Bohdan Kochtubajda

Lightning occurrence patterns over Canada and adjacent United States from lightning detection network observations

Abstract Since February 1998 the Canadian Lightning Detection Network has provided continuous lightning detection in space and time to about 65°N in the west, 55°N in the east, and offshore to about 300 km. From February 1999 the distinction between cloud‐to‐ground and cloud‐to‐cloud lightning discharges was added. Approximately 21 million lightning flashes from February 1998 to December 2000 were sorted by equal‐area square grid cells with 20‐km sides and analyzed for Canada and the adjacent United States. Both cloud‐to‐ground lightning and cloud‐to‐cloud lightning were studied, as well as total activity irrespective of lightning discharge type. We derived mean patterns of number of lightning occurrence days, flash density, fraction of cloud‐to‐cloud lightning, fraction of nocturnal‐morning lightning, duration, ratio of positive to negative cloud‐to‐ground flashes, peak current, and mean multiplicity of cloud‐to‐ground flashes. A complex national pattern of lightning occurrence was revealed, showing strong regional, diurnal and seasonal dependencies. There are substantial influences by elevated terrain features and major land‐water boundaries. Three regions where significant features were seen in lightning patterns were studied in greater detail: the Alberta foothills to western Saskatchewan, southern Ontario and vicinity, and south‐east of Nova Scotia. For these areas there were several features observed in all three years or in two of the three years. For those fields which we could compare with results of previous studies for the northern United States, our conclusions were similar even though different years were analyzed. While definition of a true Canadian lightning climatology is likely several years away, we believe we have identified many of its aspects, and where possible we suggest physical reasons for significant features seen.

A decade of cloud‐to‐ground lightning in Canada: 1999–2008. Part 1: Flash density and occurrence

Abstract Flash density and occurrence features for more than 23.5 million cloud‐to‐ground (CG) lightning flashes detected by the Canadian Lightning Detection Network (CLDN) from 1999 to 2008 are analyzed on 20 × 20 km equal area squares over Canada. This study was done to update an analysis performed in 2002 with just three years of data. Flashes were detected throughout the year, and distinct geographic differences in flash density and lightning occurrence were observed. The shape and locations of large scale patterns of lightning occurrence remained almost the same, although some details were different. Flash density maxima occurred at the same locations as found previously: the Swan Hills and Foothills of Alberta, southeastern Saskatchewan, southwestern Manitoba and southwestern Ontario. A region of greater lightning occurrence but relatively low flash density south of Nova Scotia occurred at the same location as reported previously. New areas of higher flash density occurred along the US border with northwestern Ontario and southern Quebec. These appear to be northward extensions of higher flash density seen in the previous study. The greatest average CG flash density was 2.8 flash km−2 y−1 in southwestern Ontario, where the greatest single‐year flash density (10.3 flash km−2 y−1) also occurred. Prominent flash density minima occurred east of the Continental Divide in Alberta and over the Niagara Escarpment in southern Ontario. Lightning activity is seen to be highly influenced by the length of the season, proximity to cold water bodies and elevation. The diurnal heating and cooling cycle exerted the main control over lightning occurrence over most land areas; however, storm translation and transient dynamic features complicated the time pattern of lightning production. A large portion of the southern Prairie Provinces experienced more than 50% of flashes between 22:30 and 10:30 local solar time. The duration of lightning over a 20 × 20 km square at most locations in Canada is 5–10 h y−1, although the duration exceeded 15 h y−1 over extreme southwestern Ontario. Lightning occurred on 15–30 days each year, on average, over most of the interior of the country. The greatest number of days with lightning in a single year was 47 in the Alberta foothills and 50 in southwestern Ontario. Beginning and ending dates of the lightning season show that the season length decreases from north to south; however, there are considerable east‐west differences between regions. The season is nearly year‐round in the Pacific coastal region, southern Nova Scotia, southern Newfoundland and offshore.

Summer convection and lightning over the Mackenzie river basin and their Impacts during 1994 and 1995

Abstract Lightning activity over the Mackenzie basin has been examined for the summers of 1994 and 1995. In recent years, the lightning network operating in the Northwest Territories has detected an average of 118 K strikes per season. Positive lightning strikes (defined as lightning discharges lowering positive charge to the earth) typically comprise 12% of the total. The lightning activity during 1994 was approximately 20% below normal, while in 1995, it was 53% below normal. However, the fraction of positive lightning strikes was 25.6% during 1995. The lightning was linked to synoptic conditions favouring severe storm development, especially those tied to the diurnal cycle. As a consequence of the lightning, as well as the very dry surface conditions, record forest areas were burned. In the Northwest Territories alone, forest fires burned 3 Mha in 1994 and 2.8 Mha in 1995.

Hydrometeorological features of the Mackenzie basin climate system during the 1994/95 water year: A period of record Low Discharge

Abstract The phenomena that occurred within and in the vicinity of the Mackenzie basin climate system during the 1994/95 water year are the focus of this study. This water year was characterized by a record low discharge and by a very early discharge into the Arctic Ocean. The low discharge arose because the efficiency with which the record‐low large‐scale moisture convergence over the Mackenzie basin was converted into discharge was also the lowest on record. This low efficiency occurred as a consequence of many atmospheric, surface and hydrological processes and feedbacks occurring during this period. The dry surface conditions that had developed just prior to the start of the water year were also a contributing factor. It is evident through the comprehensive examination of the overall climate system of this basin that many strongly coupled processes were occurring. Our models are able to account for some of these, but more research is needed to address a number of them.

A decade of cloud‐to‐ground lightning in Canada: 1999–2008. Part 2: Polarity, multiplicity and first‐stroke peak current

Abstract We summarize the temporal and spatial characteristics of polarity, multiplicity and first‐stroke peak current of approximately 23.5 million cloud‐to‐ground (CG) lightning flashes detected by the Canadian Lightning Detection Network for the period 1999–2008. Regional differences in these parameters reflect the complex nature and structure of thunderstorms across the country. The annual mean percentage of positive CG flashes was found to be lowest in eastern Canada (11%) and highest in northern Canada (17%). The data do not show any trends over the years in any region. The monthly distribution of positive CG flashes reflects a strong seasonality in all regions, with higher values in winter than in summer. Areas of more than 25% positive flashes are observed along the west coast of British Columbia, in Yukon extending southeast into central British Columbia, in southern Manitoba, northern Quebec, Newfoundland and off the coast of Nova Scotia. The percentages of single‐stroke positive and negative flashes for northern (western, eastern) Canada are 93% and 63%, (89% and 48%, 90% and 50%), respectively. The monthly distribution of multiplicity for negative CG flashes peaks between 2 and 2.4 strokes per flash in the summer and early fall in all regions. The multiplicity of positive flashes (slightly higher than 1 stroke per flash) shows little variation throughout the year in all regions. The annual variation of median negative and positive first‐stroke peak currents reflects a latitudinal dependence over the past decade. The lowest values for each polarity are observed in southern Canada and the highest values occur in the North. The data do not show any trends in peak currents over the years in the eastern or western regions of Canada. The monthly median first‐stroke peak currents for both polarities are strongest in winter and reach a minimum during summer in all regions. Large current flashes ≥100 kA are usually detected in summer and comprise less than 1% of the average annual CG flashes detected in Canada. Large current flashes with stroke multiplicity ≥10 are usually associated with negative polarity. These CG flashes are mostly detected in western Canada.

Hydroclimatic Aspects of the 2011 Assiniboine River Basin Flood

AbstractIn the spring and early summer of 2011, the Assiniboine River basin in Canada experienced an extreme flood that was unprecedented in terms of duration and severity. The flood had significant socioeconomic impacts and caused over

The effects of forest fires on polar bear maternity denning habitat in western Hudson Bay

1 billion (Canadian dollars) in damage. Contrary to what one might expect for such an extreme flood, individual precipitation events before and during the 2011 flood were not extreme; instead, it was the cumulative impact and timing of precipitation events going back to the summer of 2010 that played a key role in the 2011 flood. The summer and fall of 2010 were exceptionally wet, resulting in above-normal soil moisture levels at the time of freeze-up. This was followed by record high snow water equivalent values in March and April 2011. Cold temperatures in March delayed the spring melt, resulting in the above-average spring freshet occurring close to the onset of heavy rains in May and June. The large-scale atmospheric flow during May and June 2011 favored increased cyc...

A Numerical Study of the June 2013 Flood-Producing Extreme Rainstorm over Southern Alberta

The effects of forest fires on polar bear (Ursus maritimus) maternity denning habitat in western Hudson Bay were determined by comparing the physical characteristics of 48 burned and 101 unburned maternity den sites from September 2001 to October 2003. Fire significantly altered vegetation composition and increased the depth of the active layer, resulting in a decrease in the stability of den sites, the collapse of dens, and degradation of the surrounding habitat. Although bears investigated burned areas, analysis of mark and recapture data, satellite telemetry, radio-telemetry, and field observations all demonstrated that bears did not use burned areas for denning. While peat denning habitat is likely not limiting at this time, the re-use and occupancy of peat den sites during the summer may be an important means of energy conservation for pregnant female bears in western Hudson Bay. Increased energy expenditures in association with increased search times for suitable den sites and the excavation of new dens can potentially affect reproductive success. Predicted increases in forest fire activity as a result of climate change, along with the long-term recovery of denning habitat may reduce the amount of suitable denning habitat in the future. Resource managers need to be aware of the possibility of a shift in the distribution of denning bears and further loss of maternity denning habitat in western Hudson Bay.

Ensemble lightning prediction models for the province of Alberta, Canada

AbstractA devastating, flood-producing rainstorm occurred over southern Alberta, Canada, from 19 to 22 June 2013. The long-lived, heavy rainfall event was a result of complex interplays between topographic, synoptic, and convective processes that rendered an accurate simulation of this event a challenging task. In this study, the Weather Research and Forecasting (WRF) Model was used to simulate this event and was validated against several observation datasets. Both the timing and location of the model precipitation agree closely with the observations, indicating that the WRF Model is capable of reproducing this type of severe event. Sensitivity tests with different microphysics schemes were conducted and evaluated using equitable threat and bias frequency scores. The WRF double-moment 6-class microphysics scheme (WDM6) generally performed better when compared with other schemes. The application of a conventional convective/stratiform separation algorithm shows that convective activity was dominant during ...

Forest Fires and Climate Change in the Northwest Territories

Lightning is a major cause of wildland fires in Canada. During an average year in the province of Alberta, 330 000 cloud-to-ground lightning strikes occur. These strikes are responsible for igniting 45% of reported wildfires (~450 fires) and 71% of area burned (~105 000 ha). Lightning-caused wildland fires in remote areas have large suppression costs and a greater chance of escaping initial attack when compared with human-caused fires, which are often located close to infrastructure and suppression resources. In this study, geographic and temporal covariates were paired with meteorological reanalysis and radiosonde observations to generate a series of 6-h and 24-h lightning prediction models valid from April to October. These models, based on cloud-to-ground lightning from the Canadian Lightning Detection Network, were developed and validated for the province of Alberta, Canada. The ensemble forecasts produced from these models were most accurate in the Rocky Mountain and Foothills Natural Regions, achieving hits rates of 85%. The Showalter index, latitude, elevation, longitude, Julian day and convective available potential energy were found to be highly important predictors. Random forest classification is introduced as a viable modelling method to generate lightning forecasts.