Search

This study represents the first detailed analysis of multi-year, near-surface turbulence observations for an urban area located in highly complex terrain. Using 4 years of eddy covariance measurements over the Alpine city of Innsbruck, Austria, the effects of the urban surface, orographic setting and mountain weather on energy and mass exchange are investigated. In terms of surface controls, the findings for Innsbruck are in accordance with previous studies at city centre sites. The available energy is partitioned mainly into net storage heat flux and sensible heat flux (each comprising about 40 % of the net radiation, Q* , during the daytime in summer). The latent heat flux is small by comparison (only about 10 % of Q*) due to the small amount of vegetation present but increases for short periods (6–12 h) following rainfall. Additional energy supplied by anthropogenic activities and heat released from the large thermal mass of the urban surface helps to support positive sensible heat fluxes in the city all year round. Annual observed CO 2 fluxes (5.1 kg C m -2 yr -1) correspond well to modelled emissions and expectations based on findings at other sites with a similar proportion of vegetation. The net CO 2 exchange is dominated by anthropogenic emissions from traffic in summer and building heating in winter. In contrast to previous urban observational studies, the effect of the orography is examined here. Innsbruck's location in a steep-sided valley results in marked diurnal and seasonal patterns in flow conditions. A typical valley wind circulation is observed (in the absence of strong synoptic forcing) with moderate up-valley winds during daytime, weaker down-valley winds at night (and in winter) and near-zero wind speeds around the times of the twice-daily wind reversal. Due to Innsbruck's location north of the main Alpine crest, southerly foehn events frequently have a marked effect on temperature, wind speed, turbulence and pollutant concentration. Warm, dry foehn air advected over the surface can lead to negative sensible heat fluxes both inside and outside the city. Increased wind speeds and intense mixing during foehn (turbulent kinetic energy often exceeds 5 m 2 s -2) help to ventilate the city, illustrated here by low CO 2 mixing ratios. Radiative exchange is also affected by the orography – incoming shortwave radiation is blocked by the terrain at low solar elevation. The interpretation of the dataset is complicated by distinct temporal patterns in flow conditions and the combined influences of the urban environment, terrain and atmospheric conditions. The analysis presented here reveals how Innsbruck's mountainous setting impacts the near-surface conditions in multiple ways, highlighting the similarities with previous studies in much flatter terrain and examining the differences in order to begin to understand interactions between urban and orographic processes. [ABSTRACT FROM AUTHOR]

This is one of the first case studies of a snowstorm at Lake Constance, located between Austria, Germany, and Switzerland, which assesses the influence of the lake and the orography on the generation of heavy precipitation. The analysis is based on surface and radar observations and numerical simulations with the Weather Research and Forecasting (WRF) Model. On 8 February 2013, a rather stationary and banded radar reflectivity pattern was observed during postfrontal conditions with northwesterly flow. The associated snowband affected the downstream shore and the adjacent mountainous region with 36 mm of precipitation within 5 h at the shore. Surface observations show a convergence in the wind field over the lake during the period of banded precipitation. The control simulation captures the formation of a convergence line and a snowband near the shoreline and over the downstream orography. A lake-induced, low-level conditionally unstable layer is essential for the snowband formation. Orographically and thermally induced convergence provides the lifting to release conditional instability and to trigger convection. Orographic enhancement of precipitation occurs downstream of the lake. Sensitivity experiments with modified orography, land use, and lake surface temperature show that the lake is a crucial factor controlling the amount and distribution of snowfall. However, neither the lake nor the orography alone would have been able to form a snowband. This study highlights the complex interaction between lake and orographic effects and shows that Lake Constance is large enough to impact the formation of precipitation. [ABSTRACT FROM AUTHOR]