Role of land state in a high resolution mesoscale model for simulating the Uttarakhand heavy rainfall event over India

Abstract

In 2013, Indian summer monsoon witnessed a very heavy rainfall event (>30 cm/day) over Uttarakhand in north India, claiming more than 5000 lives and property damage worth approximately 40 billion USD. This event was associated with the interaction of two synoptic systems, i.e., intensified subtropical westerly trough over north India and north-westward moving monsoon depression formed over the Bay of Bengal. The event had occurred over highly variable terrain and land surface characteristics. Although global models predicted the large scale event, they failed to predict realistic location, timing, amount, intensity and distribution of rainfall over the region. The goal of this study is to assess the impact of land state conditions in simulating this severe event using a high resolution mesoscale model. The land conditions such as multi-layer soil moisture and soil temperature fields were generated from High Resolution Land Data Assimilation (HRLDAS) modelling system. Two experiments were conducted namely, (1) CNTL (Control, without land data assimilation) and (2) LDAS, with land data assimilation (i.e., with HRLDAS-based soil moisture and temperature fields) using Weather Research and Forecasting (WRF) modelling system. Initial soil moisture correlation and root mean square error for LDAS is 0.73 and 0.05, whereas for CNTL it is 0.63 and 0.053 respectively, with a stronger heat low in LDAS. The differences in wind and moisture transport in LDAS favoured increased moisture transport from Arabian Sea through a convectively unstable region embedded within two low pressure centers over Arabian Sea and Bay of Bengal. The improvement in rainfall is significantly correlated to the persistent generation of potential vorticity (PV) in LDAS. Further, PV tendency analysis confirmed that the increased generation of PV is due to the enhanced horizontal PV advection component rather than the diabatic heating terms due to modified flow fields. These results suggest that, two different synoptic systems merged by the strong interaction of moving PV columns resulted in the strengthening and further amplification of the system over the region in LDAS. This study highlights the importance of better representation of the land surface fields for improved prediction of localized anomalous weather event over India.

Assessment of the Weather Research and Forecasting (WRF) model for simulation of extreme rainfall events in the upper Ganga Basin

“…Most of the studies conducted earlier over this region (Medina et al, 2010;Kumar et al, 2012;Thayyen et al, 2013;Kumar et al, 2014;Chevuturi et al, 2015;Shekhar et al, 2015;Chevuturi and Dimri, 2016;Rajesh et al, 2016;Hazra et al, 2017) are based on the general or default WRF configuration with WSM6 microphysics, a Kain-Fritsch cumulus parameterization scheme and planetary boundary layer of the Yonsei University scheme. In this paper, ensemble experiments are conducted using the WRF model with different grid spacing, four microphysics schemes, two cumulus parameterization schemes, two planetary boundary layer schemes and two land surface model conditions.…”

“…This large-scale setting is thought to have created a platform for interaction between two synoptic-scale events -a north-west moving depression from the Bay of Bengal and a pre-existing westerly trough in the mid-troposphere. Meteorological studies conducted over the region (Kotal et al, 2014;Ray et al, 2014;Chevuturi and Dimri, 2016;Rajesh et al, 2016) established that there was a monsoon low-pressure system during this period. The longitudinal time section for 850 hPa geopotential height along with anomaly averaged over 20-26 • N showed a high negative anomaly on 14 June, which migrated to the west, moving over 75 • E by 17 June.…”

“…The monsoon low provided the moisture feed and the upper-level westerly trough provided the divergence to lift the moisture. This whole system eventually led to an unanticipated heavy rainfall during 15-18 June 2013 in the Kedarnath valley and adjoining areas in the state of Uttarakhand, India (Kotal et al, 2014;Ray et al, 2014;Chevuturi and Dimri, 2016;Rajesh et al, 2016).…”

“…The synthesis of the synoptic setting of the event has been carried out in a number of studies such as Dube et al (2014), Kotal et al (2014), Ray et al (2014), Shekhar et al (2015), Chevuturi and Dimri, (2016), and Rajesh et al (2016), but the mesoscale assessment pertaining to the simulation of this rainfall event is still lacking. Therefore, the present study emphasizes quantitatively evaluating and conducting the sensitivity analysis of the WRF model in predicting extreme rainfall.…”

“…Shekhar et al (2015), Chevuturi and Dimri, (2016), and Dimri et al (2016) performed in-depth synoptic analysis of the June 2013 heavy rainfall event using the WRF model. Rajesh et al (2016) presented the role of land surface conditions in simulating the heavy rainfall event. Therefore, from the existing literature, it can be established that the regional model performs considerably well over the region.…”