;LWCs are usually less than adiabatic. Drops are found primarily in strong updrafts at T/>−8°c. Graupel are present in low concentrations in the strong updrafts and in moderate concentrations in the weak to intermediate updrafts. Planar and needle ice crystals and aggregates are present in copious concentrations in regions of low LWC and W. Strong evidence exists for production of secondary ice crystals (SICS) through a Hallett and Mossop type of mechanism involving cloud droplets >24μm in diameter.

Particle growth calculations are used in conjunction with the measurements to infer the processes of formation of drops, graupel and hail, and secondary ice crystals. Most drops of diameters <500μm found at temperatures below 0°C are inferred to form through coalescence growth and most of diameters >500μm through shedding from growing and/or melting graupel and hail. Embryos of hailstones are found to develop to 1 cm in diameter most rapidly from millimetric size drops produced from shedding and from aggregates of planar ice-crystals. Most growth of particles to 1 cm hailstones occurs in the regions of intermediate values of LWC (1-2 gm⁻³) and W (5–15 m s^minus;1) at temperatures higher than −20°C. In these regions, moderate concentrations of ice particles can develop over appreciable periods and depletion of the liquid water content due to collection by ice particles is minimal. The regions of high LWC and W are found to be the least conducive to SIC production. Initially, most SICs come from riming of aggregates in clouds which develop embedded within cloud layers and from frozen drops in clouds which develop in isolation. The SICs themselves are found to produce abundant SICs in regions of low LWC and W. Secondary ice crystal production is found to be more copious in embedded than in isolated clouds.

Abstract

A mesoscale model is used to simulate the airflow over Lake Michigan for the major lake-effect snowstorm of 10 December 1977. This storm was characterized by a land breeze circulation and a narrow shore-parallel radar reflectivity band. The model successfully simulated the major atmospheric circulation features including a mesoscale low pressure center and a land breeze front. The model also captured the general character of the observed precipitation pattern which was typified by a narrow band of heavy precipitation along the eastern shore of Lake Michigan.

Further simulations were made to examine the effects of latent heat release, lake surface temperature distribution and model grid resolution upon the simulation. Latent heat release was found to have an important effect in strengthening convection. However, the basic land-breeze circulation was found to develop for the simulated conditions even without latent beating. For a given mean lake-land temperature difference, details of the lake surface temperature distribution were found to have a small effect. Simulations with varying model grid resolution suggest that a horizontal grid scale ≳ 20 km is insufficient to resolve the observed precipitation and airflow patterns for this storm.

Abstract

Premodification of the atmosphere by upwind lakes is known to influence lake-effect snowstorm intensity and locations over downwind lakes. This study highlights perhaps the most visible manifestation of the link between convection over two or more of the Great Lakes lake-to-lake (L2L) cloud bands. Emphasis is placed on L2L cloud bands observed in high-resolution satellite imagery on 2 December 2003. These L2L cloud bands developed over Lake Superior and were modified as they passed over Lakes Michigan and Erie and intervening land areas. This event is put into a longer-term context through documentation of the frequency with which lake-effect and, particularly, L2L cloud bands occurred over a 5-yr time period over different areas of the Great Lakes region.

Abstract

The first detailed observations of the interaction of a synoptic cyclone with a lake-effect convective boundary layer (CBL) were obtained on 5 December 1997 during the Lake-Induced Convection Experiment. Lake-effect precipitation and CBL growth rates were enhanced by natural seeding by snow from higher-level clouds and the modified thermodynamic structure of the air over Lake Michigan due to the cyclone. In situ aircraft observations, project and operational rawinsondes, airborne radar, and operational Weather Surveillance Radar-1988 Doppler data were utilized to document the CBL and precipitation structure for comparison with past nonenhanced lake-effect events. Despite modest surface heat fluxes of 100–200 W m⁻², cross-lake CBL growth was greatly accelerated as the convection merged with an overlying reduced-stability layer. Over midlake areas, CBL growth rates averaged more than twice those previously reported for lake-effect and oceanic cold-air outbreak situations. Regions of the lake-effect CBL cloud deck were seeded by precipitation from higher-level clouds over the upwind (western) portions of Lake Michigan before the CBL merged with the overlying reduced-stability layer. In situ aircraft observations suggest that in seeded regions, the CBL was deeper than in nonseeded regions. In addition, average water-equivalent precipitation rates for all of the passes with seeded regions were more than an order of magnitude greater in seeded regions than nonseeded regions because of higher concentration of snow particles of all sizes. A maximum snowfall rate of 4.28 mm day⁻¹ was calculated using aircraft particle observations in seeded regions, comparable to snowfall rates previously reported for lake-effect events, often with much larger surface heat fluxes, but not interacting with synoptic cyclones.

Abstract

Lake-effect snowstorms generally develop within convective boundary layers, which are induced when cold air flows over relatively warm lakes in fall and winter. Mesoscale circulations within the boundary layers largely control which communities near the downwind shores of the lakes receive the most intense snow. The lack of quantitative observations over the lakes during lake-effect storms limits the ability to fully understand and predict these mesoscale circulations. This study provides the first observations of the concurrent spatial and temporal evolution of the thermodynamic and microphysical boundary layer structure and mesoscale convective patterns across Lake Michigan during an intense lake-effect event. Observations analyzed in this study were taken during the Lake-Induced Convection Experiment (Lake-ICE).

Aircraft and sounding observations indicate that the lake-effect snows of 13 January 1998 developed within a convective boundary layer that grew rapidly across Lake Michigan. Boundary layer clouds developed within 15 km and snow developed within 30 km of the upwind (western) shoreline. Near the downwind shore, cloud cover was extensive and snow nearly filled the boundary layer. Extensive sea smoke in the surface layer, with disorganized (or cellular) and linear features, was observed visually across the entire lake. Over portions of northern Lake Michigan, where airborne dual-Doppler radar observations were obtained, the mesoscale circulation structure remained disorganized (random or cellular) across the lake. Given observed shear and stability conditions in this region, this structure is consistent with past theoretical and numerical modeling results. To the south, where surface winds were slightly stronger and lake–air temperature differences were less, wind-parallel bands indicative of rolls were often present.

The horizontal scale of the observed mesoscale convective structures grew across Lake Michigan, in agreement with most previous studies, but less rapidly than the increase of the boundary layer depth. The decreasing ratio of convective horizontal size to boundary layer depth (aspect ratio) is contrary to many recent studies that found a positive correlation between boundary layer depth and aspect ratio.

Abstract

A numerical cloud model has been used to simulate convective storm development on 17 July 1987 in northeast Colorado. The study involves the simulation of convergence along atmospheric boundaries and the subsequent development of convection. The model was initialized using observed conditions for this case day from the Convection Initiation and Downburst Experiment (CINDE).

A two-dimensional version of the Clark NCAR nested grid model is employed. Results indicate that convection in boundary line collision cases can be successfully simulated by using actual observed atmospheric data. Gradual deepening of the moisture layer in the convergence zone was shown to destabilize the local atmosphere leading to the initiation of deep convection on this day. The modeled storm approximated the depth and intensity of the observed storms and displayed many of the features of the actual event.

Sensitivity studies revealed that the timing and intensity of convection along boundaries is greatly affected by alterations in cross-line values of boundary-layer moisture or convergence and by variations in the vertical wind-shear profile within and above the boundary layer. Increasing the low-level moisture created a much stronger and taller modeled storm that developed much more rapidly. Variations in boundary-layer convergence were shown to affect the timing and character of the modeled storm. Horizontal vorticity in the boundary layer, associated with low-level vertical wind shear, was important for the production of deep convection. When the two air masses collided, deeper lifting was obtained if the opposing vorticity of the moving boundaries was balanced than if one of the vorticity sources was significantly stronger than the other. A threshold value of shear above the boundary layer was shown to inhibit the convective development of the modeled storm. These sensitivity studies emphasize the importance of considering the mesoscale variability of these key parameters in nowcasting convection.