Some studies of the growth of monsoon disturbances

Keshavamurty, R. N.; Asnani, G. C.; Pillai, P. V.; Das, Sunit

doi:10.1007/bf03182096

Cited by 6 publications

(12 citation statements)

References 4 publications

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“…This is simply done via the construction of a composite, which has been done before for, e.g. tropical cyclones (Catto et al, 2010) and monsoon depressions in observations (Godbole, 1977;Keshavamurty et al, 1978;Sarker and Choudhary, 1988;Prasad et al, 1990) and reanalysis (Hurley and Boos, 2015;Hunt et al, 2016a). Simply put, the composite is composed by taking all depression-time steps, centring the data for the field of interest on the depression centre, and then taking the average.…”

Section: Evaluation Of Composite Depression Structurementioning

confidence: 99%

The effect of horizontal resolution on Indian monsoon depressions in the Met Office NWP model

Hunt

Turner

2017

Quart J Royal Meteoro Soc

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Monsoon depressions are synoptic-scale features that are responsible for a significant fraction of the rain over northern India during the summer monsoon season, and therefore it is important to quantify their structure and behaviour in numerical weather prediction (NWP) models. It is known that increasing model resolution is strongly correlated with improved forecasts in the short term and global circulation in the longer term, as well as better representation of tropical cyclones; here, we explore the sensitivity of depressions to changes in resolution using the Met Office Unified Model. Seven NWP case-studies of depressions from 2013 to 2015 were run at eight resolutions corresponding to equatorial grid spacing of between 16 and 208 km, and compared with data for the same events from Tropical Rainfall Measuring Mission (TRMM) and ERA-Interim reanalysis. We found that, at the low-resolution end of the spectrum, increases in resolution led to improvements in the composite structure, but with diminishing returns; that is to say, the improvements in forecast track and structure become smaller. The model also persistently overestimated the depression intensity, in particular the wind speed and the warm core aloft-with the source appearing to originate in the mid-troposphere. The sensitivity of the diurnal cycle to resolution was also explored: the stratiform component was found to be very well represented by the model, whereas the convective component was described quite poorly. Improvement in most components of structure with increasing model resolution were marginal beyond N320 (63 km) and N512 (39 km) for dynamic and thermodynamic fields respectively.

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Section: Evaluation Of Composite Depression Structurementioning

confidence: 99%

The effect of horizontal resolution on Indian monsoon depressions in the Met Office NWP model

Hunt

Turner

2017

Quart J Royal Meteoro Soc

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“…Multiple previous studies have relied on quasi-geostrophic models to understand the dynamics of Indian MDs (e.g. Rao and Rajamani, 1970;Keshavamurty et al, 1978;Sanders, 1984).…”

Section: Invalidity Of the Quasi-geostrophic Approximationmentioning

confidence: 99%

Adiabatic westward drift of Indian monsoon depressions

Boos

Hurley

Murthy

2014

Quart J Royal Meteoro Soc

122

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A large fraction of the rain received by continental India is produced by cyclonic vortices with outer radii of about 1000 km that are contained within the larger scale South Asian monsoon flow. The more intense occurrences of these vortices are called monsoon depressions; these consist of bottom-heavy columns of relative vorticity that propagate to the northwest in time-mean low-level eastward flow. Previous studies have argued that this apparent upstream propagation is caused by dynamical lifting west of the vortex centre, with the resulting ascent producing vortex stretching that shifts the vortex to the west. Here, analysis of over 100 Indian monsoon depressions is used to show that low-level vortex stretching has a spatial structure inconsistent with the observed propagation and is balanced by other terms in the low-level vorticity budget. Instead, monsoon depressions are shown to consist of potential vorticity maxima that have peak amplitude in the middle troposphere and propagate westward by nonlinear, horizontal adiabatic advection (i.e. beta drift). The precipitating ascent in monsoon depressions makes a more minor contribution to the total storm motion and primarily acts to maintain the upright structure of the vortex. These results suggest a new view of Indian monsoon depressions as potential vorticity columns that propagate primarily by adiabatic dynamics.

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“…The pattern of cyclonic vorticity on the northern side (15°N–30°N) and anticyclonic vorticity to the south of 10°N in the IOD composites (Figure 8(a)–(d)) is a key point to this discussion. Several studies in the past have examined the stability characteristics of summer monsoon zonal‐flows and possible dynamical instabilities associated with monsoon depressions (Keshavamurty et al , 1978; Goswami et al , 1980; Mishra and Salvekar, 1980; Satyan et al , 1980; Dash and Keshavamurty, 1982). In particular, various instability mechanisms (viz., barotropic, baroclinic, combined barotropic–baroclinic instability) arising due to horizontal and vertical shears in the zonal‐flows have been investigated.…”

Section: Monsoon Lps Activity During Past Piod Eventsmentioning

confidence: 99%

The long‐lived monsoon depressions of 2006 and their linkage with the Indian Ocean Dipole

Krishnan

Ayantika

Kumar

et al. 2011

Intl Journal of Climatology

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A highlight of the 2006 boreal summer monsoon season was the enhanced activity of long-lived monsoon depressions and low-pressure systems (LPS) over the Indian region. Another important phenomenon during this period was the evolution of a positive Indian Ocean Dipole (PIOD) event. Although previous studies have investigated the impact of PIOD on the large-scale monsoon response, their influence on monsoon LPS activity is not well understood. Based on detailed diagnostic analyses of monsoon LPS during 2006, as well as those associated with other PIOD events during 1958-2007, the present work addresses two specific issues concerning the roles of (a) PIOD-induced large-scale circulation changes and (b) internal feedbacks between latent heating and dynamics, in sustaining the monsoon LPS activity. The results show that PIOD conditions generally favour increased propensity of long-lived (>5 days) LPS with long westward tracks extending into northwest India. The average contribution of long-lived monsoon LPS to the total is found to be approximately 12% higher during PIOD episodes as compared to non-PIOD. The PIOD events showed two important large-scale elements conducive for enhancement of LPS activity: (a) strengthening of cross-equatorial moisture transport from south-eastern tropical Indian Ocean into the Bay of Bengal and (b) enrichment of barotropic instability of monsoon flow. Estimates of latent-heating profiles from TRMM-satellite products during the 2006 LPS showed heating in the mesoscale updrafts above 600 hPa with maximum approximately 400 hPa; while cooling prevailed in lower levels. Stratiform precipitation covered approximately 70-85% of rain area during the prolonged LPS; and the large-scale monsoon Hadley-type circulation was found to be intensified with strong mid-level inflows entering the stratiform rain region. The overall findings suggest that the PIOD-induced background circulation together with internal feedbacks between mesoscale convective systems and large-scale circulation can effectively enhance the longevity of monsoon LPS. These results should serve as important inputs for numerical weather forecasting of extreme rainfall events associated with the regional monsoon phenomenon.

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Some studies of the growth of monsoon disturbances

Cited by 6 publications

References 4 publications

The effect of horizontal resolution on Indian monsoon depressions in the Met Office NWP model

The effect of horizontal resolution on Indian monsoon depressions in the Met Office NWP model

Adiabatic westward drift of Indian monsoon depressions

The long‐lived monsoon depressions of 2006 and their linkage with the Indian Ocean Dipole

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