Role of deep convection in regulating the Indian summer monsoon dynamics: a regional scale modelling study

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.11Nsciescopu

Low Level Jet stream of Asian Summer Monsoon and its Variability

The main objective of the of present study are to study the intraseasonal variability of LLJ and its relation with convective heating of the atmosphere, to establish whether LLJ splits into two branches over the Arabian sea as widely believed, the role of horizonatal wind shear of LLJ in the episodes of intense rainfall events observed over the west coast of India, to perform atmospheric modeling work to test whether small (meso) scale vortices form during intense rainfall events along the west coast; and to study the relation between LLJ and monsoon depression genesis. The results of a study on the evolution of Low Level Jetstream (LLJ) prior to the formation of monsoon depressions are presented. A synoptic model of the temporal evolution of monsoon depression has been produced. There is a systematic temporal evolution of the field of deep convection strength and position of the LLJ axis leading to the genesis of monsoon depression. One of the significant outcomes of the present thesis is that the LLJ plays an important role in the intraseasonal and the interannual variability of Indian monsoon activity. Convection and rainfall are dependent mainly on the cyclonic vorticity in the boundary layer associated with LLJ. Monsoon depression genesis and the episodes of very heavy rainfall along the west coast of India are closely related to the cyclonic shear of the LLJ in the boundary layer and the associated deep convection. Case studies by a mesoscale numerical model (MM5) have shown that the heavy rainfall episodes along the west coast of India are associated with generation of mesoscale cyclonic vortices in the boundary layer

Midlatitude-tropics interactions as seen from MST radar observations at Gadanki (13.5°N, 79.2°E) during winter

192-198The MST radar observations at Gadanki, Tirupati (13.5°N, 79.2°E), during 1995-96 winter showed an anomalous wind pattern in the troposphere and lower stratosphere. During the season the mean zonal wind between 3.6 km and 21 km region was westerly and the vertical velocity was downward. The zonal wind exhibited a change from the normal pattern of winter westerlies into easterlies in the troposphere and lower stratosphere during January 1996. The radar data combined with radiosonde and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data showed that the anomalous features are associated with the north-south movement of anticyclones during the passage of western disturbances over north-west India. Such process of a midlatitude-tropics interaction is further evidenced as intrusion of the upper air trough from the midlatitudes into the tropics

Characteristics of a persistent "pool of inhibited cloudiness" and its genesis over the Bay of Bengal associated with the Asian summer monsoon

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

WRF/ARPEGE-CLIMAT simulated climate trends over West Africa.

20 pagesInternational audienceThe 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 10A degrees 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

Quantification of Enhancement in Atmospheric CO2 Background Due to Indian Biospheric Fluxes and Fossil Fuel Emissions

© 2021. American Geophysical Union. All Rights Reserved.Regional carbon emissions impact global atmospheric carbon dioxide (CO2) background concentrations. This study quantified the enhancement in the atmospheric CO2 mole fractions due to biospheric and fossil fuel fluxes from India. Sensitivity experiments using model simulations were conducted, allowing CO2 enhancement due to biospheric and fossil fuel fluxes from India to diffuse into the global atmospheric background. The areal extent of column-averaged enhancement of 0.2 ppm and above due to Indian fluxes are spread over a larger area covering the Indian subcontinent, neighboring Asian regions, and the north Indian Ocean in all four seasons. The boundary layer CO2 enhancement due to biospheric fluxes (fossil fuel fluxes) have a maximum range of −2.6 to +1.4 ppm (1.8–2.0 ppm) most time of the year. At higher altitude, the amplitudes of enhancement are reduced from ±1.8 to ±0.6 ppm as we go up from 850 to 500 hPa due to diffusion by prevailing atmospheric dynamics and convection. With the information of the areal extent of >0.2 ppm CO2 enhancement due to Indian fluxes, we have evaluated the representativeness of satellite observations (GOSAT and OCO-2) in capturing those enhancements. Both the satellite coverage show a similar number of observations (0.1 per day) during all seasons except for June to August, wherein the cloud screening eliminates almost all the satellite data over the region. Within this areal extent, the satellite XCO2 shows average anomalies of nearly ±2.0 ppm; it is a valuable piece of information because it is well above the retrieval uncertainty, yet capturing the potential enhancement due to fluxes from India. The study implies that the regions of enhancement greater than 0.2 ppm can be considered locations for surface observations representing Indian fluxes. Similarly, the region with enhancement greater than one ppm could be covered by satellites/airborne observations to discern enhancement in the atmospheric CO2 mole fractions due to Indian fluxes.11Nsciescopu

Quantification of Enhancement in atmospheric CO 2

© 2021. American Geophysical Union. All Rights Reserved.Regional carbon emissions impact global atmospheric carbon dioxide (CO2) background concentrations. This study quantified the enhancement in the atmospheric CO2 mole fractions due to biospheric and fossil fuel fluxes from India. Sensitivity experiments using model simulations were conducted, allowing CO2 enhancement due to biospheric and fossil fuel fluxes from India to diffuse into the global atmospheric background. The areal extent of column-averaged enhancement of 0.2 ppm and above due to Indian fluxes are spread over a larger area covering the Indian subcontinent, neighboring Asian regions, and the north Indian Ocean in all four seasons. The boundary layer CO2 enhancement due to biospheric fluxes (fossil fuel fluxes) have a maximum range of −2.6 to +1.4 ppm (1.8–2.0 ppm) most time of the year. At higher altitude, the amplitudes of enhancement are reduced from ±1.8 to ±0.6 ppm as we go up from 850 to 500 hPa due to diffusion by prevailing atmospheric dynamics and convection. With the information of the areal extent of >0.2 ppm CO2 enhancement due to Indian fluxes, we have evaluated the representativeness of satellite observations (GOSAT and OCO-2) in capturing those enhancements. Both the satellite coverage show a similar number of observations (0.1 per day) during all seasons except for June to August, wherein the cloud screening eliminates almost all the satellite data over the region. Within this areal extent, the satellite XCO2 shows average anomalies of nearly ±2.0 ppm; it is a valuable piece of information because it is well above the retrieval uncertainty, yet capturing the potential enhancement due to fluxes from India. The study implies that the regions of enhancement greater than 0.2 ppm can be considered locations for surface observations representing Indian fluxes. Similarly, the region with enhancement greater than one ppm could be covered by satellites/airborne observations to discern enhancement in the atmospheric CO2 mole fractions due to Indian fluxes.11Nsciescopu

An observing system simulation experiment for Indian Ocean surface pCO2 measurements

An observing system simulation experiment (OSSE) is conducted to identify potential locations for making surface ocean pCO2 measurements in the Indian Ocean using the Bayesian Inversion method. As of the SOCATv3 release, the pCO2 data is limited in the Indian Ocean. To improve our modeling of this region, we need to identify where and what observation systems would produce the most good or benefit for their cost. The potential benefits of installing pCO2 sensors in the existing RAMA and OMNI moorings of the Indian Ocean, the potential of Bio-Argo floats (with pH measurements), and the implementation of the ship of opportunity program (SOOP) for underway sampling of pCO2 are evaluated. A cost function of dissolved inorganic carbon as a model state vector and CO2 flux mismatch as the source of error is minimized, and the basin-wide CO2 flux uncertainty reduction is estimated for different seasons. The maximum flux uncertainty reduction achievable by installing pCO2 sensors in the existing RAMA and OMNI moorings is limited to 30% during different seasons. One may consider that around 20 Bio-Argos are still the right choice over installing mooring based pCO2 sensors and achieve uncertainty reduction up to 50% with additional benefit of profiling the sub-surface upto 1000 & ndash;2000 m. However, a single track SOOP has the potential to reduce the uncertainty by approximately 62%. This study identifies vital RAMA and OMNI moorings and SOOP tracks for observing Indian Ocean pCO2. Plain Language Summary. Surface ocean partial pressure of CO2 (pCO2) information is vital for estimating sea-to-air CO2 exchanges. This parameter is least available from the Indian Ocean as compared to other global tropical and southern oceans. There has been no effort made so far to measure surface ocean pCO2 in the Indian Ocean with routine monitoring such as by mounting instruments to moorings or by underway sampling via any ship of opportunity program. Therefore there is a considerable demand to start pCO2 observations in the Indian Ocean. However, one key question that emerges is where to deploy pCO2 instruments in the Indian Ocean to learn the most with limited resources. This study addresses this question with inverse modeling techniques. The study finds that the existing moorings of the Indian Ocean are capable of hosting pCO2 sensors, and data from those are useful to reduce the uncertainty in the surface sea-to-air CO2 flux estimation by a quarter magnitude. In contrast, the Bio-Argo floats with pH sensors, and the ship of opportunity underway sampling of pCO2 may benefit from reducing the same up to 50% and 62%, respectively