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Patrick King

Abstract

Rural population densities and tornado incidence (confirmed and probable categories) are compared for noncoastal regions in the southwestern Ontario peninsula. No apparent relationship is found between rural population density and F0–F4 categories as indicated by a linear correlation coefficient of −0.11. The statistical breakdown for F0–F2 groupings and F3 and F4 groupings of confirmed and probable categories, respectively, when linearly regressed against rural population densities produced correlation coefficients of −0.22 and +0.33, respectively;neither of which is statistically significant (5% confidence level). It is concluded that no apparent relationship exists between rural population density and tornado incidence for the confirmed and probable categories. However, regression analysis of rural population density versus F0–F2 events classified as possible tornadoes produced a correlation coefficient of +0.47 (significant at the 5% confidence level but not the 1% level). Since the possible category may be dominated by nontornadic events, a relationship between population density and severe nontornadic weather is possible. Some evidence is presented that lake effects may extend far enough inland to cause some of the apparent anomalies noted in the spatial distribution of tornadoes in southwestern Ontario. Other factors are noted that may account, in part, for the results obtained.

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Y. Helen Yang and Patrick King

Abstract

The potential for using radar echo reflectivity to forecast cloud-to-ground (CG) lightning initiation in the 0–1-h time frame was investigated in southern Ontario, Canada. The main purpose of this investigation was to determine a reflectivity threshold at an isothermal altitude and a threshold for echo tops that best predict CG lightning initiation. The study examined lightning, radar, and upper-air sounding data for only airmass-type convection during the summer of 2008. The best predictor of the onset of CG lightning was found to be a 40-dBZ reflectivity level detected at an altitude with an environmental temperature of −10°C, with an average lead time of 17 min. Echo tops reaching or exceeding 7 km were a necessary condition prior to or at the time of the first CG lightning occurrence. Also, certain differences were observed depending on the polarity of the initial lightning flashes. Positive lightning flashes, when compared to negative ones, tended to deliver stronger electric currents and to be farther away from the locations of highest reflectivity on maximum reflectivity (MAXR) radar products. Lead times were observed to be shorter for positive lightning, which might suggest that positive-lightning-producing storm clouds became strongly electrified faster than their negative counterparts. Findings indicate the potential to develop a lightning nowcast algorithm suitable for Canadian forecast operational use.

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Ronald E. Stewart and Patrick King

Abstract

Rain–snow boundaries in two southern Ontario storms are examined. Radar and satellite information were used to illustrate the nature and extent of the associated precipitation and cloud regions. The deepest radar echoes and clouds occurred close to the boundary. Surface temperature and pressure were related to the boundary; some of the changes in these parameters were shown to be attributable to melting snowflakes. These radar, satellite, and surface observations are consistent with a mesoscale circulation driven by melting snow.

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Ronald E. Stewart and Patrick King

Abstract

The mesoscale storm structure and the evolution of precipitation type are examined during freezing precipitation episodes over southern Ontario. Precipitation bands linked to both warm and cold fronts were mainly responsible for the precipitation during these episodes. One feature detected by radar and related to freezing rain and/or ice pellets in most cases was the initial radar overhang. Observations of mixed precipitation types, including snow and ice pellets with freezing rain, are partially a consequence of size-dependent differences in melting and refreezing within an upper level invention and a lower level subfreezing region, respectively. Inadequate time for refreezing in the lower subfreezing region may, however, lead to particles at the ground being composed of a mixture of water and ice. Prediction techniques for this type of severe weather need to account for its mesoscale nature and for the actual types of precipitation involved.

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Mariusz Pagowski, Ismail Gultepe, and Patrick King

Abstract

In this study, a dense fog episode that occurred near Windsor, Ontario, Canada, on 3 September 1999 is investigated. The fog patch, with a spatial scale of several kilometers, reduced visibility on a major highway to a few meters and led to a series of collisions and loss of life. Satellite imagery and surface observations are used to analyze the physics of the event, and several hypotheses on the origin of the fog are presented. A series of simulations of the event with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) suggest that the fog formed because of convergence of land breezes developing along the shores of a lake and subsequent advection of moisture over the site of the accident. Tests indicate that the small scale of the modeled event contributes to sensitivity of the results to a broad range of factors. Sensitivity to the initial and boundary conditions, including initial soil moisture content and parameterization of turbulence, is discussed.

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Patrick King, Tsoi-Ching Yip, and J. David Steenbergen

Abstract

RAINSAT uses under data to calibrate GOES visible and infra data in terms of probability of rain. It produces probability of rain maps and 3 h forecast probability of rain maps by extrapolation.

An evaluation is made of RAINSAT probability of rain analyses and forecasts for the year 1985, with emphasis on the summer months, using both radar data and raingauge data for verification. In the daytime RAINSAT has skill in separating cloudy areas with near zero probability of rain from cloudy areas with a significant probability of rain. There is some skill in splitting the latter category into different probability levels. Using infrared data only. during day or night, results in a significant drop in skill. Forecasts show some skill out to 6 h.

Season-to-season and within-season comparisons of monthly probability of rain relationships (PoRRs) derived from radar are made. Within-season variability is small, especially in summer and winter. There are large day-to-day variations in the occurrence of rain as a function of visible and infrared values.

Regional variations in PoRRs are assessed in two ways: (1) Regional analyses based on a remote radar PoRR are verified against surface data. (2) Analyses for each region trained on local surface data are compared with those trained on the remote radar. Both approaches support the use of radar data to train the system in regions remote from the radar.

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Patrick W. S. King, William D. Hogg, and Philip A. Arkin

Abstract

Data from the first Algorithm Intercomparison Project(AIP/1) collected over Japan and surrounding waters in June, July, and August 1989 are used in this study to assess the importance of visible data in satellite rain estimation techniques. The purpose of the project was to compare different methods for estimating rainfall using satellite measurements. Radar and surface gauge data provided the validation set.

RAINSAT, an estimation technique using both visible (VIS) and infrared (IR) data, achieved the highest correlation with the validation data. In this paper rainfall estimates from RAINSAT (VIS+IR) am compared with two IR-only techniques to deduce the effectiveness of VIS data. Some estimates are also made using a VIS-only technique. Comparisons am made on both a spatial and diurnal basis.

Cloud climatologies for a subset of the AIP/1 data and the southern Ontario data on which RAINSAT was trained showed a marked similarity. It is found that the total volume of rain as a function of albedo is very similar for both Japanese and Ontario data.

The VIS data generally produced higher correlations with the validation data than did the IR data. This was especially the case when rain fell from warm, orogaphically induced rainfall. When rain fell from cold bright clouds. especially over the ocean, the correlations of the two types of data with the validation data were similar.

It is also shown that normalization of VIS data by the cosine of solar zenith data was inadequate to remove diurnal variations in apparent brightness.

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Gerald G. Mace, Yuying Zhang, Steven Platnick, Michael D. King, Patrick Minnis, and Ping Yang

Abstract

The Moderate Resolution Imaging Spectroradiometer (MODIS) on board the NASA Terra satellite has been collecting global data since March 2000 and the one on the Aqua satellite since June 2002. In this paper, cirrus cloud properties derived from ground-based remote sensing data are compared with similar cloud properties derived from MODIS data on Terra. To improve the space–time correlation between the satellite and ground-based observations, data from a wind profiler are used to define the cloud advective streamline along which the comparisons are made. In this paper, approximately two dozen cases of cirrus are examined and a statistical approach to the comparison that relaxes the requirement that clouds occur over the ground-based instruments during the overpass instant is explored. The statistical comparison includes 168 cloudy MODIS overpasses of the Southern Great Plains (SGP) region and approximately 300 h of ground-based cirrus observations. The physical and radiative properties of cloud layers are derived from MODIS data separately by the MODIS Atmospheres Team and the Clouds and the Earth’s Radiant Energy System (CERES) Science Team using multiwavelength reflected solar and emitted thermal radiation measurements. Using two ground-based cloud property retrieval algorithms and the two MODIS algorithms, a positive correlation in the effective particle size, the optical thickness, the ice water path, and the cloud-top pressure between the various methods is shown, although sometimes there are significant biases. Classifying the clouds by optical thickness, it is demonstrated that the regionally averaged cloud properties derived from MODIS are similar to those diagnosed from the ground. Because of a conservative approach toward identifying thin cirrus pixels over this region, the area-averaged cloud properties derived from the MODIS Atmospheres MOD06 product tend to be biased slightly toward the optically thicker pixels. This bias tendency has implications for model validation and parameterization development applied to thin cirrus retrieved over SGP-like land surfaces. A persistent bias is also found in the derived cloud tops of thin cirrus with both satellite algorithms reporting cloud top several hundred meters less than that reported by the cloud radar. Overall, however, it is concluded that the MODIS retrieval algorithms characterize with reasonable accuracy the properties of thin cirrus over this region.

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Hua Lu, Lesley J. Gray, Patrick Martineau, John C. King, and Thomas J. Bracegirdle

Abstract

A new regime index is constructed to capture the seasonal development of the stratospheric polar vortex in the northern winter, based on the standard deviation of Ertel’s potential vorticity in the upper stratosphere in November–December. The narrow-jet flow regime is characterized by a polar vortex that is more confined to high latitudes in the early winter upper stratosphere. This upper-level vortex configuration is more susceptible to the disturbances of upward propagating planetary-scale Rossby waves; the stratospheric polar vortex thus weakens earlier and is vertically shallower. The wide-jet flow regime is characterized by a broader-than-average polar vortex that extends further into the subtropics in the early winter upper stratosphere. The polar night jet then gradually strengthens, moves poleward, and penetrates deep into the lowermost stratosphere, with a sharply defined polar vortex edge due to more frequent Rossby wave breaking. Composite difference analyses show that the wide- and narrow-jet regimes, defined in the uppermost stratosphere in November–December, lead to different circulation anomalies of the lower stratosphere and the troposphere in January–February, offering the potential for improved predictability of subseasonal to seasonal forecasts up to two months ahead. The lower-tropospheric responses in January–February are zonally asymmetric. The narrow-jet regime projects most strongly over the North Atlantic, with an equatorward-shifted and/or broader midlatitude westerly jet. The wide-jet-regime response is characterized by a weakened midlatitude westerly jet over the North Pacific. The two flow regimes also differ in their impacts on the storm track over western Europe and the east coast of North America, which may have implications for extreme weather events in those regions.

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Patrick W. S. King, Michael J. Leduc, David M. L. Sills, Norman R. Donaldson, David R. Hudak, Paul Joe, and Brian P. Murphy

Abstract

Geostationary Operational Environmental Satellite (GOES) imagery is used to demonstrate the development of lake-breeze boundaries in southern Ontario under different synoptic conditions. The orientation of the gradient wind with respect to the shorelines is important in determining the location of such lines. When moderate winds (5–10 m s−1) are parallel to straight sections of coastlines, cloud lines can extend well inland. In the region between Lakes Huron and Erie lake-breeze lines merge frequently, sometimes resulting in long-lasting stationary storms and attendant heavy rain and flooding. The influence of the lakes is apparent in the tornado climatology for the region: tornadoes appear to be suppressed in regions visited by lake-modified air and enhanced in regions favored by lake-breeze convergence lines. The cloud patterns in the case of a cold front interacting with merging lake-breeze boundaries are shown to be similar to those on a major tornado outbreak day. Two of the cases discussed are used as conceptual models to explain many of the features in the patterns of tornado touchdown locations. In general, it appears that the lakes suppress tornadoes in southern Ontario, compared with neighboring states and in particular in areas where southwest winds are onshore, but enhance tornado likelihood locally in areas of frequent lake-breeze activity.

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