Orographic precipitation and air mass transformation: An Alpine example

Abstract

SUMMARYA case of orographic precipitation in the Alps on 20 September 1999 was studied using several models, along with rain-gauge and radar data. The objective of the study is to describe the orographic transformation of an air mass, including multi-scale aspects. Several new and some conventional diagnostic quantities are estimated, including drying ratio, precipitation ef ciency, buoyancy work, condensed-water residence time, parcel changes in heat, moisture and altitude, and dominant space-and time-scales.For the case considered, the drying ratio was about 35%. Precipitation ef ciency values are ambiguous due to repeated ascent and descent over small-scale terrain. The sign of buoyancy work changed during the event, indicating a shift from stratiform orographic to weak convective clouds. Cloud-water residence times are different for the two mesoscale models (400 compared to 1000 s) due to different cloud-physical formulations. The two mesoscale models agree that the dominant spatial-scale of lifting and precipitation is about 10 km; smaller than the scale of the main Alpine massif. Trajectory analysis of air crossing the Alps casts doubt on the classic model of föhn. Few parcels exhibit classic pattern of moist ascent followed by dry descent. Parcels that gain latent heat descend only brie y, before rising into the middle troposphere. Parcels that descend along the lee slope, originate in the middle troposphere and gain little, or even lose, latent heat during the transit. As parcels seek their proper buoyancy level downstream, a surprising scrambling of the air mass occurs.Radar data con rm the model prediction that the rainfall eld is tightly controlled by local terrain on scales as small as 10 km, rather than the full 100 km cross-Alpine scale. A curious pulsing of the precipitation is seen, indicating either drifting moisture anomalies or weak convection.

Quantitative precipitation forecasting in the Alps: The advances achieved by the Mesoscale Alpine Programme

“…Besides the above-mentioned deficiency in the cloud microphysical scheme (Smith et al, 2003), the freeatmosphere temperature and wind fields were found to be heavily disturbed by grid-scale numerical noise over the Alps despite heavy smoothing of the model topography. Schär et al (2002) suggested that numerical errors related to the terrain-following coordinate system (Gal-Chen and Somerville, 1975) were responsible for these undesirable noisy structures.…”

“…Benoit et al (2002) show that the MC2 real-time forecasts captured the timing and the structure of the precipitation events quite well but underpredicted the magnitude of the precipitation by a factor of 2. The systematic underprediction was subsequently attributed to an error in the cloud microphysical scheme, leading to an excessive residence time of cloud water in the model (Smith et al, 2003). Unfortunately, due to the early development stage of the MC2, no final conclusion could be drawn regarding the merit of high-resolution models that work without parametrization for cumulus convection.…”

Sensitivity of historical orographically enhanced extreme precipitation events to idealized temperature perturbations

“…However, a change in precipitation efficiency could also lead to a change in precipitation amounts. Smith et al (2003) found that precipitation efficiency can lead to artificially inflated condensation rates over multiple ridges. To evaluate how efficient Langfjella generates precipitation, the drying ratio (DR, the fraction of incoming water vapor flux that is rained out as air passes over the terrain) was examined.…”

NORA–Nowcasting of Orographic Rainfall by means of Analogues

“…Rain-gauge networks are typically not dense enough to reproduce such high spatial variability; even in the European Alps, one of the regions with the densest rain-gauge network, typical spacing between stations is 10 km, whereas the precipitation distribution varies at scales smaller than 10 km (e.g. Frei and Schär, 1998;Smith et al, 2003). Ground-based radars, on the other hand, are designed to monitor precipitation over large areas with high spatial and temporal resolution.…”