UoR device IMPROVE logo Leeds device

Indian Monsoon PRecipitation over Orography: Verification and Enhancement of understanding (IMPROVE) September 2019-March 2021

The IMPROVE team

IMPROVE was a collaborative project between the National Centre for Atmospheric Science (NCAS) / Department of Meteorology at University of Reading and the School of Earth and Environment at Univesity of Leeds.

IMPROVE is led by PI Dr Andy Turner at University of Reading, supported by Dr Reinhard Schiemann and Dr Thorwald Stein, with the bulk of the research to be carried out by Dr Kieran Hunt.

The team at Leeds is led by Dr Jennifer Fletcher, with the support of Dr Andrew Ross and Dr Stephen Griffiths. The new research at Leeds will be carried out by a PDRA to be appointed.

Funding information

IMPROVE is funded by the Weather and Climate Science for Services (WCSSP) India scheme administered by the Met Office on behalf of the Newton Fund. Projects funded under this programme aim to ...advanc[e] scientific understanding and modelling capabilities that can be translated into services, to protect lives and livelihoods across India

Project background

IMPROVE aims to address the following research objectives:

  1. Determine the drivers of typical and extreme orographic precipitation in the Western Ghats and Himalaya focus regions in observation-based datasets;
  2. Determine how mechanisms of orographic precipitation and precipitation extremes are represented in models ranging from the convection-permitting, kilometre scale to global, parameterized convection models;
  3. Determine what mechanisms controlling orographic rainfall and its extremes are missing from models at a range of scales and inform suggestions for model development.


Publications arising from IMPROVE are listed below and will be updated periodically. If you are not a subscriber to these journals, email me for an offprint.

  1. Synoptic-scale precursors of landslides in the western Himalaya and Karakoram. Hunt, K. M. R. and A. P. Dimri (2021). Science of the Total Environment, 776: 145895, published online 18 February 2021. |
    In the Upper Indus Basin (UIB), precipitation associated with synoptic-scale circulations impinges on the complex and steep orography of the western Himalaya and Karakoram. Heavy rainfall often falls over the foothills, frequently triggering landslides there. This study explores the role of these synoptic-scale circulations - extratropical western disturbances (WDs) and tropical depressions (TDs) - in producing the conducive conditions necessary to trigger landslides, using data from the NASA Global Landslide Catalog and WD and TD track databases. During the winter (October to April), UIB landslides peak in February and occur at a rate of 0.05 day-1, 61% of which are associated with the passage of a WD. They are most common when a WD is located within a few hundred kilometres of 30°N, and significantly rarer if the WD is north of 40°N. WDs provide moist southwesterly flow from the Arabian Sea and Mediterranean Sea to the UIB, resulting in large-scale precipitation, but landslide probability is not related to WD intensity. Non-WD winter landslides are associated with small-scale orographic precipitation that we hypothesise is due to cloudbursts. During the summer (May to September), UIB landslides peak in August and occur at a rate of 0.11 day-1, 60% of which are associated with TD activity. Many of these TDs are found over central India, slightly south of the climatological monsoon trough, where they provide strong monsoonal southeasterlies to the UIB flowing along the Himalayas. Increased landslide frequency is also associated with TD activity over the southern Bay of Bengal (BoB), and it is hypothesised that this is related to monsoon break conditions. Landslide frequency is significantly correlated with TD intensity. Non-TD landslides are associated with a northwestward extension of the monsoon trough, providing southeasterly barrier flow to the UIB. Implications for forecasting and climate change are discussed.

  2. How interactions between tropical depressions and western disturbances affect heavy precipitation in South Asia. Hunt, K. M. R., A. G. Turner and R. K. H. Schiemann (2021). Monthly Weather Review, submitted 24 April 2020, accepted 24 March 2021, published online 5 April 2021. |
    Interactions over South Asia between tropical depressions (TDs) and extratropical storms known as western disturbances (WDs) are known to cause extreme precipitation events, including those responsible for the 2013 floods over northern India. In this study, existing databases of WD and TD tracks are used to identify potential WD-TD interactions from 1979-2015; these are filtered according to proximity and intensity, leaving 59 cases which form the basis of this paper. Synoptic charts, vorticity budgets, and moisture trajectory analyses are employed to identify and elucidate common interaction types among these cases. Two broad families of interaction emerge. Firstly, a dynamical coupling of the WD and TD, whereby either the upper- and lower-level vortices superpose (a vortex merger), or the TD is intensified as it passes into the entrance region of a jet streak associated with the WD (a jet-streak excitation). Secondly, a moisture exchange between the WD and TD, whereby either anomalous moisture is advected from the TD to the WD, resulting in anomalous precipitation near the WD (a TD-to-WD moisture exchange), or anomalous moisture is advected from the WD to the TD (a WD-to-TD moisture exchange). Interactions are most common in the post-monsoon period as the subtropical jet, which brings WDs to the subcontinent, returns south; there is a smaller peak in May and June, driven by monsoon onset vortices. Precipitation is heaviest in dynamically-coupled interactions, particularly jet-streak excitations. Criteria for automated identification of interaction types are proposed, and schematics for each type are presented to highlight key mechanisms.

  3. Modes of coastal precipitation over southwest India and their relationship to intraseasonal variability. K. M. R. Hunt, A. G. Turner, T. H. M. Stein, J. K. Fletcher and R. K. H. Schiemann (2020) Quarterly Journal of the Royal Meteorological Society, submitted 1 June 2020, revised version submitted 27 August 2020, accepted 16 September 2020. |
    The west coast of India, dominated by the Western Ghats mountain range, is among the rainiest places in the tropics. The interaction between the land-sea contrast of the coast, the monsoonal westerlies, and the oblique mountains is subject to complex intraseasonal variability, which has not previously been explored in depth. This study investigates that variability from the perspective of the land-sea contrast, using empirical orthogonal function analysis to discern regimes of onshore and offshore rainfall over south-west India and the eastern Indian Ocean. Locally, it is found that the rainfall is most sensitive to mid-tropospheric hu- midity: when this is anomalously high, deep convection is encouraged; when this is anomalously low, it is suppressed. A moisture tracking algorithm is employed to determine the primary sources of the anomalously wet and dry mid-tropospheric air. There are important secondary contributions from low-level vorticity and cross-shore moisture flux. The dominant control on intraseasonal variability in coastal precipitation is found to be the BSISO: over 75% of the strongest offshore events occur during phases 3 and 4; and about 40% of the strongest onshore events occur during phases 5 and 6. The location of monsoon low-pressure systems is also shown to be important in determining the magnitude and location of coastal rainfall.

  4. The 2018 Kerala floods: a climate change perspective. K. M. R. Hunt and A. Menon (2020). Climate Dynamics, 54: 2433-2446. Authors' preprint | UoR blog article about this work |
    In August 2018, the Indian state of Kerala received an extended period of very heavy rainfall as a result of a low-pressure system near the beginning of the month being followed several days later by a monsoon depression. The resulting floods killed over 400 people and displaced a million more. Here, a high resolution setup (4 km) of the Weather Research and Forecasting (WRF) model is used in conjunction with a hydrological model (WRF-Hydro, run at 125 m resolution) to explore the circumstances that caused the floods. In addition to a control experiment, two additional experiments are performed by perturbing the boundary conditions to simulate the event in pre-industrial and RCP8.5 background climates. Modelled rainfall closely matched observations over the study period, and it is found that this would this would have been about 18% heavier in the pre-industrial due to recent weakening of monsoon low-pressure systems, but would be 36% heavier in an RCP8.5 climate due to moistening of the tropical troposphere. Modelled river streamflow responds accordingly: it is shown the six major reservoirs that serve the state would have needed to have 34% more capacity to handle the heavy rainfall, and 43% had the deluge been amplified by an RCP8.5 climate. It is further shown that this future climate would have significantly extended the southern boundary of the flooding. Thus it is concluded that while climate change to date may well have mitigated the impacts of the flooding, future climate change would likely exacerbate them.

Page constructed by Dr Andy Turner; last updated March 2020.