Gerhard W. Reuter

Transport of Atmospheric Moisture during Three Extreme Rainfall Events over the Mackenzie River Basin

Lagrangian trajectories were computed for three extreme summer rainfall events (with rainfall exceeding 100 mm) over the southern Mackenzie River basin to test the hypothesis that the low-level moisture feeding these rainstorms can be traced back to the Gulf of Mexico. The three-dimensional trajectories were computed using the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT). For all three events, parcel trajectories were identified that originated near the Gulf of Mexico and terminated over the southern Mackenzie River basin. Specifically, the transport of low-level moisture was found to occur along either quasi-continuous or stepwise trajectories. The time required to complete the journey varied between 6 and 10 days. Closer examination of the data suggests that, for the three cases in question, the transport of modified Gulf of Mexico moisture to high latitudes was realized when the northward extension of the Great Plains low-level jet to the Dakotas occurred in synch with rapid cyclogenesis over Alberta, Canada. In this way, modified low-level moisture from the Gulf of Mexico arrived over the northern Great Plains at the same time as a strong southerly flow developed over the Dakotas and Saskatchewan, Canada, in advance of the deepening cutoff low over Alberta. This moist air was then transported northward over Saskatchewan and finally westward over the southern Mackenzie River basin, where strong ascent occurred.

Modeling Maximum Hail Size in Alberta Thunderstorms

Abstract A one-dimensional steady-state cloud model was combined with a time-dependent hail growth model to predict the maximum hailstone size on the ground. Model runs were based on 160 proximity soundings recorded within the Alberta Hail Project area for three summers between 1983 and 1985. The forecast hail sizes were verified against reports of maximum hail size gathered from a high-density observation network within the project area. The probability of detection (POD), the false-alarm ratio (FAR), and the Heidke skill score (HSS) were computed for the hail model forecasts and were compared with the skill scores for a nomogram method developed to forecast hail size in Alberta. The hail model was skillful in forecasting hail (POD = 0.85, FAR = 0.26, HSS = 0.64). On days with hail larger than 2 cm in diameter, the hail model performed slightly better (POD = 0.90, FAR = 0.40, HSS = 0.67). Analysis of the skill scores and hail-size forecasts suggests that employing a coupled cloud and hail model noticeabl...

WRF Model Simulation of Two Alberta Flooding Events and the Impact of Topography

AbstractThis study examines simulations of two flooding events in Alberta, Canada, during June 2005, made using the Weather Research and Forecasting Model (WRF). The model was used in a manner readily accessible to nonmeteorologists (e.g., accepting default choices and parameters) and with a relatively large spatial resolution for rapid model runs. The simulations were skillful: strong storms were developed having the correct timing and location, generating precipitation rates close to observations, and with precipitation amounts near that observed. The model was then used to examine the sensitivity of the two storms to the topography of the Rocky Mountains. Comparing model results using the actual topographic grid with those of a reduced-mountain grid, it is concluded that a reduction in mountain elevation decreases maximum precipitation by roughly 50% over the mountains and foothills. There was little sensitivity to topography in the precipitation outside the mountains.

Fire-growth modelling using meteorological data with random and systematic perturbations

The focus of this investigation is to quantify the effects of perturbations in the meteorological data used in a fire-growth model. Observed variations of temperature, humidity, wind speed, and wind direction are applied as perturbations to hourly values within a simulated weather forecast to produce several forecasts. In turn, these are used by a deterministic eight-point fire-growth model to produce an ensemble of possible final fire perimeters. Two studies were conducted to assess the value of applying perturbations. In the first study, fire growth using detailed, one-minute data was compared to growth based on the more commonly used hourly data. Results showed that the detailed weather produced fire growth larger and wider than the hourly based data. By applying perturbations, variations in the flank and back-fire spread were captured by the random-perturbation model while the forward spread fell within the 20 to 30% probability prediction. A sensitivity analysis based on the observed variations showed that wind speed accounted for a 44% difference in area burned, while temperature accounted for only a 16% difference. In the second study, case studies were conducted on four observed forest fires in Wood Buffalo National Park. Results showed that daily fire-growth predictions using simulated weather forecasts over-predicted fire growth using actual hourly weather observations by 27%. Systematic-perturbation models best compensated for this with most fire growth falling within the predicted range of the models (52 out of 63 days).

Forecasting Tornadic Thunderstorm Potential in Alberta Using Environmental Sounding Data. Part II: Helicity, Precipitable Water, and Storm Convergence

Abstract Sounding parameters are examined to determine whether they can help distinguish between Alberta, Canada, severe thunderstorms that spawn significant tornadoes (F2–F4), weak tornadoes (F0–F1), or nontornadic severe storms producing large hail. Parameters investigated included storm-relative helicity (SRH), precipitable water (PW), and storm convergence. The motivation for analyzing these parameters is that, in theory, they might affect the rate of change of vertical vorticity generation through vortex stretching, vortex tilting, and baroclinic effects. Precipitable water showed statistically significant differences between significant tornadic storms and those severe storms that produced weak tornadoes or no tornadoes. All significant tornadic cases in the dataset used had PW values exceeding 22 mm, with a median value of 24 mm. Values of PW between 19 and 23 mm were generally associated with weak tornadic storms. Computed values of storm convergence, height of the lifted condensation level, and n...

Spatial Forecasts of Maximum Hail Size Using Prognostic Model Soundings and HAILCAST

Abstract Forecasting the occurrence of hail and the maximum hail size is a challenging problem. This paper investigates the feasibility of producing maps of the forecast maximum hail size over the Canadian prairies using 12-h prognostic soundings from an operational NWP model as input for a numerical hail growth model. Specifically, the Global Environmental Multiscale model run by the Canadian Meteorological Center is used to provide the initial data for the HAILCAST model on a 0.5° × 0.5° grid. Maps depicting maximum hail size for the Canadian prairies are generated for 0000 UTC for each day from 1 June to 31 August 2000. The forecast hail-size maps are compared with radar-derived vertically integrated liquid data over southern Alberta and surface hail reports. Verification statistics suggest that the forecast technique is skillful at identifying the occurrence of a hail day versus no-hail day up to 12 h in advance. The technique is also skillful at predicting the main threat areas. The maximum diameter ...

An approach to operational forest fire growth predictions for Canada

This paper presents an operational approach to predicting fire growth for wildland fires in Canada. The approach addresses data assimilation to provide predictions in a timely and efficient manner. Fuels and elevation grids, forecast weather, and active fire locations are entered into a fire-growth model; then predicted fire perimeters are mapped and presented on the web. The Moderate Resolution Imaging Spectroradiometer (MODIS) and the National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA/AVHRR) satellite-based detection systems are used to detect current wildland fires (referred to as hotspots). For selected regions, fire-growth simulation environments are assembled. Fuel type data from several fire management agencies are available in grid format at a resolution of 100 m or less; in areas where such data are not available, a national fuels map based on Satellite Pour l’Observation de la Terre Vegetation sensor (SPOT VGT) land cover and forest inventory is used. Similarly, terrain data are available from a variety of sources. Current hotspots are used as ignition points while past hotspots are used to delineate area burned. Surface wind, temperature, and dew-point values (forecast by Environment Canada) are used to determine the fire weather conditions at the fire location. A case study of two large fires in Canada consisting of 54 fire simulation days is used to test these hypotheses.

Forecasting Tornadic Thunderstorm Potential in Alberta Using Environmental Sounding Data. Part I: Wind Shear and Buoyancy

Abstract This study investigates, for Alberta, Canada, whether observed sounding parameters such as wind shear and buoyant energy can be used to help distinguish between thunderstorms with significant (F2–F5) tornadoes, thunderstorms with weak (F0–F1) tornadoes, and nontornadic severe thunderstorms. The observational dataset contains 87 severe convective storms, all of which occurred within 200 km of the upper-air site at Stony Plain, Alberta, Canada. Of these storms, 13 spawned significant (F2–F5) tornadoes, 61 spawned weak (F0–F1) tornadoes, and 13 had no reported tornadoes yet produced 3 cm or larger hailstones. The observations suggest that bulk shear contained information about the probability of tornado formation and the intensity of the tornado. Significant tornadic storms tended to have stronger shear values than weak tornadic or nontornadic severe storms. All significant tornado cases had a wind shear magnitude in the 900–500-mb layer exceeding 3 m s−1 km−1. Combining the 900–500-mb shear with th...

Reflectivity‐rain rate relationships for convective rainshowers in Edmonton: Research note

Abstract A reflectivity‐rainfall rate (Z‐R) relationship is derived from Carvel radar and Edmonton rain gauge measurements. Our analysis indicates that the traditional point‐by‐point comparison method is not accurate for Alberta summertime precipitation due to timing errors in fast moving convective storms. The Window Probability Matching Method (WPMM) was superior and provided a robust Z‐R relationship in the form of Z = 32.5 R1.65.

Organization of cloud and precipitation in an Alberta storm

Abstract On 16–18 July 1986, a cold core low pressure system developed over central Alberta that produced heavy rainfall. Measurements from a polar-orbiting satellite, weather radar scans, synoptic surface reports and sounding data were analyzed to document the evolution of this system. The focus was on the spatial organization of cloud and precipitation. the cold low produced strong easterly flow to the north of its centre, thereby forcing the airmass over increasingly higher ground to create an extensive and thick layer of nimbostratus cloud with embedded cloud towers. The radar data showed that the strongest precipitation occurred in two narrow bands aligned in the direction of windshear. We analyzed the sounding data and found the atmosphere to be stable for vertical convection, except near the ground. However, the air in the 40–60 kPa layer was slightly unstable or neutral for slantwise convection, indicating that the precipitation bands were probably caused by the release of the slantwise convective instability.