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 therefore it is important to quantify their structure and behaviour in numerical weather prediction (NWP) 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 to 2015 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 Tropical Rainfall Measuring Mission (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; that is to say, the improvements in forecast track and structure become smaller. 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.

Monsoon depression amplification by moist barotropic instability in a vertically sheared environment

“…Interestingly, we were able to get reasonable simulations up to about 25 km grid spacing, perhaps because synoptic‐scale dynamics organize the moist convection on larger scales. For monsoon depressions themselves, this outcome is consistent with the results of Hunt and Turner (), who found only marginal improvements in the structure of monsoon depressions simulated in the Met Office Unified Model as the resolution was refined beyond about a 39‐km grid spacing. However, since their simulation included a convective parametrization, it may not be directly comparable.…”

The evolution, seasonality and impacts of western disturbances

“…This method for representing the three-dimensional distribution of cloud cover is presented in full detail in Figure 17 of Hunt and Turner (2017), but we shall summarize briefly it here. As in that study, we use the ERA-Interim definitions for low, medium and high cloud height, i.e.…”

The relationship between Indian monsoon rainfall and low-pressure systems

“…Here, we use the algorithm of Hunt et al (2016a), which looks for positive anomalies in 850 hPa relative vorticity, as modified by Hunt and Turner (2017) and Hunt et al (2018a) with one adjustment. It will be necessary to differentiate between depressions and the weaker but more common low-pressure systems (LPAs); to this end, we employ the definitions outlined by the India Meteorological Department (http://imd.gov.in/secti on/nhac/wxfaq .pdf).…”