Occurrence of Summertime Convective Precipitation and Mesoscale Convective Systems in Finland during 2000-01

摘要

There are few if any studies of statistics of deep convection occurring in high-latitude regions. In this study, the basic characteristics of convective precipitation and mesoscale convective systems (MCSs) in Finland and nearby regions have been investigated by using composite reflectivity data from seven operational C-band weather radars. The period of study covered summers 2000 and 2001 from the beginning of April to the end of September. During the study period, the fraction of days with convective precipitation (with reflectivity exceeding 40 dBZ) occurring anywhere in the study region was 88%, and the fraction of days with heavy convective precipitation (with reflectivity exceeding 50 dBZ) was 61%. An MCS was defined by means of radar reflectivity as follows: a continuous area of stratiform precipitation (18-40 dBZ), with a long axis of 100 km or more in at least one direction, must exist for at least 4 h, and during the lifetime of the system the maximum reflectivity must exceed 40 dBZ during at least two consecutive hours. All precipitation features that met these criteria were considered MCSs whether they occurred in extratropical cyclones or not. An MCS was qualified as intense if the maximum reflectivity exceeded 50 dBZ during at least two consecutive hours. The total amount of MCSs observed in the study region was 341, out of which 32% were intense. Over 50% of all intense MCSs occurred in July when the majority of systems traveled along south-north- or southeast-northwest-oriented paths. For instance, in July 2001, 34 intense MCSs developed in the study region. The majority of intense systems arrived in the region of study from a channel outlined by the Sarema and the Karelian Isthmus. The average duration of all MCSs was 11.1 h with a monthly minimum of 9 h occurring in July. Generally, the intense and midsummer MCSs were clearly modulated by the diurnal heating-cooling cycle whereas the nonintense and springtime systems were less dependent on the time of the day.