Abstract
In common with many global models, the Met Office Unified Model (MetUM) climate simulations show large errors in Indian summer monsoon rainfall, with a wet bias over the equatorial Indian Ocean, a dry bias over India, and with too weak low‐level flow into India. The representation of moist convection is a dominant source of error in global models, where convection must be parametrized, with the errors growing quickly enough to affect both weather and climate simulations. Here we use the first multi‐week continental‐scale MetUM simulations over India, with grid spacings that allow explicit convection, to examine how convective parametrization contributes to model biases in the region.Some biases are improved in the convection‐permitting simulations with more intense rainfall over India, a later peak in the diurnal cycle of convective rainfall over land, and a reduced positive rainfall bias over the Indian Ocean. The simulations suggest that the reduced rainfall over the Indian Ocean leads to an enhanced monsoon circulation and transport of moisture into India. Increases in latent heating associated with increased convection over land deepen the monsoon trough and enhance water vapour transport into the continent. In addition, delayed continental convection allows greater surface insolation and, along with the same rain falling in more intense bursts, generates a drier land surface. This increases land–sea temperature contrasts, and further enhances onshore flow. Changes in the low‐level water vapour advection into India are dominated by these changes to the flow, rather than to the moisture content in the flow. The results demonstrate the need to improve the representations of convection over both land and oceans to improve simulations of the monsoon.
Highlights
The Indian monsoon (Sperber et al, 2013) is the largest annual reversal in synoptic patterns of wind and rainfall in the world
To investigate the role convective parametrization plays in the development of these systematic model biases, convection-permitting simulations with grid spacings of 2.2, 4, 8 and 12 km, and convection-parametrized simulations with grid spacings of 8, 12, 24, and 120 km, are compared with model analyses and satellite and ground station observations
The simulations are of a 3 week period during August and September 2011, with a domain that covers the subcontinent and its surrounding oceans, and captures the monsoon circulation over the subcontinent
Summary
The Indian monsoon (Sperber et al, 2013) is the largest annual reversal in synoptic patterns of wind and rainfall in the world. Most of India receives more than 80% of its annual rainfall during the summer monsoon months of June to September (Venkateswarlu and Rao, 2013). In May 2002 there was no indication from any empirical or atmospheric general circulation model that all-India rainfall in June and July would be 30% below normal (19% deficit for June to September) with a similar failure in 2004, when there was a seasonal (June to September) rainfall deficit of 13% (Gadgil et al, 2002, 2005). Improving forecasts for the Indian summer monsoon, on all time-scales, has been linked to a need for a better understanding of the role of deep convection in the Tropics (Gadgil et al, 2003)
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More From: Quarterly Journal of the Royal Meteorological Society
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