Understanding the importance of vertical levels resolution in forecasting an orographic heavy rainfall event

Approximation of vertical turbulence depends on the ability of the forecast model to resolve as many vertical levels as possible. There might be times when forecast inaccuracy relies on a certain atmospheric level, especially, under complex terrain cases such as orographic flows. Therefore, this is the point when vertical resolution becomes vital in predicting phenomena such as turbulence, convective transport and clouds that are partially resolved otherwise by coarser vertical resolution.

It is here investigated how by altering the number of vertical levels at specific altitudes above earth surface impacts forecast accuracy. The global non-hydrostatic Conformal Cubic Atmospheric Model (CCAM) is used, with a global stretched grid targeting high resolution over part of North Greece (1-km) and the wider region (8-60-km). The grid configuration in CCAM allows the placement of a high-resolution face centred over the domain of interest with gradually reduced resolution away from this region. This more seamless grid configuration allows coupling between the global and regional spatial scales on the same grid and may provide benefits for the representation of storms as they approach the domain of interest. CCAM offers the ability of selecting different number of vertical levels ranging between 0-43,900m from surface.

The importance of vertical levels resolution was made obvious during an extreme rainfall event in Northern Greece when coarse approximation failed to predict the heavy rain event but only became evident when the number of vertical levels was improved. The area is characterized by complex morphology with orographic flows adjacent to sea breezes from Aegean Sea. In the first instance 27 vertical levels (0-35,000m) were selected underestimating intense rainfall of the following three days. On the contrary, the 54 vertical levels (0-43,700m) run predicted efficiently the rain event due to finer vertical resolution at certain altitudes. This was proved by comparing daily and hourly rain levels by differing forecast intervals before the event.

Acknowledgements: Marcus Thatcher,^, Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere, Aspendale, VIC, Australia