A case‐study of land–atmosphere coupling during monsoon onset in northern India

Abstract

This article presents a land–atmosphere case‐study for a single day during monsoon onset, incorporating data from a research aircraft, satellite products and model outputs. The unique aircraft observations reveal temperature and humidity contrasts of up to 5 K and 4 g/kg in the planetary boundary layer induced by spatial variations in soil moisture. Both antecedent rain and irrigation were found to be drivers of this atmospheric variability. There is also evidence of soil moisture‐induced mesoscale circulations above some surfaces. This is the first time such responses have been observed in situ over India. Soil moisture‐driven temperature anomalies are larger than those found in previous observational studies in the African Sahel. Moreover, irrigation in the region is extensive, unlike in the Sahel, and has a similar atmospheric effect to antecedent rainfall. This implies that historical changes in irrigation practices are likely to have had an important influence on mesoscale processes within the Indian monsoon. We also examine evidence linking soil moisture and cloud formation. Above wetter soils we observed a suppression of shallow cloud, whilst the initiation of deep convection occurred mostly in areas affected by wet–dry soil moisture boundaries. To investigate the impact of soil moisture heterogeneity on large‐scale wind flow, three model depictions of the day are assessed: the European Centre for Medium‐Range Weather Forecasts ERA‐Interim and ERA5 reanalyses, and a high‐resolution (1.5 km) simulation generated using the Indian National Centre for Medium Range Weather Forecasting regional convection‐permitting Unified Model. We find evidence indicating surface flux uncertainties in the models lead to ∼3.5 hPa anomalies in the monsoon trough. This does affect the simulation of monsoon circulation and rainfall. Better representation of mesoscale land–atmosphere coupling is likely to improve the depiction of convection within weather and climate models over India.

Performance of the NCMRWF convection‐permitting model during contrasting monsoon phases of the 2016 INCOMPASS field campaign

“…More details of the NCUM‐R land–atmosphere coupling during the campaign period is discussed in the recent work of Barton et al . (). For the active monsoon conditions (Figure c), the profile over the continental location (Lucknow) from the model is found to contain stronger updraughts; specifically Diag gives a maximum updraught in the mid‐levels, which is missing in the PC2 simulations.…”

Interaction of convective organization with monsoon precipitation, atmosphere, surface and sea: The 2016 INCOMPASS field campaign in India

“…These demonstrate the importance of agricultural practices as well as meteorology in determining the partition between sensible-and latent-heat fluxes. At smaller scales, Barton et al (2020) have demonstrated that gradients in soil moisture, whether caused by antecedent rainfall or irrigation practices, lead to mesoscale convergence patterns that can initiate storms. Finally, using the IIT Kanpur flux tower observations, Chakraborty et al (2019) found that the NOAH land-surface model significantly underestimates the latent-heat flux over the region during the monsoon onset period, which was partly addressed by improving the vegetation parametrization in the model.…”

“…At smaller scales, Barton et al . () have demonstrated that gradients in soil moisture, whether caused by antecedent rainfall or irrigation practices, lead to mesoscale convergence patterns that can initiate storms. Finally, using the IIT Kanpur flux tower observations, Chakraborty et al .…”

“…En route, the impacts of antecedent soil moisture or irrigation patterns on the atmosphere could be assessed (e.g. in the study of Barton et al, 2020). Flights from Lucknow were possible both prior to (two flights: B956 and B957) and after the onset in northern India (eight flights: B968-B973, B975 and B976).…”

Preface to the INCOMPASS Special Collection

“…Other work published so far in the Special Collection includes a description of the modelling and forecast tools employed in guiding the flight missions (Martin et al ., 2020) and an evaluation of convective‐scale model performance in comparison to flight observations (Jayakumar et al ., 2020). There is a proof‐of‐concept of the importance of mesoscale soil moisture gradients over northern India in initiating convection, based on flight observations and modelling (Barton et al ., 2020), and the new identification of distinct regimes of onshore and offshore convection around the Western Ghats, based on flight and flux‐tower measurements (Fletcher et al ., 2020) which could lead to new insights in forecasting.…”