Comparison of total ozone retrieved from NOAA satellite and measured from Dobson spectrophotometer over Indian region

An attempt has been made to compare the total ozone retrieved from HIRS channel-9 of NOAA-12 satellite using ITPP software at the facility of lMDPS, New Delhi with that of conventional Dobson spectrophotometer over Indian network stations. The satellite-retrieved total ozone agrees within an accuracy of +-8% with that of Dobson-measured total ozone except during the passage of a weather system over the Indian region. It is seen that whenever a western disturbance is passing over north India and neighbourhood, the difference between the satellite-retrieved and Dobson-measured total ozone becomes more than +- 8% (or +-20 DU).

Synoptic processes of winter precipitation in the Upper Indus Basin

Precipitation in the Upper Indus Basin is triggered by orographic interaction and the forced uplift of a cross-barrier moisture flow. 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 time series 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 related not only 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 and 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.

Synoptic processes of winter precipitation in the Upper Indus Basin

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.

The relationship of Western Disturbances to precipitation in the upper Indus River basin

<p>The upper Indus River basin is characterised by heavy precipitation falling near the foothills of the major mountain ranges, during two wet seasons: winter and summer. Winter precipitation is known to be related to the passing of upper-level synoptic systems embedded in the subtropical westerly jet called Western Disturbances. Here, we investigate the precipitation variability in relation to the Western Disturbances at the synoptic scale, using ERA5 reanalysis data. We take advantage of the results of a previous study that showed that the precipitation is mostly triggered by the forced uplift of a low-level moisture-rich southerly flow across the ranges. We show that the low-level southerly wind triggering the precipitation is produced by the interaction of a Western Disturbance with a baroclinic front located between the Iranian plateau and the Arabian Sea. Ahead of the Western Disturbance, low-level winds draw moisture from the extreme north of the Arabian Sea, the Persian Gulf, and to a lower extent, the Red Sea. At the rear, moisture is depleted by the advection of continental dry air in the Indus River basin. However, the balance between moisture drawing and depletion depends on the characteristics of the Western Disturbance, leading to differences in precipitation intensity. We found the jet position and western Russia blockings to play a role in this. These findings offer clues to understand the longer-term precipitation variability in the area.</p>