Three-dimensional modeling of synthetic cold fronts approaching the Alps

Summary A numerical model was used to study the behaviour of prototype cold fronts as they approach the Alps. Two fronts with different orientations relative to the Alpine range have been considered. One front approaches from west, a second one from northwest. The first front is connected with southwesterly large-scale air-flow producing pre-frontal foehn, whereas the second front is associated with westerly largescale flow leading to weak blocking north of the Alps.Model simulations with fully represented orography and parameterized water phase conversions have been compared with control runs where either the orography was cut off or the phase conversions were omitted. The results show a strong orographic influence in case of pre-frontal foehn which warms the pre-frontal air and increases the cross-frontal temperature contrast leading to an acceleration of the front along the northern Alpine rim. The latent heat effect was found to depend much on the position of precipitation relative to the surface front line. In case of pre-frontal foehn precipitation only falls behind the surface front line into the intruding cold air where it partly evaporates. In contrary, precipitation already appears ahead of the front in the case of blocking. Thus, the cooling effect of evaporating rain increases the cross-frontal temperature difference only in the first case causing an additional acceleration of the front.List of symbols C_pd specific heat capacity of dry air at constant pressure (C_pd=1004.71 J kg^–1 K^–1) - C_pv specific heat capacity of water vapour at constant pressure (C_pv=1845.96 J kg^–1 K^–1) - C_f propagation speed of a front - x, y horizontal grid spacing (cartesian system) - , horizontal grid spacing (geographic system) - t time step - E turbulent kinetic energy - f Coriolis parameter - g gravity acceleration (g=9.81 ms^–1) - h terrain elevation - H height of model lid (H=9000 m) - k Karman constant (k=0.4) - K_Mh horizontal exchange coefficient of momentum - K_Hh horizontal exchange coefficient of heat and moisture - K_Mz vertical exchange coefficient of momentum - K_Hz vertical exchange coefficient of heat and moisture - l mixing length - l_c specific condensation heat (l_c=2500.61 kJ kg^–1) - l_f specific freezing heat (l_f=333.56 kJ kg^–1) - l_s specific sublimation heat (l_s=2834.17 kJ kg^–1) - longitude - m₁,m₂,m₃ metric coefficients - p pressure - Exner function - Pr Prandtl number - latitude - _M profile function - q_v specific humidity - q_c specific content of cloud droplets - q_i specific content of cloud ice particles - q_R specific content of rain drops - q_S specific content of snow - R_d gas constant of dry air (R_d=287.06 J kg^–1 K^–1) - R_v gas constant of water vapour (R_v=461.51 J kg^–1 K^–1) - r_E radius of earth (r_E=6371 km) - Ri_F flux Richardson number - density of dry air - t time - T temperature - _dia period of diastrophy - potential temperature - _v virtual potential temperature - _e equivalent potential temperature - U relative humidity - u, v, w cartesian wind components - u_F,v_F front-normal and front-parallel wind components - x, y, z cartesian coordinates - w^* transformed vertical wind component - W_R speed of falling rain - W_S speed of falling snow - z^* transformed vertical coordinateAbbreviations GND (above) ground level - MSL (above) mean sea levelWith 12 Figures