A new Western Disturbance Index for the Indian winter monsoon

Midhuna, T. M.; Kumar, Pramod; Dimri, A. P.

doi:10.1007/s12040-019-1324-1

Cited by 50 publications

(34 citation statements)

References 44 publications

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“…4 Results on the wind contribution 4.1 WD characteristics 4.1.1 The upper troposphere disturbance High W 700 is associated with a negative geopotential anomaly (i.e. cyclonic disturbances) with the minimum located at 36 • N 66 • E, just north of the Hindu Kush, and north-west of the UIB (Figure 2-B), and near the tropopause, at around 300 hPa (Figure 3-B), in agreement with previous studies (Hunt et al, 2018a;Midhuna et al, 2020). The lead/lag analysis (Figures 2-A Non-significant anomalies at the level 95% are shown in white (result of a t-test on the means).…”

Section: Investigating Variability In Wd Structuresupporting

confidence: 87%

“…The geopotential height at 300 hPa has been used for tracking analysis (Cannon et al, 2016) and WD indices (Madhura et al, 2015;Midhuna et al, 2020) to characterise WDs. However, only using geopotential heights as predictors limits the predictive skill (R 2 = 0.678), and leads to an underestimation of W 700 for values above 2.5 m s −1 (Baudouin, 2020).…”

Section: Relating Cross-barrier Wind To Western Disturbancesmentioning

confidence: 99%

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Synoptic processes of winter precipitation in the Upper Indus Basin

Baudouin

Herzog

Petrie

2021

Preprint

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Abstract. Precipitation in the Upper Indus Basin is triggered by cross-barrier moisture transport. Winter precipitation events are particularly active in this region and are driven by an approaching upper troposphere Western Disturbance. Here statistical tools are used to decompose the winter precipitation timeseries into a wind and a moisture contribution. The relationship between each contribution and the Western Disturbances are investigated. We find that the wind contribution is not only related to the intensity of the upper troposphere disturbances but also to their thermal structure through baroclinic processes. Particularly, a short-lived baroclinic interaction between the Western Disturbance and the lower altitude cross-barrier flow occurs due to the shape of the relief. This interaction explains both the high activity of Western Disturbances in the area, as well as their quick decay as they move further east. We also revealed the existence of a moisture pathway from the Red Sea, to the Persian Gulf and the north of the Arabian Sea. A Western Disturbance strengthens this flow and steers it towards the Upper Indus Plain, particularly if it originates from a more southern latitude. In cases where the disturbance originates from the north-west, its impact on the moisture flow is limited, since the advected continental dry air drastically limits the precipitation output. The study offers a conceptual framework to study the synoptic activity of Western Disturbances as well as key parameters that explain their precipitation output. This can be used to investigate meso-scale processes or intra-seasonal to inter-annual synoptic activity.

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Section: Investigating Variability In Wd Structuresupporting

confidence: 87%

Section: Relating Cross-barrier Wind To Western Disturbancesmentioning

confidence: 99%

Synoptic processes of winter precipitation in the Upper Indus Basin

Baudouin

Herzog

Petrie

2021

Preprint

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“…Midhuna et al (2020 collated seasonal WD frequency from daily weather reports issued by the India Meteorological Department from 1987 to 2016, for which there was a significant, declining trend. In each of Shekhar et al (2010), , and Midhuna et al (2020), the observed trend in frequency was a decline of about 1% per year. Other authors have used tracking algorithms to identify WDs in reanalysis datasets; both Cannon et al (2016), using CFSR, and Hunt et al (2018a), using ERA-Interim, identified insignificant (though declining) trends in winter western disturbance frequency.…”

Section: Observed Trends Of Wdsmentioning

confidence: 93%

“…Shekhar et al (2010), also looking at the western Himalaya, found no significant trend in either Nov-Apr or Jan-Mar WD frequency over the period , and noted that this contrasted with the significant decline in snowfall days over the same periods. However, noted a significant decrease in Dec-Mar WD frequency over the Central Himalaya from 1977. Midhuna et al (2020 collated seasonal WD frequency from daily weather reports issued by the India Meteorological Department from 1987 to 2016, for which there was a significant, declining trend.…”

Section: Observed Trends Of Wdsmentioning

confidence: 99%

“…They found that this field exhibits a significant positive trend in both ERA-40 (1958ERA-40 ( -2002 and ERA-Interim (1979 reanalyses. These disagreements with more direct counting methods are in part explained by Midhuna et al (2020) who tried to derive an index of WD activity based on 850 hPa and 200 hPa geopotential height EOFs, but could only achieve a correlation coefficient of 0.18 between that and observed counts, suggesting that even well thought out proxy techniques are currently inadequate.…”

Section: Observed Trends Of Wdsmentioning

confidence: 99%

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The impacts of climate change on the winter water cycle of the western Himalaya

2020

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Some 180 million people depend on the Indus River as a key water resource, fed largely by precipitation falling over the western Himalaya. However, the projected response of western Himalayan precipitation to climate change is currently not well constrained: CMIP5 GCMs project a reduced frequency and vorticity of synoptic-scale systems impacting the area, but such systems would exist in a considerably moister atmosphere. In this study, a convection-permitting (4 km horizontal resolution) setup of the Weather Research and Forecasting (WRF) model is used to examine 40 cases of these synoptic-scale systems, known as western disturbances (WDs), as they interact with the western Himalaya. In addition to a present-day control run, three experiments are performed by perturbing the boundary and initial conditions to reflect pre-industrial, RCP4.5 and RCP8.5 background climates respectively. It is found that in spite of the weakening intensity of WDs, net precipitation associated with them in future climate scenarios increases significantly; conversely there is no net change in precipitation between the pre-industrial and control experiments despite a significant conversion of snowfall in the pre-industrial experiment to rainfall in the control experiment, consistent with the changes seen in historical observations. This shift from snowfall to rainfall has profound consequences on water resource management in the Indus Valley, where irrigation is dependent on spring meltwater. Flux decomposition shows that the increase in future precipitation follows directly from the projected moistening of the tropical atmosphere (which increases the moisture flux incident on the western Himalaya by 28%) overpowering the weakened dynamics (which decreases it by 20%). Changes to extreme rainfall events are also examined: it is found that such events may increase significantly in frequency in both future scenarios examined. Two-hour maxima rainfall events that currently occur in 1-in-8 WDs are projected to increase tenfold in frequency in the RCP8.5 scenario; more prolonged (1-week maxima) events are projected to increase fiftyfold.

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