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K. K. Szeto
and
R. E. Stewart

Abstract

Frontal precipitation systems are simulated with a 2D cloud model including ice-phase microphysics. Despite the use of idealized frontogenetic forcing in the simulations, some observed characteristics of frontal zones and their associated cloud and precipitation fields are reproduced in the simulations.

The effects of melting snow on surface frontogenesis is investigated. It is found that the cooling effects of melting snow significantly accelerate surface frontogenesis in winter storm environments, especially when the melting layer is close to the surface. However, the steady-state surface frontal strength in the model is not sensitive to the melting effects.

Finescale thermal and kinematic perturbations inside the frontal zone near the melting level, quite similar to those recently reported in the literature, are evident in the model results. Analysis of the model results suggests that cooling from melting snow may induce these thermal and kinematic perturbations and may enhance baroclinicity, resulting in accelerating frontogenesis. These frontogenetic effects should be strongest when the melting layer is near the surface, thus explaining the often observed coincidence of surface fronts with the surface rain–snow boundary in winter storms.

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Kit K. Szeto
,
Charles A. Lin
, and
Ronald E. Stewart

Abstract

The melting of snow extracts latent heat of fusion from the environment. The basic response of the atmosphere to this cooling-by-melting mechanism is investigated by using a nonlinear two-dimensional numerical model. It is found that the resultant melting-induced circulations consist of a forced downdraft which spreads out laterally like a gravity current and transients which are gravity waves. The characteristics of these mesoscale thermally driven circulations are studied under idealistic atmospheric conditions. Model results show that the melting associated with realistic precipitation rates (up to 10 mm h−1) can induce horizontal wind perturbations of several meters per second and vertical motions of tens of centimeters per second. Since the gravity waves and the cold outflow current propagate away from the source, they can have significant dynamic effects on the environment remote from the precipitation region. Moreover, the melting-induced, near O°C isothermal layer in the atmosphere alters the local static stability. It is inferred that thew melting-induced effects may significantly influence the momentum and moisture transports in mesoscale precipitation systems.

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Kit K. Szeto
,
Ronald E. Stewart
, and
Charles A. Lin

Abstract

Various authors have proposed that the cooling associated with melting precipitation contributes significantly to the dynamics of mesoscale precipitation systems. In this study, we use the numerical model described in Part I of this paper to investigate the effects of the cooling-by-melting mechanism in three specific situations: rain/snow boundaries, the production of deep 0°C isothermal layers, and the trailing stratiform region associated with mesoscale convective systems.

It is found that melting in the vicinity of a rain/snow boundary produces a thermally indirect mesoscale vertical circulation that may be responsible for enhanced precipitation near a rain/snow boundary. Melting in the presence of warm air advection above the melting layer and cold advection at and below it are necessary for producing deep 0°C layers within realistic times. The dynamic effects of cooling associated with melting and evaporation in the stratiform region of a mature squall line system produce a mesoscale circulation qualitatively similar to that recently reported in the literature.

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K. K. Szeto
,
A. Tremblay
,
H. Guan
,
D. R. Hudak
,
R. E. Stewart
, and
Z. Cao

Abstract

A severe ice storm affected the east coast of Canada during the Canadian Atlantic Storms Project II. A hierarchy of cloud-resolving model simulations of this storm was performed with the objective of enhancing understanding of the cloud and mesoscale processes that affected the development of freezing rain events. The observed features of the system were reasonably well replicated in the high-resolution simulation. Diagnosis of the model results suggests that the change of surface characteristics from ocean to land when the surface warm front approaches Newfoundland disturbs the (quasi-) thermal wind balance near the frontal region. The cross-frontal circulation intensifies in response to the thermal wind imbalance, which in turn leads to the development of an extensive above-freezing inversion layer in the model storm. Depending on the depth of the subfreezing layer below the inversion, the melted snow may refreeze within the subfreezing layer to form ice pellets or they may refreeze at the surface to form freezing rain. Such evolution of surface precipitation types in the model storm was reasonably well simulated in the model. Model results also show that the horizontally differential cooling by melting near the nose of the above-freezing inversion layer enhances the local baroclinicity, which in turn induces perturbations on the cross-front flow. Depending on stability of the ambient flow, such local flow perturbations may trigger symmetric or convective overturning above the region and consequently enhance the local precipitation production via a positive feedback mechanism.

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John M. Hanesiak
,
Ronald E. Stewart
,
Kit K. Szeto
,
David R. Hudak
, and
Henry G. Leighton

Abstract

On 30 September 1994 an Arctic low pressure system passed over the southern Beaufort Sea area of northern Canada and research aircraft observations were made within and around the warm front of the storm. This study is unique in that the warm front contained subzero centigrade temperatures across the entire frontal region. The overall structure of the warm front and surrounding region was similar to midlatitude storms; however, the precipitation rates, liquid water content magnitudes, horizontal and vertical winds, vertical wind shear, turbulence, and thermal advection were very weak. In addition, a low-level jet and cloud bands were aligned parallel to the warm front, near-neutral stability occurred within and around the front, and conditional symmetric instability was likely occurring. A steep frontal region resulted from strong Coriolis influences that in turn limited the amount of cloud and precipitation ahead of the system. The precipitation efficiency of the storm was high (60%) but is believed to be highly dependent on the stage of development. The mesoscale frontogenetic forcing was primarily controlled by the tilting of isentropic surfaces with confluence/convergence being the secondary influence. Sublimation contributions may have been large in the earlier stages of storm development. Satellite and aircraft radiometers underestimated cloud top heights by as much as 4 km and this was mostly due to the near transparency of the lofted ice layer in the upper portion of the storm. Maximum surface solar radiation deficits ranged between 91 W m−2 and 187 W m−2 at two surface observing sites. This common type of cloud system must have a major impact on the water and energy cycles of northern Canada in the autumn and therefore must be well accounted for within climate models.

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