The effect of horizontal resolution on Indian monsoon depressions in the Met Office NWP model

Abstract

Monsoon depressions are synoptic scale features that are responsible for a significant fraction of the rain over northern India during the summer monsoon season, and, as such, it is important to quantify their structure and behaviour in numerical weather prediction models. It is known that increasing model resolution is strongly correlated with improved forecasts in the short term and global circulation in the longer term, as well as better representation of tropical cyclones; here, we explore the sensitivity of depressions to changes in resolution using the Met Office Unified Model. Seven NWP case studies of depressions from 2013-15 were run at eight resolutions corresponding to equatorial grid spacing of between 16 and 208 km, and compared with data for the same events from TRMM and ERA-Interim reanalysis. We found that at the low resolution end of the spectrum, increases in resolution led to improvements in the composite structure, but with diminishing returns. The model also persistently overestimated the depression intensity, in particular the wind speed and the warm core aloft -- with the source appearing to originate in the mid-troposphere. The sensitivity of the diurnal cycle to resolution was also explored: the stratiform component was found to be very well represented by the model, whereas the convective component was described quite poorly. Improvement in most components of structure with increasing model resolution were marginal beyond N320 (63 km) and N512 (39 km) for dynamic and thermodynamic fields respectively