A 9‐year summary of radar characteristics of mesocyclonic storms and of deep convection in southern Québec
Abstract Nine summers of reflectivity and Doppler data, (1993–2001), archived by the Marshall Radar Observatory of McGill University have been processed for the purpose of identifying the location, strength and frequency of occurrence of severe weather events associated with convective activity. A mesocyclone detection and tracking algorithm has identified 329 features with a lifetime of at least 10 minutes. The distribution of their duration, path length, diameter, depth, rotational velocity and shear are provided, as well as their hourly and monthly frequency. Their geographical distribution reveals an increased relative probability of occurrence over the western area of the McGill radar coverage as well as to the north, near Mirabel, and a reduced probability of occurrence near the metropolitan area of Montréal. The analysis of deep convection has been performed in terms of upper level Vertically Integrated Liquid water content maps (UVIL) where the vertical integration of reflectivity begins at a height of 5 km. UVIL maps are preferred among other types of reflectivity‐based radar products because they are the least affected by radar artefacts such as bright band, ground echoes, anomalous propagation and beam blocking. An inherent bias, present in the standard generation of UVIL maps, and due to an oxygen attenuation overcorrection and to the vertical profile of reflectivity of thunderstorms as sampled by a broadening beamwidth, has been identified and corrected. The geographical distribution of UVIL measurements in terms of the time that a UVIL threshold of 10 and 15 kg m−2 is exceeded has been obtained for both the individual years and for the entire 9‐year period. Areas of preferred convection have been identified in the latter, particularly along the flatlands of the St. Lawrence River Basin. A relative minimum is quite noticeable along a broad corridor running north and south of the radar over heavily forested mountainous regions. This combination results in a non‐negligible correlation of the order of ‐0.2 between the height of the topography (m) and deep convection. Curves revealing the relative severity of the nine years in terms of a continuous UVIL threshold have also been derived. The years 1994, 1998 and 1999 emerged as the most active years while 1996 and 2000 were clearly very quiet.
VIL Density as a Hail Indicator
Abstract In current severe thunderstorm warning operations, forecasters frequently use the vertically integrated liquid water content (VIL) product from the WSR-88D to estimate thunderstorm severity and, particularly, hail size. Since VIL varies greatly based on airmass characteristics, forecasters have typically determined a threshold VIL to be used for each new thunderstorm event. A product that is independent of airmass characteristics, and thus independent of season and geographic location, would be more desirable in an operational warning environment. It has been observed that high-topped thunderstorms with high VILs do not always produce large hail. It has also been observed that low-topped thunderstorms with low VILs occasionally do produce large hail. However, the maximum reflectivity in both high-topped and low-topped thunderstorms is similar when both produce similar-sized hail. From this, it was hypothesized that dividing the VIL by the echo top would “normalize” the VIL and produce a common va...
The Use of Compact Discs for the Storage of Large Meteorological and Oceanographic Data Sets
The amount of data created by the ever-expanding array of meteorological and oceanographic observing systems is steadily, if not exponentially, increasing. Satellite platforms and radar facilities are producing gigabytes of data each day, and planned networks of doppler radars (Milner, 1986) and vertical profilers (Balsley and Gage, 1982) will provide a new flood of information. These data sources as well as numerous types of more-conventional surface and upper-air observations must be archived and distributed. Today, the predominant method for such data handling depends on half-inch magnetic tapes. Unfortunately, this medium possesses several drawbacks. First, even high-density (6250 bits per inch) magnetic tapes can hold only about 130 megabytes, a small amount compared to the current daily stream of data. Second, tapes need to be in physical contact with recording or playback heads and can snag, break, age, and wear. Experience has shown that tapes should be copied every few years to assure acceptable error rates. Considering the large number of tapes that have been and will be created, such periodic tape maintenance is a serious and costly problem. Third, tape drives are complex electro-mechanical devices that are not only expensive to purchase, but also require frequent and costly servicing. Fourth, tape storage does not allow random access to the data; sometime tens of minutes are required to secure a particular piece of information on a high-density tape. As a microprocessors have gotten faster and cheaper, it has become possible to decentralize many of the small and moderate processing tasks from university or organizational mainframes to local microprocessor-based workstations. Unfortunately, the price of tape drives has not fallen at nearly the same rate as central processing units and are generally too expensive to serve