Open AccessPrediction of Clouds and Rain Using a z-Coordinate Nonhydrostatic Model
Author(s) -
J. Steppeler,
Heinz-Werner Bitzer,
Zavisă Janjić,
U. Schättler,
P. Prohl,
U. Gjertsen,
L. Torrisi,
J. Parfinievicz,
Euripides Avgoustoglou,
U. Damrath
Publication year - 2006
Publication title -
monthly weather review
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.862
H-Index - 179
eISSN - 1520-0493
pISSN - 0027-0644
DOI - 10.1175/mwr3331.1
Subject(s) - orography , mesoscale meteorology , terrain , radiosonde , meteorology , precipitation , grid , coordinate system , representation (politics) , environmental science , atmospheric model , atmosphere (unit) , climatology , geology , geodesy , mathematics , geometry , geography , cartography , politics , political science , law
The most common option for numerical models of the atmosphere is to use model layers following the surface of the earth, using a terrain-following vertical coordinate. The present paper investigates the forecast of clouds and precipitation using the z-coordinate nonhydrostatic version of the Lokalmodell (LM-z). This model uses model layers that are parallel to the surface of the sphere and consequently intersect the orography. Physical processes are computed on a special grid, allowing adequate grid spacing even over high mountains. In other respects the model is identical to the nonhydrostatic terrain-following version of the LM, which in a number of European countries is used for operational mesoscale forecasting. The terrain-following version of the LM (LM-tf) is used for comparison with the forecasts of the LM-z. Terrain-following coordinates are accurate when the orography is shallow and smooth, while z-coordinate models need not satisfy this condition. Because the condition of smooth orography is rarely satisfied in reality, z-coordinate models should lead to a better representation of the atmospheric flow near mountains and consequently to a better representation of fog, low stratus, and precipitation. A number of real-data cases, computed with a grid spacing of 7 and 14 km, are investigated. A total of 39 real-data cases have been used to evaluate forecast scores. A rather systematic improvement of precipitation forecasts resulted in a substantial increase of threat scores. Furthermore, RMS verification against radiosondes showed an improvement of the 24-h forecast, both for wind and temperature. To investigate the possibility of flow separation at mountain tops, the flow in the lee of southern Italy was investigated.