Sub-synoptic circulation variability in the Himalayan extreme precipitation event during June 2013

This study investigates the sub-synoptic scale circulation aspects associated with the extreme rainfall event occurred over the North Indian state of Uttarakhand located in the western Himalayas (WH) during the 15–18 June 2013 period. A diagnosis based on hourly ERA5 reanalyzed circulation products archived on finer grids reveals that sustenance of heavy rains during the event period is supported by a propensity of cyclonic vorticity sources channeled toward the WH region through a narrow quasi-steady conduit in the lower troposphere from the ISM circulation. The equatorward segregating mesoscale potential vorticity (PV) structures from the quasi-stationary upper level PV anomaly (trough) during the event administered two pathways for vorticity sources. The first pathway is from the base of the trough culminating into longer horizontal conduit path from the western Arabian Sea, lending perpetual cyclonic vorticity support to the ISM environment. The second pathway is from the right flank of the trough, which promotes sustained environment of deeper mesoscale convergence zone, potentially unstable atmosphere and strong ascent over the Uttarakhand region. The convergence zone is potentially viewed as a region for strong monsoon and extratropical circulation interactions to occur on finer horizontal scales of motion, where significant vertical synchronization of positive PV advection is realized during the 16–17 June 2013 period. In addition to orographic precipitation enhancements, deeper advective synchronization noticed at sub-synoptic time periods is accredited to the nearly doubling 24-h rainfall amounts in the foothill region of Uttarakhand during the event period. The ERA5 diagnosed diabatic heating additionally indicates that precipitating systems at higher (foothill) elevations contribute to upper (lower) tropospheric heat sources

Re-examination of the I-5 dust storm

The infamous dust storm over the thanksgiving holiday of 1991 that led to loss of life from numerous automobile accidents on Interstate 5 (I-5) has been re-examined. Pauley et al. (1996) conducted an earlier investigation of this dust storm following the tenets of Danielsen's paradigm—a paradigm that links the tropopause fold phenomenon and a balanced thermally indirect circulation about the upper level jet stream. However, a cursory examination of mesoscale structures in the storm from the North American Regional Reanalysis (NARR) indicated evidence of a low-level unbalanced thermally direct circulation that demanded further investigation using a high-resolution Weather Research and Forecasting (WRF) model simulation. Principal results from the present study follow: (1) Although the model simulation showed evidence of a weak indirect circulation in the upper troposphere in support of the Danielsen's paradigm, the dynamic control of the storm stemmed from the lower tropospheric mesoscale response to geostrophic imbalance. (2) A lower tropospheric direct circulation led to mass/temperature adjustments that were confirmed by upper air observations at locations in proximity to the accident site, and (3) boundary layer deepening and destabilization due to these mesoscale processes pinpointed the timing and location of the dust storm. Although the present study does not underestimate the value of analyses that focus on the larger/synoptic scales of motion, it does bring to light the value of investigation that makes use of the mesoscale resources in order to clarify synoptic-mesoscale interactions

Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon

Anomalous interactions between the Indian summer monsoon (ISM) circulation and subtropical westerlies are known to trigger breaks in the ISM on subseasonal time-scales, characterised by a pattern of suppressed rainfall over central-north India, and enhanced rainfall over the foothills of the central–eastern Himalayas (CEH). An intriguing feature during ISM breaks is the formation of a mid-tropospheric cyclonic circulation anomaly extending over the subtropical and mid-latitude areas of the Asian continent. This study investigates the mechanism of the aforesaid Asian continental mid-tropospheric cyclonic circulation (ACMCC) anomaly using observations and simplified model experiments. The results of our study indicate that the ACMCC during ISM breaks is part of a larger meridional wave train comprising of alternating anticyclonic and cyclonic anomalies that extend poleward from the monsoon region to the Arctic. A lead–lag analysis of mid-tropospheric circulation anomalies suggests that the meridional wave-train generation is linked to latent heating (LH) anomalies over the CEH foothills, Indo-China, and the Indian landmass during ISM breaks. By conducting sensitivity experiments using a simplified global atmospheric general circulation model forced with satellite-derived three-dimensional LH, it is demonstrated that the combined effects of the enhanced LH over the CEH foothills and Indo-China and decreased LH over the Indian landmass during ISM breaks are pivotal for generating the poleward extending meridional wave train and the ACMCC anomaly. At the same time, the spatial extent of the mid-latitude cyclonic anomaly over Far-East Asia is also influenced by the anomalous LH over central–eastern China. While the present findings provide interesting insights into the role of LH anomalies during ISM breaks on the poleward extending meridional wave train, the ACMCC anomaly is found to have important ramifications on the daily rainfall extremes over the Indo-China region. It is revealed from the present analysis that the frequency of extreme rainfall occurrences over Indo-China shows a twofold increase during ISM break periods as compared to active ISM conditions. © 2019, The Author(s)

Invloed van verontreiniging (kust en scheepvaart) op het ecosysteem in kustwater en open zee

An intriguing feature associated with ‘breaks’ in the Indian summer monsoon is the occurrence of intense/flood-producing precipitation confined to central-eastern parts of the Himalayan (CEH) foothills and north-eastern parts of India. Past studies have documented various large-scale circulation aspects associated with monsoon-breaks, however the dynamical mechanisms responsible for anomalous precipitation enhancement over CEH foothills remain unclear. This problem is investigated using diagnostic analyses of observed and reanalysis products and high-resolution model simulations. The present findings show that the anomalous precipitation enhancement over the CEH foothills during monsoon-breaks emerges as a consequence of interactions between southward intruding mid-latitude westerly troughs and the South Asian monsoon circulation in its weak phase. These interactions facilitate intensification of mid-tropospheric cyclonic vorticity and strong ascending motion over the CEH foothills, so as to promote deep convection and concentrated rainfall activity over the region during monsoon-breaks. Mesoscale orographic effects additionally tend to amplify the vertical motions and precipitation over the CEH foothills as evidenced from the high-resolution model simulations. It is further noted from the model simulations that the coupling between precipitation and circulation during monsoon-breaks can produce nearly a threefold increase of total precipitation over the CEH foothills and neighborhood as opposed to active-monsoon conditions

Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon

Anomalous interactions between the Indian summer monsoon (ISM) circulation and subtropical westerlies are known to trigger breaks in the ISM on subseasonal time-scales, characterised by a pattern of suppressed rainfall over central-north India, and enhanced rainfall over the foothills of the central–eastern Himalayas (CEH). An intriguing feature during ISM breaks is the formation of a mid-tropospheric cyclonic circulation anomaly extending over the subtropical and mid-latitude areas of the Asian continent. This study investigates the mechanism of the aforesaid Asian continental mid-tropospheric cyclonic circulation (ACMCC) anomaly using observations and simplified model experiments. The results of our study indicate that the ACMCC during ISM breaks is part of a larger meridional wave train comprising of alternating anticyclonic and cyclonic anomalies that extend poleward from the monsoon region to the Arctic. A lead–lag analysis of mid-tropospheric circulation anomalies suggests that the meridional wave-train generation is linked to latent heating (LH) anomalies over the CEH foothills, Indo-China, and the Indian landmass during ISM breaks. By conducting sensitivity experiments using a simplified global atmospheric general circulation model forced with satellite-derived three-dimensional LH, it is demonstrated that the combined effects of the enhanced LH over the CEH foothills and Indo-China and decreased LH over the Indian landmass during ISM breaks are pivotal for generating the poleward extending meridional wave train and the ACMCC anomaly. At the same time, the spatial extent of the mid-latitude cyclonic anomaly over Far-East Asia is also influenced by the anomalous LH over central–eastern China. While the present findings provide interesting insights into the role of LH anomalies during ISM breaks on the poleward extending meridional wave train, the ACMCC anomaly is found to have important ramifications on the daily rainfall extremes over the Indo-China region. It is revealed from the present analysis that the frequency of extreme rainfall occurrences over Indo-China shows a twofold increase during ISM break periods as compared to active ISM conditions

Anhörigas uppfattning om delaktighet vid samordnad vårdplanering

The origin of two separate Southern High Plains dust storms, which occurred over a 2 day period in February 2007, is traced to an interaction between the subtropical jet (STJ) and the polar jet (PJ). A large-scale thermal wind imbalance resulting from the confluence of these two jets led to a series of mesoscale circulations that ultimately produced the dust storms. Understanding the connectivity between the dust storms with differing geometries is central to the present investigation. The study rests on the interpretation of analyses from upper air and surface observations complemented by imagery from satellites, the 32 km gridded data set from the North American Regional Reanalysis, and a fine-resolution (6 km grid) simulation from the Weather Research and Forecasting model. Principal assertions from the present study are (1) scale interaction is fundamental to the creation of an environment conducive to dust storm development, (2) low to middle tropospheric mass adjustment is the primary response to a large-scale imbalance, (3) the mesoscale mass adjustment is associated with circulations about a highly accelerative jet streak resulting from the merger of the PJ and STJ, (4) the structure of the jet streak resulting from this merger governs the evolution of the geometry of the dust plumes, with plumes that initially had a straight-line orientation developing a semicircular geometry, and (5) it is concluded that improvements in dust storm prediction will depend on an augmentation to the upper air network in concert with a flow-dependent data assimilation strategy

Subtropical-polar jet interactions in Southern Plains dust storms

[1] The origin of two separate Southern High Plains dust storms, which occurred over a 2 day period in February 2007, is traced to an interaction between the subtropical jet (STJ) and the polar jet (PJ). A large-scale thermal wind imbalance resulting from the confluence of these two jets led to a series of mesoscale circulations that ultimately produced the dust storms. Understanding the connectivity between the dust storms with differing geometries is central to the present investigation. The study rests on the interpretation of analyses from upper air and surface observations complemented by imagery from satellites, the 32 km gridded data set from the North American Regional Reanalysis, and a fine-resolution (6 km grid) simulation from the Weather Research and Forecasting model. Principal assertions from the present study are (1) scale interaction is fundamental to the creation of an environment conducive to dust storm development, (2) low to middle tropospheric mass adjustment is the primary response to a large-scale imbalance, (3) the mesoscale mass adjustment is associated with circulations about a highly accelerative jet streak resulting from the merger of the PJ and STJ, (4) the structure of the jet streak resulting from this merger governs the evolution of the geometry of the dust plumes, with plumes that initially had a straight-line orientation developing a semicircular geometry, and (5) it is concluded that improvements in dust storm prediction will depend on an augmentation to the upper air network in concert with a flow-dependent data assimilation strategy

Monsoon-extratropical circulation interactions in Himalayan extreme rainfall

Extreme precipitation and flood episodes in the Himalayas are oftentimes traced to synoptic situations involving connections between equatorward advancing upper level extratropical circulations and moisture-laden tropical monsoon circulation. While previous studies have documented precipitation characteristics in the Himalayan region during severe storm cases, a comprehensive understanding of circulation dynamics of extreme precipitation mechanisms is still warranted. In this study, a detailed analysis is performed using rainfall observations and reanalysis circulation products to understand the evolution of monsoon-extratropical circulation features and their interactions based on 34 extreme precipitation events which occurred in the Western Himalayas (WEH) during the period 1979–2013. Our results provide evidence for a common large-scale circulation pattern connecting the extratropics and the South Asian monsoon region, which is favorable for extreme precipitation occurrences in the WEH region. This background upper level large-scale circulation pattern consists of a deep southward penetrating midlatitude westerly trough, a blocking high over western Eurasia and an intensifying Tibetan anticyclone. It is further seen from our analysis that the key elements of monsoon-midlatitude interactions, responsible for extreme precipitation events over the WEH region, are: (1) midlatitude Rossby wave breaking, (2) west-northwest propagation of monsoon low-pressure system from the Bay of Bengal across the Indian subcontinent, (3) eddy shedding of the Tibetan anticyclone, (4) ageostrophic motions and transverse circulation across the Himalayas, and (5) strong moist convection over the Himalayan foothills. Furthermore, high-resolution numerical simulations indicate that diabatic heating and mesoscale ageostrophic effects can additionally amplify the convective motions and precipitation in the WEH region

Linkage of water vapor distribution in the lower stratosphere to organized Asian summer monsoon convection

Accumulation of water vapor in the upper troposphere/lower stratosphere (UT/LS) over the Asian continent is a recognized feature during the boreal summer monsoon. While there has been a debate on the role of monsoon convective intensities on the UT/LS water vapor accumulations, there are ambiguities with regard to the effects of organized monsoon convection on the spatial distribution of water vapor. We provide insights into this aspect using high precision balloon measurements of water vapor from a high-elevation site Nainital (29.4° N, 79.5° E), India, located in the Himalayan foothills and satellite retrievals of water vapor from the Microwave Limb Sounder (MLS). We also use precipitation estimates from the Tropical Rainfall Measuring Mission (TRMM) satellite (i.e., merged product 3B42 and precipitation radar 3A25 estimates of rain rate and rain type viz convective/stratiform), reanalysis circulation data, as well as numerical model simulations. We first evaluate the MLS estimates of water vapor mixing ratios with in situ high precision hygrometer balloon observations over Nainital. It is seen from our analyses of the MLS data that the LS water vapor distribution is closely linked to the organization of the South Asian monsoon convection and its influence on the UT/LS circulation. This link between LS water vapor distribution and organized monsoon convection is also captured in the in situ observations on 3 August 2016. It is evidenced that periods of organized summer monsoon convective activity over the Indian subcontinent and Bay of Bengal promote divergence of water vapor flux in the UT/LS; additionally the Tibetan anticyclonic circulation causes widespread distribution of the UT/LS water vapor. In addition to the effects of Asian monsoon convection, we also note that global climate drivers such as El Niño-Southern Oscillation (ENSO), Brewer–Dobson circulation (BDC), and Quasi-Biennial Oscillation (QBO) can contribute to nearly 38% of the UT/LS water vapor variability over the Asian monsoon region. The main result of our study indicates that widespread spatial distribution and accumulation of water vapor in the LS (about 80% of total accumulation between May and August months) tend to co-occur with organized monsoon convection, intensified divergence of water vapor flux in the UT/LS and intensified Tibetan anticyclone. On the other hand, the circulation response and LS water vapor distribution to pre-monsoon localized deep convection tend to have a limited spatial scale confined to Southeast Asia. Results from model experiments suggest that the UT/LS circulation pattern to organized monsoon convection has resemblance to stationary Rossby waves forced by organized latent heating, with the westward extending response larger by about 15° longitudes as compared to that of the pre-monsoon localized deep convection