Monsoon Low-level Jet Research Articles

The Monsoon Low-Level Jet: Climatology and Impact on Monsoon Rainfall over the West Coast and Central Peninsular India

The Monsoon Low-Level Jet: Climatology and Impact on Monsoon Rainfall over the West Coast and Central Peninsular India

Is the Monsoon climatology observed over the National Capital Region, Delhi indicative of an Urban -Monsoon linkage on rainfall modification?

The Indian Summer Monsoon (ISM) plays a vital role in the lives of the National Capital Region (NCR) - Delhi, the second biggest urban settlement globally. This study attempt to assess the spatio-temporal characters and decadal changes of monsoonal features in detail. The climatology of Monsoonal rainfall and Monsoon Low Level Jet (MLLJ) are investigated. NCR Delhi exhibits a ∼ 28.5% in the recent decade and ∼ 19.7% in the 1997–06 decade reduction in rainfall is noticed compared with 1987–96 decade. The study reveals that the precipitation reduction is alongside the weakening of the MLLJ and marginal reduction of core height over the region. Also, the magnitude of surface wind declined with the directional shift of both MLLJ and surface winds notable in southerlies in the 2007–16 decade. Rainfall islands parallel to the urban heat islands (UHI) are seen likely as coupled monsoon-urban induced effects under the weakened synoptic regime evidenced through MLLJ. The Land Surface Temperature difference (UHI) of ∼ 2.5 °C and more than 3.5 °C is observed from the urban centre to the surrounding cropland during the months of August (core monsoon) and September, respectively. In addition to the rainfall islands during August, islands of rainfall in both the upwind and downwind directions of the urban centre are noted in September. The rainfall patterns point out the signal of monsoon-rainfall modification due to NCR urbanization. Further, the influence of crosswinds with Aravali ridges and the presence of Himalayan foothills are also notable in the NCR rainfall patterns.

Dynamic downscaling ensemble forecast of an extreme rainstorm event in South China by COSMO EPS

In this study, the high-resolution ensemble prediction system COSMO (Consortium for Small Scale) EPS is used to predict the extreme rainstorm that occurred from 27 to 31 August 2018 in Guangdong Province, China, which leads to intensities exceeding historical extreme values. COSMO EPS is run with a 2.8-km grid spacing, allowing for an explicit treatment of deep convection, and 24 members of the EPS are initialized and laterally driven by the ICON (ICOsahedral Nonhydrostatic) global model. We compare the predictions of COSMO EPS against observations derived from the global precipitation measurement (GPM) and with ensemble forecasts of both mesoscale EPS and global EPS provided by GRAPES (Global and Regional Assimilation and PrEdiction System), and with the deterministic forecasts of global models ICON and ECMWF (European Centre for Medium-Range Weather Forecasts). Model performances are evaluated both by gridpoint-based scores, such as the equitable threat score (ETS), and by the Method for Object-based Diagnostic Evaluation (MODE) for spatial verification. According to our results, COSMO EPS could perform better forecasts for the rainstorms taken place in eastern Guangdong than other models. However, the location and coverage area of its predicted rainstorm is eastward and smaller in contrast with the observations. Therefore, COSMO EPS exhibits relative high performance by object-based spatial evaluations, while it could not display evident superiority in terms of the gridpoint-based scores. The cause analysis of this extreme rainstorm shows that Guangdong Province of China is mainly affected by monsoon depression. Southwesterly and southerly winds continuously transport water vapor from the South China Sea to Guangdong Province. The southwest monsoon low-level jet advances northward over time, which promotes the occurrence and development of continuous heavy precipitation in the coastal areas of Guangdong. In an additional experiment, we investigate the benefit of assimilation of radar data, by applying the latent heat nudging (LHN) approach based on surface-based radar observations to the COSMO EPS. Subsequently, the prediction by assimilation of radar data more reasonably reproduces the spatial distribution of precipitation observations, while the coverage and intensity of the rainstorm in eastern Guangdong are still not reflected satisfactorily.

Observed features of monsoon low-level jet and its relationship with rainfall activity over a high-altitude site in Western Ghats, India

Observed features of monsoon low-level jet and its relationship with rainfall activity over a high-altitude site in Western Ghats, India

Impact of Aeolus horizontal line of sight wind observations in a global NWP system

This study evaluates the impact of Aeolus horizontal line-of-sight (HLOS) wind observations in the National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Model (NCUM) assimilation and forecast system from 20 May to 22 June 2020. The quality of both Rayleigh clear and Mie cloudy Aeolus HLOS winds are validated during the study period and found to be useful for assimilation. Observing System Experiment (OSE) and adjoint-based Forecast Sensitivity to Observation Impact (FSOI) methods are used to assess the impact of the Aeolus HLOS wind observations in the global NCUM system. The Indian Summer Monsoon onset and the life cycle of a severe cyclonic storm Nisarga formed over the eastern Arabian Sea are investigated. The FSOI study clearly shows that HLOS wind observations significantly impact the model forecast, especially over the Tropics and Southern Hemisphere extra-tropics. The impact of HLOS winds in the upper troposphere and above is more marked. Forecasts with the assimilation of the HLOS winds enhanced the circulation during the onset of the Indian summer monsoon and weakened the Monsoon Low Level Jet (MLLJ) when the Nisarga cyclone formed over the Arabian Sea. The high-resolution wind observations from Aeolus reduced the position error of the tropical cyclone Nisarga in the NCUM analysis.

Role of unusual moisture transport across Equatorial Indian Ocean on the extreme rainfall event during Kerala flood 2018

Southwestern Indian state, Kerala experienced extreme devastating statewide flood event of the century during 2018 monsoon season. In this study, an attempt has been made to bring out the salient dynamical factors contributed to the Kerala flood. There were 3 active spells over Kerala during 2018 Monsoon season. All the three spells were accustomed with the intrinsic factors of low frequency components of the active spells such as strength of monsoon Low Level Jet (LLJ), Monsoon depressions in the Bay of Bengal, favorable Madden-Julian oscillation (MJO) phases and Western Pacific systems. Though all the common ingredients remain same, the third spell is distinct with the less evaporation flux over Western and Central Arabian Sea and unusual moisture transport from maritime continent through South Equatorial Indian ocean (SEIO) towards the Kerala coast across Equator. Strong meridional pressure gradient force created by the combined effect of high pressure anomaly oriented Northwest-Southeast direction across tropical Indian ocean and anomalous low pressure along monsoon trough might have contributed to this unusual moisture transport across SEIO originating from west of Australia. The anomalous high pressure in South Indian ocean was greatly influenced by the position of the Mascarene high. Subtropical Indian ocean dipole (SIOD) also exhibits an influential role by altering tropical Southern Indian ocean dynamics in favor of the unusual moisture transport. The position of the monsoon depression and presence of typhoons in Western Pacific might have aided to this moisture transport. However, the normal moisture transport from Central Arabian sea towards Kerala coast by virtue of the strong LLJ along with additional moisture transport directly from South of maritime continent through SEIO across the Equator towards Kerala coast might have played a dominant role in the historical flood event over entire Kerala state.

Oceanic Mixing over the Northern Arabian Sea in a Warming Scenario: Tug of War between Wind and Buoyancy Forces

AbstractA consensus of the twenty-first-century climate change in the ocean is surface warming, stratification due to extreme freshening and subsequent weakening of mixing, overturning circulation, and biological production. Counterintuitively, certain parts of the tropical ocean may develop a resistance to changes in mixing, where the climate change impacts of atmosphere and ocean are complementary to each other. Under the poleward shift of monsoon low-level jet (LLJ) in the twenty-first century, a part of the northern Arabian Sea has a tendency to maintain the mixed layer depth intact. The process is studied using a set of high-resolution regional ocean model downscaling experiments for the present and future climate. It is found that the wind intensification caused by the shift in LLJ tends to counteract the stratification gained by surface ocean warming and maintains the mixing process in a warming scenario. The mixing energetics shed light on the way in which this is achieved. Intensified winds promote shear production and surface ocean warming demotes buoyancy production of turbulent kinetic energy (TKE), with a net effect of an increase in TKE. However, TKE appears to be dissipating quickly because of the presence of a larger number of small-scale eddies. This causes the mixing length and mixed layer depth to remain intact. Therefore, the interpretations of impacts of future climate change in ocean mixing should be viewed with caution, at least regionally, by focusing on the detailed changes of the governing mechanisms.

Existence of ‘mini-break’ like condition during early Indian monsoon onset years

In the context of increasing probability of advancement in onset date of Indian Summer Monsoon during recent years, precipitation and circulation features of early monsoon onset events are investigated in detail using reanalysis data and model simulations. During the onset pentad (± 2 days from the onset day), an anomalous increase in precipitation is observed over the west coast of peninsular India and Bay of Bengal during early onset years compared to normal onset years. The monsoon low level jet (LLJ) also becomes stronger during the onset pentad of early onset years particularly over the southern peninsular India. However, after a week from the onset day of early years, precipitation is considerably decreased and the LLJ become weaker. Moreover, convection over the eastern Arabian Sea and western peninsular India suppresses during that period which in turn inhibits the systematic northward propagation of monsoon rainbands. This induces a ‘mini break’ like condition immediately after a week from the onset day during early onset years. Analysis of the sea surface temperature (SST) shows that, over the north eastern Arabian Sea, a lower value of SST exists during early onset years compared to normal onset years which may have a triggering role in the formation of ‘mini break’ like condition. This aspect is examined using sensitivity simulations with the Advanced Research Weather Research and Forecasting model. Simulations with reduced SST over the north eastern Arabian Sea during normal onset years exhibits considerable reduction in precipitation over the west coast of India and weakens the LLJ over the peninsular India. Whereas simulations with enhanced SST over the north eastern Arabian Sea during early onset years show increase in the convective activities over the eastern Arabian Sea and west coast of peninsular India, and the existence of ‘mini break’ like situation disappears from those years. Thus numerical simulations confirm the relation between SST over the north eastern Arabian Sea and the formation of ‘mini break’ during early onset years.

Remote influences on the Indian monsoon low-level jet intraseasonal variations

A strong low-level jet (LLJ), also known as the Findlater jet, develops over the Arabian Sea during the Indian summer monsoon. This jet is an essential source of moisture for monsoonal rainfall over the densely-populated Indian subcontinent and is a key contributor to the Indian Ocean oceanic productivity by sustaining the western Arabian Sea upwelling systems. The LLJ intensity fluctuates intraseasonally within the ~ 20- to 90-day band, in relation with the northward-propagating active and break phases of the Indian summer monsoon. Our observational analyses reveal that these large-scale regional convective perturbations only explain about half of the intraseasonal LLJ variance, the other half being unrelated to large-scale convective perturbations over the Indian Ocean. We show that convective fluctuations in two regions outside the Indian Ocean can remotely force a LLJ intensification, four days later. Enhanced atmospheric deep convection over the northwestern tropical Pacific yields westerly wind anomalies that propagate westward to the Arabian Sea as baroclinic atmospheric Rossby Waves. Suppressed convection over the eastern Pacific/North American monsoon region yields westerly wind anomalies that propagate eastward to the Indian Ocean as dry baroclinic equatorial Kelvin waves. Those largely independent remote influences jointly explain ~ 40% of the intraseasonal LLJ variance that is not related to convective perturbations over the Indian Ocean (i.e. ~ 20% of the total), with the northwestern Pacific contributing twice as much as the eastern Pacific. Taking into account these two remote influences should thus enhance the ability to predict the LLJ.

Variability of monsoon low‐level jet and associated rainfall over India

AbstractThe structure and climatology of the monsoon low‐level jet (MLLJ) is studied based on dynamically downscaled simulations over a 37‐year period (1980–2016) using the Weather Research Forecasting (WRF) model. The simulations are conducted by adopting a continuous initialization method with daily re‐initializations using ERA‐Interim data as initial and boundary conditions. Validation of the downscaled fields with radiosonde data shows that the model has reasonable skill in reproducing MLLJ characteristics. Analysis of the simulations suggests that the MLLJ exhibits systematic diurnal variation: maximum winds of the synoptically induced large‐scale monsoon jet occur during the daytime, and the orographic channelled winds through the mountains of East Africa, Hejaz, and Western Ghats in the night‐time. These diurnal changes in monsoon winds modulate the moisture convergence process and the associated evolution of rainfall over India. Seasonal and monthly climatology of monsoon winds shows that the model accurately reproduces the spatial pattern of winds and slightly overestimates (2–3 m/s) the mean monthly winds over the Bay of Bengal and Arabian Seas. Analysis of wind variability and the trends using 37 years simulations suggests that the MLLJ exhibits an increasing trend in wind speed on both seasonal and monthly scales, except for August which shows a decreasing trend. The weakening of the MLLJ in August has a profound influence on the number of monsoon depressions forming over the Bay of Bengal (which are decreasing), and on the number of break days (which are increasing) and associated precipitation reduction over the central Indian region.

Revisiting Moisture Transport During the Indian Summer Monsoon Using the Moisture River Concept

The moisture river (MR) concept is used here to revisit the characteristics of moisture transport during the Indian Summer Monsoon (ISM) season from June to September. The study is carried out using Modern-Era Retrospective analysis for Research and Applications (MERRA) data products for the 30-year period from 1982 to 2011. MRs are detected from vertically integrated vapor transport (IVT) computed using validated MERRA mixing ratios and horizontal wind speeds at vertical levels from the surface to 300 hPa. The characteristics of MR and moisture transport associated with major rainfall episodes of the 2011 ISM are studied in detail. The large moisture content, together with high wind speeds associated with the monsoon low-level jet, contributes to the observed structure of the MR. The outflux of moisture from the Arabian Sea is on average 40% larger than the cross-equatorial flux from the southern Indian Ocean, indicating increased contribution from the Arabian Sea. The upward movement of moisture to the layers above the jet is a decisive factor in the strengthening of the river and major rainfall episodes. Back-trajectory analyses reveal that the path of moisture transport from the southern Indian Ocean is confined to the lower levels, whereas higher-level trajectories originate from the Red Sea, Arabian Sea and southern Indian Ocean. The MR statistics show significant intra-seasonal and inter-annual variability in occurrence and strength, with a near-linear relationship between the IVT strength and the corresponding daily ISM rainfall, especially over the west coast. Wavelet cross-spectral analyses carried out using the daily Arabian Sea IVT maximum and rainfall over the west coast and central India during a deficit and surplus monsoon year depict significant differences in the dominant periodicities, associated with intra-seasonal and inter-annual monsoon variations.

A Study on the Structural Transformation of the Monsoon Low-Level Jet Stream on Its Passage Over the South Asian Region

A comprehensive study of the cross-sectional structure of the cross-equatorial low-level jet stream (LLJ) at various locations over the Asian summer monsoon region has been carried out for the period 1979–2014 using a reanalysis dataset. Structural transformation of the LLJ along its path over the south Indian Ocean (SIO), cross-equatorial region, Arabian Sea, Indian land region and Bay of Bengal (BoB), has been investigated by taking the vertical cross section of the jet stream perpendicular to the direction of its flow. During the passage of the LLJ over the monsoon area, the core of the LLJ is lower over the oceanic region, whereas the core height increases over land. The LLJ has its maximum wind speed at a lower height (925 hPa) over the Indian Ocean region, whereas the core height raises to around 850 hPa over the Indian sub-continent. The strength of the monsoon LLJ is found to be varying in different temporal scales. The LLJ is much stronger in the active monsoon spells than in break periods, except in the SIO sector and BoB. During monsoon breaks, the LLJ axis shifts poleward over the Arabian Sea, and the intensity of wind over the SIO and BoB increases. The strength of the LLJ is greater during wet monsoon years than in dry monsoon years, except in the BoB zone. The changes noted in the monsoon LLJ during dry monsoon years is possibly due to the increased number of break days observed in the recent decades.

Changes in the La Niña teleconnection to the Indian summer monsoon during recent period

The interannual variability of Indian Summer Monsoon (ISM) rainfall is strongly associated with the El Niño–Southern Oscillation (ENSO), experiencing below and above normal rainfall during El Niño and La Niña years respectively. However, during recent La Niña years, particularly after 1980, the above normal ISM rainfall events are decreased compared to pre-1980 years. The strength of prominent monsoon circulations such as monsoon low level jet and tropical easterly jet are also decreased during recent La Niña years compared to pre-1980. In the spatial scale, the decrease in rainfall observes more over the monsoon trough region and the western part of Indian subcontinent. The number of active monsoon rainfall days, which is an identification of the intraseasonal ISM variability, are considerably reduced during post-1980 La Niña years. The warming over the Indian ocean during recent years influence the anomalous cooling observe over the western Indian ocean during La Niña events. The Indian ocean warming leads to an enhancement of convection over the equatorial region. Latent heat release from the enhanced equatorial convection warms the equatorial troposphere and weakens the meridional tropospheric temperature gradient over the monsoon region. These processes degrade the ascending motion over the Indian region, resulting weaker monsoon circulation and reduced rainfall.

UHF wind profiler observations of monsoon low-level jet (MLLJ) and its association with rainfall over a tropical Indian station

High resolution data of horizontal winds profiles (zonal and meridional) in the lower troposphere, derived from a UHF wind profiler at a tropical Indian station, Pune (18o 32’ N, 73o 51’ E, 559 masl) during a 3-yr period (June 2003-May 2006) has been utilized to study seasonal and intra-seasonal variability of winds. Winds display a systematic seasonal evolution with behavior opposite in phase in the two altitude regimes below and above a height of 4-5km. In the lower region, during the southwest monsoon months (June to September) winds are predominantly westerly with a peak in the 1.5-3.0km range indicating the occurrence of the monsoon low-level jet (MLLJ). Soon after September, winds in this height region change from westerly to easterly and these easterlies continue in winter months (December to February). Above a height of 4km, westerlies are observed during post-monsoon (October to November) and winter periods. The MLLJ is observed to be strong during normal/good monsoon years. On a day-to-day scale during southwest monsoon months, winds exhibit considerable intra-seasonal variability and periods of strong MLLJ seem to be associated with occurrence of spells of rainfall over the region.

Intensification of upwelling along Oman coast in a warming scenario

AbstractThe oceanic impact of poleward shift in monsoon low‐level jet (MLLJ) is examined using a Regional Ocean Modeling System (ROMS). Two sets of downscaling experiments were conducted using ROMS with boundary and initial conditions from six CMIP5 models. While outputs from the historical run (1981–2000) acts as forcing for the first, the second uses RCP8.5 (2080–2099). By comparing the outputs, it is found that Oman coast will experience an increase in upwelling in tune with MLLJ shift. Consistent with the changes in upwelling and zonal Ekman transport, temperature, salinity, and productivity show significant changes near the Oman coast. The changes in MLLJ causes the coastal wind to angle against the Oman coast in such a fashion that the net upwelling increases in the next century and so does the marine productivity. This study contrasts the general view of weakening of upwelling along the Arabian coasts due to the weakening of monsoon winds.

Monsoon low-level jet over the gateway of Indian summer monsoon: a comparative study for two distinct monsoon years

High-resolution radiosonde measurements are used to study the characteristics and dynamics of monsoon low-level jet at the monsoon onset region of Cochin (10.04 $$^\circ$$ N; 76.32 $$^\circ$$ E) in India under two contrasting monsoon years, 2013 and 2015. The core speed and core height of the low-level jet is significantly higher during the strong monsoon year of 2013 than for the monsoon-deficient year of 2015. The average core heights for these years are seen to exist at 2.03 and 2.20 km, respectively. The low-level jet-modulated parameters such as moisture flux, momentum flux and kinetic energy flux show higher values during monsoon of 2013 as compared to 2015. Among the monsoon low-level jet parameters, the moisture flux has the strongest influence on the observed rainfall over Cochin. Also, an exponential function is seen to best explain the moisture flux–rainfall relationship. The weakening of monsoon during 2015 is attributed most likely to an eastward shift of the core convective activity from the Indian subcontinent as revealed from satellite observation of the upper tropospheric humidity. A close association is seen between the rainfall over Cochin and the convective activity over the Indian subcontinent. Observational studies such as this, which links monsoon rainfall, monsoon low-level jet parameters and convective activity, are expected to enhance the understanding of monsoon processes in general and subsequently improve the forecasting skill of models.

Changes in the south Asian monsoon low level jet during recent decades and its role in the monsoon water cycle

June–September mean wind at 850hPa from ERA-Interim, MERRA and NCEP2 reanalyses shows an increasing trend in the south Asian monsoon Low Level Jet (LLJ) during 1980–2014. In the sub-seasonal scale, the LLJ during July and September exhibits increasing trend, while August shows a decreasing trend. Lesser changes in surface pressure over heat low region and weaker Bay of Bengal convection lead to weakening of LLJ during August while an intense heat low during September results stronger LLJ. The associated moisture transport changes affect the monsoon hydrological cycle with decreasing precipitation during August and increasing precipitation during September.

Role of surface and boundary layer processes in the temporal evolution of Monsoon Low Level Jet (MLLJ) observed from high resolution Doppler wind lidar measurements

Monsoon Low Level Jet (MLLJ) is one of the important components of Indian summer monsoon. Using high-resolution measurements of boundary layer wind profiles from a Doppler wind lidar at Mahbubnagar (16.73°N, 77.98°E and 445m above mean sea level), India, the temporal evolution of the MLLJ has been investigated. Both jet core height and jet speed show clear diurnal variation during the monsoon season. Jet core starts descending down in the evening hours and remains at a height between 600m and 900m (above surface level) during nighttime. Soon after local sunrise, jet core starts ascending and reaches heights above 1800m by afternoon hours. Jet core speed starts strengthening in the nighttime and attains maximum intensity in the early morning hours and then the core speeds decrease around the time when the jet core is at its maximum height. Simultaneous diurnal variations of surface temperature, sensible heat flux, latent heat flux and Richardson number show that daytime heating and turbulence in surface layers enhance mixing in the daytime boundary layer which helps in lifting of the jet core. Shear produced turbulence and momentum fluxes also play a significant role in the diurnal variation of MLLJ. Factors influencing the evolution of the convective boundary layer and factors responsible for formation of nocturnal boundary layer are closely associated with the diurnal variation of MLLJ occurring over the low-latitude south Indian peninsular region during monsoon season. The results emphasize the importance of continuous and high spatial-temporal resolution wind profile measurements in the monsoon boundary layer from a Doppler wind lidar.

Diurnal variations of the low-level jet over peninsular India during the onset of Asian summer monsoon

The evidence for diurnal variation of monsoon low-level jet (MLLJ) over the southern tip of India is illustrated from observations, a mesoscale model results, and long-term reanalysis products. This illustration showed that MLLJ has a diurnal variation in its strength and location of maxima. Diurnal signature of MLLJ is not found to be location-specific. Stronger jets are formed at a lower elevation during the nighttime, and the jets are weaker during the day but are higher elevated. During the daytime, a secondary jet is also observed along with the MLLJ. Both the jets were numerically simulated during the onset phase of southwest monsoon over the southwest coast of India. Thirty-year climatological analysis from Modern Era Retrospective analysis for Research and Applications (MERRA) showed that the secondary jet is shallow and weak during the pre-onset phase of monsoon. With the progress of monsoon, the MLLJ gets stronger with increased wind speed and elevated location of its core. In occasions where the jet is strong before the onset, the strongest jets proceed after the onset. Daytime MLLJ is higher elevated compared to the nocturnal MLLJ due to the frictional decoupling.

Time evolution of monsoon low-level jet observed over an Indian tropical station during the peak monsoon period from high-resolution Doppler wind lidar measurements

Doppler wind lidar measurements of horizontal winds at an Indian tropical station, Mahbubnagar (16.73°N, 77.98°E, 445 m above mean sea level), were used to investigate the time evolution of the monsoon low-level jet (MLLJ) during the southwest monsoon season. Vertical profiles of zonal wind in the altitude range of 100 to 3000 m above surface (at every 50 m height interval and 5 min time averaged) obtained during the period 25 July to 23 August 2011 are considered for the analysis. The zonal winds in the altitude up to 3000 m above ground are predominantly westerly throughout the period and on almost all the days there is a westerly wind speed maximum around 500 m above ground during nighttime. Soon after local sunrise, the core of this wind speed maximum (jet) gets lifted up and by afternoon, the westerly wind maximum is shifted to a higher altitude of 2000 m–2500 m without much change in its magnitude. Analysis of the high-resolution lidar data strongly indicates that the same nocturnal LLJ seems to be moving up and evolving into a daytime westerly MLLJ reported in several previous studies. Wind speed and direction derived from the wind lidar agree reasonably well with simultaneously observed GPS upper air sounding wind measurements. Further analysis shows that the time-height evolution of the jet core is closely associated with daytime convection and boundary layer growth. The presence of clouds over the region seems to inhibit this type of time evolution.