Abstract
AbstractAccurately predicting the Indian monsoon is limited by inadequate understanding of the underlying processes, which feed into systematic model biases. Here we aim to understand the dynamic and thermodynamic features associated with the progression of the monsoon, using 2016 as a representative year, with the help of convection‐permitting simulations of the Met Office Unified Model. Simulations are carried out in a 4 km resolution limited‐area model, nested within a coarser global model. Two major processes thought to influence the northwestward progression of the monsoon are: (a) the interaction between the low‐level monsoon flow and a mid‐tropospheric dry‐air intrusion from the northwest, and (b) land–atmosphere interactions. We find that the 4 km limited‐area model simulates the mid‐tropospheric moistening that erodes the northwesterly dry intrusion, pushing the northern limit of moist convection northwestwards. The surface soil moisture also plays a major role at the leading edge of the monsoon progression. The heavy rains associated with the local onset wet the soil, reducing the sensitivity of surface fluxes to soil moisture and weakening the land influence on further progression of monsoon rains. The 4 km model is tested with an alternative land‐surface configuration to explore its sensitivity to land‐surface processes. We find that the choice of soil and vegetation ancillaries affects the time‐scales of soil moisture–precipitation feedback and the timing of diurnal convection, thereby affecting the local onset. We further compare these simulations with a parametrized convection run at 17 km resolution to isolate the effects of convective parametrization and resolution. The model with explicit convection better simulates the dynamic and thermodynamic features associated with the progression of the monsoon.
Highlights
The Indian monsoon provides water for agriculture, industry and the livelihoods of more than a billion people in India
This study focuses on the local processes that affect Indian monsoon progression using 2016 as an example, given the observational and modelling efforts already performed for the year
The low-θe layer became shallower as the mid-tropospheric relative humidity (RH) increased, eroding the dry-air intrusion as the onset approached (Figure 9b,e). These features are consistent with the theory of Parker et al (2016) and the results shown by Menon et al (2018): as the onset approaches, shallow clouds started to form at the altocumulus layer near the freezing level and these clouds penetrated to higher levels, moistening the tropospheric profile over time, thereby eroding the dry-air intrusions
Summary
The Indian monsoon provides water for agriculture, industry and the livelihoods of more than a billion people in India. High-resolution model simulations using the Advanced Research Weather Research and Forecasting Model (ARW-WRF) show that an increase in soil moisture to the immediate north of the Indian monsoon onset front, from lighter rains emanating in the anvils of deep convective clouds, results in an increase in buoyancy and development of new clouds towards the north of the front which is steered to the north and eventually to the northwest by a divergent circulation (Krishnamurti et al, 2012) This progression is highly sensitive to the parametrization of soil moisture and non-convective anvil rains. We aim to understand the impact of resolution and parametrization on the role of mid-level dry-air intrusions and land-surface wetting on the northwestward progression of the monsoon using high-resolution, season-long model simulations for 2016.
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