Interaction of valleys and circulation patterns (CPs) on spatial precipitation patterns in southern Germany

Abstract

Abstract. Topography exerts influence on the spatial precipitation distribution over different scales, known typically at the large scale as the orographic effect, and at the small scale as the wind-drift rainfall (WDR) effect. At the intermediate scale (1 ∼ 10 km), which is characterized by secondary mountain valleys, topography also demonstrates some effect on the precipitation pattern. This paper investigates such intermediate-scale topographic effects on precipitation patterns, focusing on narrow-steep valleys in the complex terrain of southern Germany, based on the daily observations over a 48 yr period (1960 ∼ 2007) from a high-density raingauge network covering two sub-areas, Baden-Wuerttemberg (BW) and Bavaria (BY). Precipitation data at the valley and non-valley stations are compared under consideration of the daily general circulation patterns (CPs) classified by a fuzzy rule-based algorithm. Scatter plots of precipitation against elevation demonstrate a different behavior of valley stations comparing to non-valley stations. A detailed study of the precipitation time series for selected station triplets, each consisting of a valley station, a mountain station and an open station have been investigated by statistical analysis with the Kolmogorov-Smirnov (KS) test supplemented by the Oneway analysis of variance (One-way ANOVA) and a graphical comparison of the mean precipitation amounts. The results show an interaction of valley orientation and the direction of the CPs at the intermediate scale, i.e. when the valley is shielded from the CP which carries the precipitation, the precipitation amount within the valley is comparable to that on the mountain crest, and both larger than the precipitation at the open station. When the valley is open to the CP, the precipitation within the valley is similar to the open station but much less than that on the mountain. Such phenomenon where the precipitation is "blind" to the valleys at the intermediate scale conditioned on CPs is defined as the "narrowvalley effect" in this work. Such an effect cannot be captured by the widely used elevation-precipitation relationship, which implies that the traditional geostatistical interpolation schemes, e.g. ordinary kriging (OK) or external drift kriging (EDK) applying digital elevation model (DEM) as external information, are not sufficient. An interpolation experiment applying EDK with orographic surrogate elevation defined in this paper as auxiliary information to account for the valley effects shows improvement for the cross-validation.

Global daily 1 km land surface precipitation based on cloud cover-informed downscaling

“…Such dry valleys are situated in areas where the wet air masses flow over an orographic barrier and are prevented from flowing into deep valleys 75 . These effects are however mainly confined to large mountain ranges, and are not as prominent in intermediate mountain ranges 72 . To account for these effects, we used a variant of the windward-leeward equations with a linear search distance of 300 km in steps of 5° from 0° to 355° circular for each grid cell.…”

“…where d WHi and d LHi refer to the horizontal distances between the focal 3 km grid cell in windward and leeward direction and d WZi and d LZi are the corresponding vertical distances compared with the focal 3 km cell following the wind trajectory. Distances are summed over a search distance of 75 kilometers as orographic airflows are limited to horizontal extents between 50-100 km 71,72 LHi lead to a longer-distance impact of leeward rain shadow. The final wind-effect parameter, which is assumed to be related to the interaction of the large-scale wind field and the local-scale precipitation characteristics, is calculated as:…”

“…with z max being the maximum distance between the boundary layer height B z at elevation z and all grid cells at a 30 arc sec resolution falling within a respective 0.25° grid cell, h being a constant of 9000 m, and z being the respective elevation from the Global Multi-resolution Terrain Elevation Data (GMTED2010) 72 with:…”

Added value of an atmospheric circulation pattern‐based statistical downscaling approach for daily precipitation distributions in complex terrain

“…The third tested set of variables is a combination of mean sea level pressure and the u‐ and v‐wind components at 10 m (MSL + 10 m wind). According to Liu et al (2013) the wind direction influences the spatial distribution of precipitation directly due to valley interactions with near surface level atmosphere caused by the complex topography in the study area. Therefore the inclusion of surface wind components is tested due to the assumption of being superior to the first two combinations.…”

CHELSA-TraCE21k – high-resolution (1 km) downscaled transient temperature and precipitation data since the Last Glacial Maximum

“…Such valleys are situated in areas where the wet air masses flow over an orographic barrier and are prevented from flowing into deep valleys. These effects are mainly confined to large mountain ranges and are not as prominent in small-to intermediate-sized mountain ranges (Liu et al, 2013). To account for these effects, we used a variant of the windward-leeward equations with a linear search distance of 300 km in circular steps of 5 • from 0 to 355 • for each grid cell.…”