Evaluation of monsoon depressions in climate simulations of the South Asian Summer Monsoon: the role of model resolution and large-scale circulation

Monsoon depressions and low pressure systems are synoptic-scale systems which contribute substantially to monsoon rainfall over central and northern India. These are areas where MetUM (Met Office Unified Model) climate simulations, as well as many CMIP5 models, typically show deficient monsoon rainfall. Tracking of synoptic-scale depressions and lows is performed using TRACK software (Hodges, 1994) designed for Tropical Cyclone tracking, and is performed on 6-hourly 850hPa vorticity fields. The parameters have been re-tuned for monsoon depressions and lows, and additional filters applied to remove effects of sub-synoptic-scale features. This method has been validated against the IMD e-Atlas of monsoon depressions, and picks up most monsoon depressions in this database, and additionally those monsoon lows that travel at least a set distance (5 degrees) from the source. Systems in MetUM global simulations are found to form in the correct position in the head of the Bay of Bengal, but generally die out far too quickly after landfall, while there is also a deficient number of systems generated compared to observations. This results in deficient rainfall from monsoon depressions and lows across the monsoon trough region in the MetUM. Both the number of systems and their associated rainfall increases with resolution (from ~130km to ~25km), which also coincides with strengthening of mean state monsoon over India. However, resolution is not the only factor. Regional model experiments with different domain sizes, and forced at the boundaries by ERA interim re-analysis, highlight that the simulation of monsoon depressions is particularly sensitive to correction of the large-scale circulation biases as the domain size is reduced (see Figure). Furthermore it is found that this domain size sensitivity is far more dominant than any intrinsic sensitivity to horizontal model resolution. This suggests that a more realistic representation of monsoon depressions is possible even at lower resolution if the larger-scale systematic error pattern in the monsoon is improved. The horizontal and vertical structure of monsoon depressions and lows, and how these are affected by resolution and large-scale circulation will be further examined. Figure caption: Monsoon depression and low trajectories for JJAS season and contribution to seasonal mean precipitation (coloured contours), 850hPa winds (vectors), and mean MSLP conditions during system lifetime (dash-dot contours)