Search Results

You are looking at 1 - 10 of 23 items for

  • Author or Editor: R. E. Stewart x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
R. List, N. R. Donaldson, and R. E. Stewart

Abstract

The evolution of raindrop spectra by collisional breakup is examined analytically and modelled in box and 1-dimensional shaft models, using the parameterization of Low and List. The significant analytical result shows that equilibrium drop size distributions occur in families that are multiples of one another:
fDRRD
where D is the drop diameter, R is the rainfall rate, f(D,R) the number density distribution in terms of D and R and ψ is a shape function.

For the Low-List breakup scheme the shapes are trimodal, with peaks in the number distributions at diameters of 264, 790, and 1760 μm. Similar structures were found by Valdez and Young, and Brown for box models. These peaks are expected to exist wherever spectra approach equilibrium, independently of the rainfall rate. In this paper the development of these peaks from non-equilibrium spectra is examined, together with the effect of periodically varying rainfall rates.

In box and one-dimensional shaft models, nonequilibrium spectra quickly develop features similar to those at equilibrium, but times and/or heights to reach true equilibrium are in excess of 30 minutes, or 3 km for all but the very heaviest rainfall rates. The peaks, however, should be identifiable in a matter of minutes, thus encouraging field verification under favorable conditions. In the absence of evaporation, spectral evolution below a cloud is dominated by the large drops, which produce the accompanying small drops by breakup.

Evaporation, while basically affecting the smallest drops, is quickly spread over the whole spectrum by the collision process and reduces the total liquid water content The drop spectrum shape however, remains unchanged.

Full access
R. E. Stewart, C. A. Lin, and S. R. Macpherson

Abstract

On 22 February 1986 Nova Scotia experienced heavy precipitation in the form of snow, freezing precipitation, and rain from a storm having a central pressure no lower than 99.3 kPa. Using observations obtained during the Canadian Atlantic Storms Program (CASP) field project, the mesoscale structure of this storm was investigated. Throughout much of the storm, the lowest 1–3 km of the atmosphere over the coastline was near 0°C as a result of the diabatic process of melting and refreezing. Convergent flow aloft and the trajectories of particles undergoing terminal velocity changes contributed to enhanced precipitation near the coastline that was sometimes detected by radar as a precipitation band straddling the coastline. A mesoscale circulation, driven by melting and forced to remain linked to the coastline between the warm ocean and the cold land, is consistent with the observations.

Full access
R. E. Stewart, R. W. Crawford, N. R. Donaldson, T. B. Low, and B. E. Sheppard

Abstract

Precipitation and environmental conditions occurring during accretion in Canadian east coast winter storms are described and investigated. Accretion is generally associated with snow, freezing rain, and ice pellets within saturated conditions. Precipitation types are sometimes invariant but usually evolve during individual accretion events. Accretion events are also generally associated with moderate wind speeds (average of 7.5 m s−1) and warm temperatures (between −1° and 0°C are most common). Remote sensing of particle shapes and terminal velocities are capable of identifying some of the features of these precipitation types. Model calculations indicate that a detailed understanding of precipitation characteristics, such as the nature of wet snow, is needed to accurately simulate accretion.

Full access
G. B. Raga, R. E. Stewart, and J. W. Strapp

Abstract

The present study discusses the meso- and microscale structures of Precipitation regions within a midlatitude winter storm over the North Atlantic, observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic. Two wide regions of precipitation separated by a narrow band were observed at low levels by airborne radar. These regions were aligned parallel to the cold front and were sampled by aircraft at three different levels. The calculated mesoscale frontogenetical forcing is dominated at low levels by confluence and at mid-levels by the tilting term. The absolute magnitudes are smaller than those reported by Shapiro, and Bond and Fleagle, and are consistent with the broader and less intense front in this study. The frontogenetical forcing due to melting of ice crystals was estimated from observations of precipitation particles. The analysis indicates that the cooling due to melting of ice particles is not a dominant frontogenetical forcing at the observed stage in storm evolution. Precipitation rates larger than those observed (by a factor of 3) behind the cold front are needed before the thermal impact of melting could contribute to frontogenesis as much as confluence at the same level. The region of precipitation ahead of the cold front appears to be linked to convective instability observed in the warm sector. The observed precipitation region to the west of the cold front is consistent with the trajectories of failing particles carried by the relative wind flowing toward the back of the system. The decrease in precipitation rate observed right behind the front can be interpreted as ice particles failing through a deep region in which temperatures are close to 0°C. The presence of such a region leads to a nonuniform precipitation distribution, with areas that would appear as precipitation bands in radar images, and others in which precipitation is reduced.

Full access
Graciela B. Raga, Ronald E. Stewart, and Norman R. Donaldson

Abstract

The microphysical characteristics of a precipitation type transition region within a midlatitude winter storm are discussed in relation to the background thermodynamic and kinematic fields. A deep region in which the temperature was close to 0°C (the transition region) was observed along the Atlantic coastline of Nova Scotia. This transition region was approximately 30 km wide and about 2 km deep. At 80 kPa, a large horizontal temperature gradient marked the boundary between the transition region and the colder air. The observed thermal structure is linked to diabatic processes, and in particular, to the freezing of small droplets, the refreezing of semi-melted particles and the melting of precipitation. Large, partially melted aggregates were located just downwind of the deep transition region. Particle trajectories near the transition region are very sensitive to the background temperature and wind fields and may lead to regions of reduced and enhanced concentrations at the surface and aloft. A conceptual model of the flow fields suggests that this case resembles warm and cold conveyor belts similar to those found in synoptic systems, but on a smaller scale. The transition region in this case is located at the boundary between the warm and cold conveyor belts.

Full access
R. Paul Lawson, Ronald E. Stewart, and Leigh J. Angus

Abstract

The Canadian Atlantic Storms Program (CASP II) field experiment was conducted near St. John’s, Newfoundland, Canada, during January–March 1992, and it focused on the nature of winter storms. Analyses of CASP II aircraft, surface, satellite, and radar observations collected during an intensive study of the origin and development of 9 mm h−1 precipitation containing 4–5-cm diameter snowflakes are compared in this article with results of the MM5 (mesoscale) and Mitchell (microphysical) models. MM5 simulations of the thermal, kinematic, and bulk microphysical fields were in good agreement with the observations; this comparison provided the basis for extending the spatial and temporal scales of the aircraft observations to a larger-scale domain using the model results. The Mitchell analytical–numerical model was used to improve the understanding of the microphysical processes that led to the development of the very large snowflakes. A synthesis of results using the different techniques leads to the conclusion that the snowflakes originated as 3–5-mm dendritic crystals in an area of weak convective instability at 5 km and were transported downwind in a strongly sheared airflow. The dendrites aggregated, fell into an existing snowzone (supported in some regions by vertical motion with velocities ranging from 0.2–0.6 m s−1), and continued to descend along a deep, downward sloping layer with temperatures near 0°C. Rapid aggregation occurred in the near 0°C region in particular and without appreciable particle breakup. An exponential fit to the particle size distribution in the region of very large snowflakes had a slope parameter on the order of 100 m−1.

Full access
B. E. Sheppard, R. E. Stewart, G. A. Isaac, and T. B. Low

Abstract

Atmospheric temperature profiles and integrated water vapor and liquid are retrieved from ground-based microwave radiometric measurements using both nonlinear optimal estimation (NLOE) and statistical inversion (SI) methods. The results obtained from both methods are compared with collocated radiosonde observations during the Canadian Atlantic Storms Program field project in 1986. In general, the NLOE was superior to the SI method when clouds with high liquid water contents or when precipitation was present. Under these conditions, temperature profiles derived using NLOE had smaller root-mean-square differences from radiosonde observations than those retrieved using SI. Also, the overestimation of integrated vapor retrieved using the SI method was eliminated using the NLOE method. The radiometric observations were used in two case studies of winter cyclonic storms striking Atlantic Canada.

Full access
R. E. Stewart, R. W. Shaw, and G. A. Isaac

The field phase of the Canadian Atlantic Storms Program (CASP) was conducted from 15 January to 15 March 1986. The principal objective of the meteorological component of the program was to begin the process of improving the understanding and prediction of mesoscale features within East Coast storms as well as the storms themselves. The project area, instrumentation platforms used, real-time forecasts, and the linkage of the program to the American Genesis of Atlantic Lows Experiment (GALE) are discussed. Sixteen storms were sampled during the field phase. A number of mesoscale features such as fronts, precipitation bands, heavy snow, and freezing precipitation were sampled. These features and the storms themselves will be studied over the next several years. It is anticipated that scientific progress in understanding the nature of these winter systems and experience gained with new forecasting tools will lead to improved weather forecasts.

Full access
K. K. Szeto, H. Tran, M. D. MacKay, R. Crawford, and R. E. Stewart

Abstract

This study represents the first attempt at developing a comprehensive climatology of atmospheric and surface water and energy budgets for the Mackenzie River basin (MRB). Different observed, remotely sensed, (re)analyzed, and modeled datasets were used to obtain independent estimates of the budgets. In particular, assimilated datasets, including the National Centers for Environmental Prediction Global Reanalysis 2 (NCEP-R2), the global 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), the NCEP North American Regional Reanalysis (NARR), and the Canadian Meteorological Centre (CMC) operational regional analysis as well as results from the Canadian Regional Climate Model (CRCM) simulations, are used in the study. Apart from the development of state-of-the-art budget estimates for the MRB, the relative merits of current models, data assimilation systems, and global blended datasets in representing aspects of the water and energy cycle of this northern and data-sparse region were also assessed. In addition, the levels of uncertainty in assessing the budgets as well as their sources are discussed. The regional water budget for the MRB is closed within 10% of the observed runoff by using the moisture flux convergence from ERA-40, NARR, CMC, or CRCM. While these are noted improvements over previous water closure assessments for the region, magnitudes of the residuals in balancing the budgets are often comparable to the budget terms themselves in all the model and analysis datasets, and the spreads of budget estimates from the different datasets are also typically large, suggesting that substantial improvements to the models and observations are needed before the assessments of water and energy budgets for this northern region can be vastly improved.

Full access