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An investigation of the characteristics of monsoon low pressure systems in the present climate and their sensitivity to topography and climate change
An investigation of the characteristics of monsoon low pressure systems in the present climate and their sensitivity to topography and climate change
animation_final.aviMonsoon LPS in CESM 1.2.2 simulation at 50km resolution
The Global Circulation Model (CESM) is used to simulate world climate at 50km resolution. The white patches represent clouds and the colors represent precipitation (mm/day). LPS is represented as blobs of heavy rainfall (with a maximum of 500mm/day) forming over Bay of Bengal and moving north-westwards.
Monsoon Low pressure systems (LPS) are synoptic scale tropical cyclonic disturbances that periodically form over the Indian subcontinent during the monsoon months of June-September. With a lifespan of around 3-6 days and an average speed of 170km/day, around 14 of them occur in each monsoon season. They typically form over north Bay of Bengal and move north westward along the planetary scale monsoon trough causing copious rain, along the southwest side of the system, in the core monsoon region of India. More than 50% of summer-monsoon (June-September) rainfall received in the country occurs on LPS days. Apart from being a lifeline to the country by providing significant part of rainfall, the LPS triggered-rainfall causes catastrophic floods in the country. The rainfall during the summer monsoon season in Odisha, Maharashtra, Madhya Pradesh, UP, Bihar, Rajasthan and Gujarat crucially depend on the number and intensity of these low-pressure systems. There is large uncertainty associated with intensity, frequency and tracks of LPS. Hence, an improved understanding of the LPS statistics for the present and future is needed for managing/mitigating their adverse effects.
The primary goal of our study is to understand the large-scale factors that influence the intensity, frequency and tracks of LPS and hence to assess potential risk over Indian subcontinent due to LPS under the current and future climate change scenarios.
Automated tracking Algorithm using Geopotential Criteria (ATAGC)
Automated tracking Algorithm using Geopotential Criteria (ATAGC)
An automated tracking algorithm developed to determine the genesis points of LPS and to track their propagation in the Indian Subcontinent. The algorithm tracks a LPS by connecting relative vorticity peaks in consecutive time frames. The use of geopotential criteria to confirm the locations of LPS makes the algorithm unique. The obtained tracks are categorized into lows, depressions and deep depressions based on its maximum velocity and minimum geopotential drop attained along its path. Another unique feature of the algorithm is the use of a an extra filter to avoid accidental inclusion of tropical cyclones as LPS.
Track Density in ERA-Interim data using ATAGC
The number of tracks observed per year with 500km radii of each location in analysis with tracks obtained by applying ATAGC on ERA-Interim data for the period 1979-2015.
Percentage of LPS related extreme precipitation events
Percentage of extreme precipitation events (>65.4mm/day) occurring within 1000km radii of LPS tracks in analysis with tracks obtained by applying ATAGC on ERA-Interim data for the period 1979-2015.
Low Pressure Systems in ERA-Interim Data
Low Pressure Systems in ERA-Interim Data
The developed algorithm was applied on the ERA interim data product to track LPS originating in Indian Subcontinent over the period 1979-2015. Around 14 LPS form every year over Indian subcontinent spanning over 68 days (in June-September). 60-70% of summer monsoon rainfall over India occur on LPS days. Once the known features of LPS were also observed for the tracks obtained using ATAGC on ERA-Interim Data, extreme rainfall analysis was undertaken. 82% of extreme rainfall events in the country occur on LPS days of which 78% occur within the vicinity of a LPS track. It is also observed in our analysis that the intensity of extreme precipitation increases by 50-100% when they are associated with LPS.
Low Pressure Systems in CESM1.2.2
Low Pressure Systems in CESM1.2.2
The Community Earth System Model (CESM1.2.2) was run at 1- degree resolution. The developed algorithm was applied to the simulation to generate LPS tracks to enable us to understand the various features of LPS over the country. CESM simulates the mean monsoon features reasonably well, but simulated mean summer precipitation is larger by about 16%. The frequency of simulated LPS is similar to observations, but a southward latitudinal shift (by ~ 4°) is simulated in the median LPS tracks, which is likely associated with a weaker simulated meridional tropospheric temperature gradient during the pre-monsoon and monsoon seasons.
Monsoon precipitation and LPS frequency over India
Average summer monsoon precipitation (blue bars, cm) and the number of LPS forming over India (green bars, per year) during 37 year study period in ERA-Interim data and CESM simulations
Monsoon circulation
Mean 850 mb winds during the monsoon season (June-September) (m/s) over the Indian subcontinent in the ERA-Interim reanalysis and in CESM simulation with SAMs (CTRL) and without SAMs (No-MTN) simulations over a 37-year period.
Effect of Southeast Asian mountains on LPS activity over India
Effect of Southeast Asian mountains on LPS activity over India
Another simulation was performed in which the Southeast Asian mountains (SAMs) were removed to understand the influence of SAMs on LPS activity over India. Two hypotheses were tested:
1. Whether the monsoon winds will not turn around the trough, but instead flow straight over SAMs. This can lead to a reduction in the number of LPS.
2. Whether the remnants of Tropical cyclones in the Pacific can easily passover SAMs area and reamplify as LPS in BoB. This can lead to an increase in the number of LPS.
Small increase in the number of BoB LPS (0.7 LPS /year, 12.7%) and the total number of LPS (1.3 LPS /year, 9.6%) is simulated when SAMs are removed. But only a small portion of the increased LPS count could be attributed to the remnants of Pacific disturbances indicating that the mountains are not a major barrier to the passage of the remnants of the cyclonic disturbances into the BoB.
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