INCOMPASS 2015-2018
INCOMPASS Publications
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Below are publications either funded via INCOMPASS research or produced from INCOMPASS field campaign data.
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Submitted
Preprints of submitted papers may be available on request subject to author discretion.- Vertical Structure and Radiative Forcing of Monsoon Clouds over Kanpur during the INCOMPASS Campaign. G. George, C. Sarangi, S. N. Tripathi, T. Chakraborty and A. G. Turner. Journal of Geophysical Research - Atmospheres, submitted 21 September 2017.
- Reduction of Bowen ratio under episodic accumulation of absorbing aerosols over Gangetic Plains. Sarangi, C., S. N. Tripathi, T. Chakraborty, K. V. Mithun Krishna, R. Morrison, J. Evans and L. M. Mecado (2017). Geophysical Research Letters, submitted.
Refereed publications
- The role of potential vorticity anomalies in the Somali Jet on Indian Summer Monsoon Intraseasonal Variability. P. Rai, M. Joshi, A. P. Dimri and A. G. Turner (2017). Climate Dynamics, published online 23 August 2017. Authors' preprint |
The climate of the Indian subcontinent is dominated by rainfall arising from the Indian summer monsoon (ISM) during June to September. Intraseasonal variability during the monsoon is characterized by periods of heavy rainfall interspersed by drier periods, known as active and break events respectively. Understanding and predicting such events is of vital importance for forecasting human impacts such as water resources. The Somali Jet is a key regional feature of the monsoon circulation. In the present study, we find that the spatial structure of Somali Jet potential vorticity (PV) anomalies varies considerably during active and break periods. Analysis of these anomalies shows a mechanism whereby sea surface temperature (SST) anomalies propagate north/northwestwards through the Arabian Sea, caused by a positive feedback loop joining anomalies in SST, convection, modification of PV by diabatic heating and mixing in the atmospheric boundary layer, wind-stress curl, and ocean upwelling processes. The feedback mechanism is consistent with observed variability in the coupled ocean-atmosphere system on timescales of approximately 20 days. This research suggests that better understanding and prediction of monsoon intraseasonal variability in the South Asian monsoon may be gained by analysis of the day-to-day dynamical evolution of PV in the Somali Jet.
- The 2015 Indian summer monsoon onset - Phenomena, forecasting and research flight planning. Willetts, P., A. G. Turner, G. Martin, G. Mrudula, K. Hunt, D. Parker, C. Taylor, C. Birch and A. Mitra (2016) Weather, accepted 16 August 2016. Authors' preprint |
From May to July 2016, as part of the INCOMPASS project, the Facility for Airborne AtmosphericMeasurements (FAAM, jointly funded by the Met Office and NERC) BAe-146 research aircraft travelled to India to record key aspects of the Indian summer monsoon onset and evolution. As part of the planning for the campaign, partners in the UK and India took part in a dry-run forecasting exercise during 2015, to assess the reliability of the forecast products and develop a set of flight plans, in advance of the real campaign, and to get a real-time feel for the monsoon onset. 5-day forecasts from the Met Office and the Indian National Centre for Medium Range Weather Forecasting (NCMRWF) showed good skill in terms of predicting the advance of rainfall in regions key for the campaign in north and south India, and captured transitions from active (wet) monsoon conditions to break (dry) periods and back again. Key phenomena seen during the dry-run exercise include (1) a western disturbance, which had a major effect on the extreme pre-monsoon heatwave conditions over India; (2) dry intrusions, which are thought to be important in the progression of the monsoon onset against the synoptic flow; and (3) cyclones Ashobaa and Komen.
- Assessing the level of spatial homogeneity of the agronomic Indian monsoon onset.> R. G. J. Fitzpatrick, D. J. Parker and P. D. Willetts (2016). Geophysical Research Letters, 43(22): 11867-11874. Over monsoon regions, such as the Indian subcontinent, the local onset of persistent rainfall is a crucial event in the annual climate for agricultural planning. Recent work suggested that local onset dates are spatially coherent to a practical level over West Africa; a similar assessment is undertaken here for the Indian subcontinent. Areas of coherent onset, defined as local onset regions or LORs, exist over the studied region. These LORs are significant up to the 95% confidence interval and are primarily clustered around the Arabian Sea (adjacent to and extending over the Western Ghats), the Monsoon Trough (north central India), and the Bay of Bengal. These LORs capture regions where synoptic scale controls of onset may be present and identifiable. In other regions, the absence of LORs is indicative of regions where local and stochastic factors may dominate onset. A potential link between sea surface temperature anomalies and LOR variability is presented. Finally, Kerala, which is often used as a representative onset location, is not contained within an LOR suggesting that variability here may not be representative of wider onset variability.
- The interaction of moist convection and mid-level dry air in the advance of the onset of the Indian monsoon. D. J. Parker, P. Willetts, C. Birch, A. G. Turner, J. H. Marsham, C. M. Taylor, S. Kolusu and G. M. Martin (2016). Quarterly Journal of the Royal Meteorological Society, 142(699): 2256-2272. Authors' preprint |
The advance of the onset of the Indian monsoon is here explained in terms of a balance between the low-level monsoon flow and an over-running intrusion of mid-tropospheric dry air. The monsoon advances, over a period of about 6 weeks, from the south of the country to the northwest. Given that the low-level monsoon winds are westerly or southwesterly, and the midlevel winds northwesterly, the monsoon onset propagates upwind relative to midlevel flow, and perpendicular to the low-level flow, and is not directly caused by moisture flux toward the northwest. Lacking a conceptual model for the advance means that it has been hard to understand and correct known biases in weather and climate prediction models. The mid-level northwesterlies form a wedge of dry air that is deep in the far northwest of India and over-runs the monsoon flow. The dry layer is moistened from below by shallow cumulus and congestus clouds, so that the profile becomes much closer to moist adiabatic, and the dry layer is much shallower in the vertical, toward the southeast of India. The profiles associated with this dry air show how the most favourable environment for deep convection occurs in the south, and onset occurs here first. As the onset advances across India, the advection of moisture from the Arabian Sea becomes stronger, and the mid-level dry air is increasingly moistened from below. This increased moistening makes the wedge of dry air shallower throughout its horizontal extent, and forces the northern limit of moist convection to move toward the northwest. Wetting of the land surface by rainfall will further reinforce the north-westward progression, by sustaining the supply of boundary layer moisture and shallow cumulus. The local advance of the monsoon onset is coincident with weakening of the mid-level northwesterlies, and therefore weakened mid-level dry advection.
Non-refereed publications
- Solving the mysteries of the monsoon. Turner, A.G. Planet Earth (Autumn 2016) Natural Environment Research Council. | PDF of Autumn 2016 Planet Earth.