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The deep convection and associated moist processes have a major role in regulating the circulation and precipitation characteristics of the Indian summer monsoon. This aspect is examined by conducting sensitivity experiments with the Weather Research and Forecast model. Three active monsoon cases during the periods 16–25 June 2015, 20–29 July 2010 and 1–9 August 2007 are selected for the study. Control simulations using reanalysis data as initial and lateral boundary conditions reveal that the model could simulate mean features of the precipitation and circulation pattern during those active monsoon periods. In sensitivity experiments, microphysical latent heat release in the model is switched off and all other conditions are kept same as that of control simulations. The removal of latent heat release in the model suppresses development of deep convection over the monsoon domain and causes substantial reduction in precipitation. A large-scale descending motion appears in the mid-troposphere and vertical growth of clouds is hampered. As a result, thick cloud bands form in the lower atmosphere, which reduces the short-wave radiation reaching the surface and leading to a reduction in land surface temperature over the Indian region. The cessation of deep convection also affects the strength and position of monsoon low-level circulation. The lack of convective heating shifts the low-level jet core over the Arabian Sea towards north. Consequently, the low-level jet gets strengthened over the north-west India and weakens over the peninsular India. The present study unambiguously established the fact that organized deep convection and concomitant vertical heating over the monsoon domain have a prominent role in regulating monsoon dynamics.

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.

The strong cross-equatorial low level jet stream (LLJ) with its core around 850 hPa of the Asian summer monsoon (June–September) is found to have large intraseasonal variability. During the monsoon onset over Kerala, India, and during break monsoon periods, when the convective heating of the atmosphere is over the low latitudes of the Indian Ocean, the axis of the LLJ is oriented southeastward over the eastern Arabian Sea and it flows east between Sri Lanka and the equator and there is no LLJ through peninsular India. This affects the transport of moisture produced over the Indian Ocean to peninsular India and the Bay of Bengal. In contrast, during active monsoon periods when there is an east–west band of strong convective heating in the latitudes 10°–20°N from about longitude 70° to about 120°E, the LLJ axis passes from the central Arabian Sea eastward through peninsular India and it provides moisture for the increased convection in the Bay of Bengal and for the monsoon depressions forming there. The LLJ does not show splitting into two branches over the Arabian Sea. Splitting of the jet was first suggested by Findlater and has since found wide acceptance as seen from the literature. Findlater’s findings were based on analysis of monthly mean winds. Such an analysis is likely to show the LLJ of active and break monsoons as occurring simultaneously, suggesting a split. Strengths of the convective heat source (OLR) over the Bay of Bengal and the strength of the LLJ (zonal component of wind) at 850 hPa over peninsular India and also the Bay of Bengal between latitudes 10° and 20°N have the highest linear correlation coefficient at a lag of 2–3 days, with OLR leading. The LLJ crossing the equator close to the coast of East Africa will pass through India only if there is active monsoon convection in the latitude belt 10°–20°N over south Asia. The position in latitude of the LLJ axis between longitudes 70° and 100°E is decided by the south–north movement of the east–west convective cloud band of the monsoon in its 30–50-day oscillation. When there is little convection over south Asia in the latitude belt 10°–20°N, the LLJ crossing the equator curves clockwise over the Arabian Sea under conservation of potential vorticity and by-passing India passes east close to the equator. It is speculated that the cyclonic vorticity associated with this low-latitude LLJ causes convergence in the boundary layer and consequent upward motion in the atmosphere resulting in the formation of a convective cloud band there that later moves into the Bay of Bengal as part of the monsoon’s 30–50-day oscillation. Since LLJ is very important in monsoon dynamics, monsoon modelers should take adequate care to see that LLJ and the associated deep convection and their intraseasonal variability are properly simulated in their models.

Regional distribution of precipitation during the onset phase of the Indian summer monsoon (15 May–15 June) shows distinct patterns in the years 2009 and 2010, with the latter having considerably more precipitation over the southeast Arabian Sea (AS) and the west coast of peninsular India. During these years, the location and regional extent of the warm pool in the AS are also distinct. In 2009, the warm pool core is located in the equatorial region, whereas in 2010 it spreads to a wide region of the AS. Sensitivity experiments with different SST forcings have been carried out using the Weather Research and Forecasting (WRF) model to understand the influence of the AS warm pool on the monsoon onset precipitation characteristics. Simulations with actual SSTs in the AS and climatological SSTs elsewhere are able to reproduce the distinct behavior of the monsoon onset precipitation observed during 2009 and 2010. These simulations show suppressed convection over the central and northern AS in 2009, while warmer SSTs in the AS favor enhanced convection during 2010 combined with a sharp contrast in the moisture transport. The strong intrusion of drier air from the north AS effectively confines the moist air mass from the south, causing a net transport of moisture toward the southwest coast of peninsular India and leads to positive anomalies in precipitation over the region in 2010. However, during 2009, the drier air from the north mixes rather easily over the AS, which suppress the convection.

Accurate renditions of country-scale methane (CH 4 ) emissions are critical in understanding the regional CH 4 budget and essential for adapting national climate mitigation policies to curtail the atmospheric build-up of this greenhouse gas with high warming potential. India housing 30% of the Asian population is currently appraised as a region of CH 4 source based on the inventories. To date, there have not been many reported efforts to estimate the regional CH 4 emissions using direct measurements of boundary layer CH 4 concentrations at multiple locations over India. Here, 2 years (2017–2018) of in situ CH 4 observations from three distantly placed stations over the peninsular India is combined with state-of-the-art inversion using a Lagrangian particle dispersion model for the estimation of CH 4 emission. This study updates CH 4 emission over the peninsular India (land area south of 21.5°N) as ~ 10.63 Terra gram (Tg) CH 4 year −1 , which is 0.13 Tg CH 4 year −1 higher than the existing inventory-based emission. On seasonal scale, the changes from the existing CH 4 emission inventories are 0.12, 0.05, 0.055 and 0.28 Tg CH 4 year −1 during winter, pre-monsoon, monsoon and post-monsoon seasons respectively. Spatial distributions of seasonal variability of posterior emissions suggest an enhancement over the eastern region of peninsular India compared to the western part. The study with observations from three stations over the peninsular India provides an update on the inventory-based estimation of CH 4 emissions and urges the importance of more observations over the Indian region for the accurate estimation of fluxes.

The Advanced Research Weather Research and Forecasting (AR-WRF) model is used to study the influence of Western Ghats situated along the west cost of peninsular India in the mean characteristics of the Asian summer monsoon (ASM) through numerical simulations. A control simulation (CTRL) is carried out using 11-year (2000–2010) mean initial and lateral boundary conditions from the ERA-Interim reanalysis to simulate the mean atmospheric features of the ASM. The Modern-Era retrospective analysis for research and applications (MERRA) data along with the Tropical Rainfall Measuring Mission (TRMM, 3B42 daily rainfall) data are used to validate the CTRL simulation. The simulated dynamical features and precipitation characteristics during the ASM period agree well with the MERRA reanalysis and TRMM observations. In order to examine the role of Western Ghats on the mean characteristics of the ASM, a sensitivity simulation (NoWG) is carried out with orography reduced to surface over a domain bound between 5°–28°N and 72°–90°E, keeping all other conditions unchanged. This sensitivity analysis showed an enhancement in the low level monsoon flow over the Indian Ocean and peninsular India in the absence of Western Ghats. The prominent up-draft over the west coast of peninsular India observed in the CTRL simulation also decrease in the absence of Western Ghats. The simulated rainfall show a considerable decrease over the west coast and an enhancement over the east coast of peninsular India in the absence of Western Ghats. These simulations clearly depict the importance of Western Ghats in the circulation dynamics and rainfall features during the ASM period.

The Weather Regional Forecast (WRF) model is used in this study to downscale low-resolution data over West Africa. First, the performance of the regional model is estimated through contemporary period experiments (1981–1990) forced by ARPEGE-CLIMAT GCM output (ARPEGE) and ERA-40 re-analyses. Key features of the West African monsoon circulation are reasonably well represented. WRF atmospheric dynamics and summer rainfall compare better to observations than ARPEGE forcing data. WRF simulated moisture transport over West Africa is also consistent in both structure and variability with re-analyses, emphasizing the substantial role played by the West African Monsoon (WAM) and African Easterly Jet (AEJ) flows. The statistical significance of potential climate changes for the A2 scenario between 2032 and 2041 is enhanced in the downscaling from ARPEGE by the regional experiments, with substantial rainfall increases over the Guinea Gulf and eastern Sahel. Future scenario WRF simulations are characterized by higher temperatures over the eastern Tropical Atlantic suggesting more evaporation available locally. This leads to increased moisture advection towards eastern regions of the Guinea Gulf where rainfall is enhanced through a strengthened WAM flow, supporting surface moisture convergence over West Africa. Warmer conditions over both the Mediterranean region and northeastern Sahel could also participate in enhancing moisture transport within the AEJ. The strengthening of the thermal gradient between the Sahara and Guinean regions, particularly pronounced north of 10°N, would support an intensification of the AEJ northwards, given the dependance of the jet to the position/intensity of the meridional gradient. In turn, mid-tropospheric moisture divergence tends to be favored within the AEJ region supporting southwards deflection of moist air and contributing to deep moist convection over the Sahel where late summer rainfall regimes are sustained in the context of the A2 scenario regional projections. In conclusion, WRF proved to be a valuable and efficient tool to help downscaling GCM projections over West Africa, and thus assessing issues such as water resources vulnerability locally.

Using spatial and vertical distributions of clouds derived from multi-year spaceborne observations, this paper presents the characteristics of a significant \"pool of inhibited cloudiness\" covering an area of >106 km2 between 3–13° N and 77–90° E over the Bay of Bengal (BoB), persisting throughout the Asian summer monsoon season (ASM). Seasonal mean precipitation rate over the \"pool\" is <3 mm day−1 while that over the surrounding regions is mostly in the range of 6–14 mm day−1. Frequency of occurrence of clouds in this \"pool\" is ~20–40 % less than that over the surrounding deep convective regions. Zonal and meridional cross sections of the altitude distribution of clouds derived from CloudSat data reveal a vault-like structure at the \"pool\" with little cloudiness below ~7 km, indicating that this \"pool\" is almost fully contributed by the substantially reduced or near-absence of low- and middle-level clouds. This suggest the absence of convection in the \"pool\" region. Spaceborne scatterometer observations show divergence of surface wind at the \"pool\" and convergence at its surroundings, suggesting the existence of a mini-circulation embedded in the large-scale monsoon circulation. Reanalysis data shows a mini-circulation extending between the surface and ~3 km altitude, but its spatial structure does not match well with that inferred from the above observations. Sea surface at the south BoB during ASM is sufficiently warm to trigger convection, but is inhibited by the subsidence associated with the mini-circulation, resulting in the \"pool\". This mini-circulation might be a dynamical response of the atmosphere to the substantial spatial gradient of latent heating by large-scale cloudiness and precipitation at the vast and geographically fixed convective zones surrounding the \"pool\". Subsidence at the \"pool\" might contribute to the maintenance of convection at the above zones and be an important component of ASM that is overlooked hitherto.

Six-hourly soundings (GPS sonde) were carried out at the central equatorial Indian Ocean (80º–83ºE) during 25th September–10th October 2011 under the CINDY2011 (Cooperative Indian Ocean Experiment on Intra-seasonal variability in Year 2011) field campaign. One-degree interval soundings were also taken along a meridional section at 83ºE from 5ºN to 5ºS during 12–20 October 2011 to supplement the time series data. Relative humidity (RH) and meridional wind component exhibit downward propagation of air mass in bands of high and low RH associated with northerly and southerly winds, respectively. Low (20–100 day) and high (2–10 day) frequency band pass filtered OLR data (NOAA-interpolated OLR) revealed the presence of Madden and Julian Oscillation (MJO) with 20- to 40-day periodicity, and weak Mixed Rossby Gravity (MRG) waves with 4- to 5-day periodicity. Eastward (westward) propagating MJO (MRG wave) with wave numbers 3–4 (4–5), amplitudes of anomaly 1.1–1.2 Wm −2 (1.8 Wm −2 ) were observed. The asymmetric bifurcation of warm surface water by the subsurface cold water off Sumatra generate asymmetric convective regimes in the vicinity of the equator probably triggered convection with periodicity similar to MRG waves. The intermittent surface convection associated is believed to be responsible for the ascending moisture to the middle troposphere prior to the initiation of MJO. The moisture pumped to the middle troposphere makes the layer convectively more unstable leading to the state of deep convection, a situation conducive for the MJO initiation processes.

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.