The West African Monsoon Onset: a concise comparison of definitions

The onset of the West African Monsoon (WAM) marks a vital time for local and regional stakeholders. Whilst the seasonal progression of monsoon winds and the related migration of precipitation from the Guinea Coast towards the Soudan/Sahel is apparent, there exist contrasting man-made definitions of what the WAM onset means. Broadly speaking, onset can be analyzed regionally, locally or over a designated intermediate scale. There are at least eighteen distinct definitions of the WAM onset in publication with little work done on comparing observed onset from different definitions or comparing onset realizations across different datasets and resolutions. Here, nine definitions have been calculated using multiple datasets of different metrics at different resolution. It is found that mean regional onset dates are consistent across multiple datasets and different definitions. There is low inter-annual variability in regional onset suggesting that regional seasonal forecasting of the onset provides few benefits over climatology. In contrast, local onsets show high spatial, inter-annual and inter-definition variability. Furthermore it is found that there is little correlation between local onset dates and regional onset dates across West Africa implying a disharmony between regional measures of onset and the experience on a local scale. The results of this study show that evaluation of seasonal monsoon onset forecasts is far from straightforward. Given a seasonal forecasting model, it is possible to simultaneously have a good and bad prediction of monsoon onset simply through selection of onset definition and observational dataset used for comparison

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

Monsoon onset over Sudan-Sahel: Simulation by the regional scale model MM5.

International audienc

Regional scale model simulation for West Africa using a Mesoscale Model MM5

Dynamical downscaling is considered as a good tool for deriving regional climate in formation based on large scale climate conditions because they can represent surface conditions like topography and vegetation more realistic than GCM. The fifth generation Pennsylvania State University (PSU)-National Centre for Atmospheric Research (NCAR) Mesoscale Model MM5 is used to study the regional scale process that control West African rainfall, its associated circulations and other surface features. The simulations are carried out for seven months from March to September using 1.125X1.125 degree lat/lon version of ECMWF reanalysis data for defining the initial and laternal boundary conditions. The objective is to study the monsoon structure from the first rainy season in Guinea (5N) to the sahelian rainfall peak in August. The year 2000 is taken as a reference year in order to validate the model simulation for further study. The results are compared with reanalysis and observed data, then statistical methods are used to study temporal and spatial relationships among various field parameters with 60 km resolution. Preliminary results show that the model is able to reproduce a more realistic rainfall field than the reanalysis

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

Seasonal cycle and intraseasonal oscillations in the interannual variability over monsoon region

International audienceA characteristic seasonal cycle in the interannual variability (IAV) of dynamical fields over the Indian summer monsoon (ISM) region is revealed by an investigation of daily fields from a 40-year global data set from NCEP. The IAV, measured in terms of standard deviation, is remarkably suppressed over the monsoon region during June to September. Time-latitude distribution of IAV shows that a suppression of IAV in the summer months is a general feature, if not as pronounced as over ISM, of the northern hemisphere. Other new features revealed by our analysis are (a) a differential, nearly out of phase behaviour between zonal and meridional wind components north of 30N, and (b) strong intraseasonal oscillations in IAV, which are once again suppressed during the monsoon months. The low IAV implies a weaker contrast in these fields for different years; implications of this feature for long range forecasting of monsoon are discussed

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