Precipitation Efficeincy over the Sudety Mountains (SW Poland) in Specific Air Circulation Conditions

Background

The massif of the Sudetes has relatively compact and oblong shape oriented from north-west to south-east. It consists of three undulating plateaus divided by wide intermontane basins: Jeleniogórska and Kłodzka. The atmospheric circulation conditions combined with processes conditioned by land morphology and land use diversify the potential of the precipitable water content (PW) over the mountains. The differentiation is observed also for precipitation in the Sudetes. On north-western windward edge annual sums exceed 1200 mm, whereas in rain-shadow area, in mentioned basins reach from 600 to 700 mm. The general tendency is a decrease of precipitation sums from the north-west to the south east, according to main course of the Sudetes. However in case of wind direction perpendicular to the mountain ridge the airflow deformation takes place and then spatial distribution of precipitation may be significantly different. The forced ascent of air is characteristic during advection from SW and NW (north-western part of mountains), often with significant role of the “seeder-feeder” effect. High precipitation events are observed also with the advection from the NE and SE sector, especially in summer when warm and wet air masses ascend over the cold air coming from the NW and N sector. In some events daily sums of precipitation exceed 50 mm on average, with the maximum over 200 mm/day.

Aim

The study is focused on precipitation efficiency (PE), defined as the percentage of precipitable water content (PW), that falls as a measurable precipitation in the Sudetes in a given time period. General aim is to identify the spatial variability of precipitation efficiency in specific air circulation conditions. The analysis is done also to select the circulation types favorable to high precipitation efficiency events, especially for the high precipitable water content events. Comparison of the precipitation efficiency between both sides of the mountains could be a base of further discussion about mechanism of spatial distribution of precipitation.

Method

The precipitation efficiency was calculated as the percent ratio of daily precipitation sums to daily mean of precipitable water. The daily precipitation sums from period 1981-2010 were measured at 104 stations operated by the Institute of Meteorology and Water Management National Research Institute (IMGW PIB) located in the Sudetes and their forefield. Data of precipitable water content were prepared with the use of the Advance Research Weather Research and Forecasting Model (WRF) version 3.4.1, in resolution 5 x 5 km. The model was run in reanalysis mode in three one-way nested domains (Europe 45 km, Poland 15 km, SW Poland 5 km) and 3 hours temporal resolution. Daily meteorological data from WRF model from the middle domain were used to air circulation types classification. The classification is based on 5 advection sectors (SW, NW, SE, NE, XX – undefined) and 4 pairs of lower and upper vorticity indexes (AA, AC, CA, CC, where the first letter is vorticity index for lower isobaric level and the latter for the upper one, each indicating cyclonic - C or anticyclonic – A circulation). As the result it gives 20 possible air circulation types. The variability of precipitation efficiency according to circulation types were analyzed with the use of one-way analysis of variance ANOVA and post-hoc Tukey’s HSD test.

Results

In the analyzed period, the half of daily precipitation totals exceeded 1.5 mm, whereas precipitable water – 13.8 mm. Median value of PE was 11.5%. In 75% of days the precipitation efficiency did not exceed the value of 33.4%. The maximum value of precipitable water was 44.6 mm, but it did not coincide with the highest daily rainfall (200 mm). Regardless of the analyzed circulation types in some days the PE exceeded 100%. The highest noted value of PE reached 885% in NECC circulation type.

NECC, NWCC and SECA were the circulation types with the highest median value of PE. Type NWCC was connected with advection of fresh air masses from the Atlantic Ocean, whereas in NECC and SECA the air masses were often coming from the Mediterranean as well as Black Sea region. In these types, excluding SECA, the highest overrun of the precipitation sums was observed in relation to precipitation water content (above 600%). Extremely high PE concerned also lower and upper anticyclonic types: NWAA and SWAWW. Relatively the lowest values (below 5%) of precipitation efficiency were noted in rare types without advection (XXCC, XXCA, XXAC) and types with lower anticyclone during advection from east sector (NEAA, SEAA, SEAC).

During circulation types with advection from the most frequent western sector (both SW and NW), the PE was on average 3% higher than from the eastern sector. Efficiencies about 10% lower from remaining types – were noted in types without advection. In types with lower and upper cyclone the median values were about 5% higher than in anticyclone ones, but similar in both groups of types the PE exceeded sometimes the value of 100%.

In July – month with the highest daily precipitation sums – the median value of PE reached almost 40% in NECC type. Only in this type the extreme values of PE exceeded 600%. In SECA, SWCA and SWCC there were some events with PE higher than 400% but the most often PE had almost 20%.

Conclusions

The variability of PE over the Sudetes is closely related to atmospheric circulation pattern. The lowest values were noted in rare cases without advection and types with lower anticyclone during advection from east sector (NEAA, SEAA, SEAC). The highest PE values were noted in types NECC, NWCC and SECA connected with air masses flowing from the Atlantic Ocean, Mediterranean or the Black Sea, especially during forced ascent on perpendicular slopes. This situation was not always observed during the days with very high value of precipitable water. In high PW events convergence on macro scale was usually a result of intensive cyclogenesis in the southern Europe organized in the form of quasi-stationary thermal asymmetry.