Prediction of Clouds and Rain Using a z-Coordinate Nonhydrostatic Model

Abstract

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.

An upgraded version of the Eta model

“…There is a strong indication that the interest in quasi-horizontal coordinates is on the increase in recent years, originally sparked by the shaved cells approach of Adcroft et al (1997), and used subsequently by Steppeler et al (2006) and Walko and Avissar (2008). The standard step-topography eta has been used in more recent efforts as well; note Marshall et al (2004), and in particular Russell (2007).…”

A Conserved Topographical Representation Scheme Using a Thin-Wall Approximation in Z-Coordinates

“…Good et al (2014) demonstrated the undesirable influence of its nonorthogonality using idealized test cases. With realistic forecasting cases, the property of the terrain-following approach was considerably sensitive for precipitation or temperature, particularly near high mountains (Steppeler et al 2006(Steppeler et al , 2011(Steppeler et al , 2013. Satomura (1989) introduced generated coordinates, whose orthogonality was higher, to an atmospheric model in order to reduce the errors associated with the terrain-following approach.…”

“…A cut-cell approach with z-coordinates, which has recently been investigated (e.g., Adcroft 2013; Adcroft et al 1997;Good et al 2014;Lock et al 2012;Steppeler et al 2002Steppeler et al , 2006Steppeler et al , 2011Steppeler et al , 2013Walko and Avissar 2008;Yamszaki and Satomura 2010;Yamazaki et al 2016), is that topography is approximated using pricewise linear or bilinear functions. Although topography can be more accurately represented, the method requires complex boundary conditions for the pressure or the velocities on topographical surface.…”

Vertically combined shaved cell method in a z‐coordinate nonhydrostatic atmospheric model

“…In particular, they concluded that the shaved cell method is the most conducive to smooth and accurate topography representation, although the partial cell method is a good compromise for shallow slopes. The shaved cell method has also been applied to an atmospheric model by Steppeler et al (2002) and Steppeler et al (2006). Steppeler et al (2002) showed that the shaved cell method reproduces flow over a gently sloping bell-shaped mountain as precisely as a model using the terrain-following coordinates.…”

“…Steppeler et al (2002) showed that the shaved cell method reproduces flow over a gently sloping bell-shaped mountain as precisely as a model using the terrain-following coordinates. In Steppeler et al (2006), tests of precipitation forecasts over realistic mountains were performed using the shaved cell model, and compared with those using the terrain-following model. However, they did not clearly show if the shaved cell method is adaptive to steep mountains.…”