Scale decomposition of atmospheric water budget over West Africa during the monsoon 2006 from NCEP/GFS analyses

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Understanding the West African monsoon variability and its remote effects: an illustration of the grid point nudging methodology

International audienceGeneral Circulation Models still show deficiencies in simulating the basic features of the West African Monsoon at intraseasonal, seasonal and interannual timescales. It is however difficult to disentangle the remote versus regional factors that contribute to such deficiencies, and to diagnose their possible consequences for the simulation of the global atmospheric variability. The aim of the present study is to address these questions using the so-called grid point nudging technique, where prognostic atmospheric fields are relaxed either inside or outside the West African Monsoon region towards the ERA40 reanalysis. This regional or quasi-global nudging is tested in ensembles of boreal summer simulations. The impact is evaluated first on the model climatology, then on intraseasonal timescales with an emphasis on North Atlantic/Europe weather regimes, and finally on interannual timescales. Results show that systematic biases in the model climatology over West Africa are mostly of regional origin and have a limited impact outside the domain. A clear impact is found however on the eddy component of the extratropical circulation, in particular over the North Atlantic/European sector. At intraseasonal timescale, the main regional biases also resist to the quasi-global nudging though their magnitude is reduced. Conversely, nudging the model over West Africa exerts a strong impact on the frequency of the two North Atlantic weather regimes that favor the occurrence of heat waves over Europe. Significant impacts are also found at interannual timescale. Not surprisingly, the quasi-global nudging allows the model to capture the variability of large-scale dynamical monsoon indices, but exerts a weaker control on rainfall variability suggesting the additional contribution of regional processes. Conversely, nudging the model towards West Africa suppresses the spurious ENSO teleconnection that is simulated over Europe in the control experiment, thereby emphasizing the relevance of a realistic West African monsoon simulation for seasonal prediction in the extratropics. Further experiments will be devoted to case studies aiming at a better understanding of regional processes governing the monsoon variability and of the possible monsoon teleconnections, especially over Europe

Assimilation of water vapour airborne lidar observations: Impact study on the COPS precipitation forecasts

The Convective and Orographically-driven Precipitation Study (COPS) carried out in summer 2007 over northeastern France and southwestern Germany provided a fairly comprehensive description of the low-troposphere water-vapour field, thanks in particular to the deployment of two airborne differential absorption lidar systems. These lidar observations were assimilated using the 3D-Var assimilation system of the Application of Research to Operations at MEsoscale (AROME) numerical weather prediction mesoscalemodel. The assimilation was carried out for the period 4 July�3 August by running a three-hour forward intermittent assimilation cycle. First, the impact of the lidar observations was assessed by comparing the analyses with a set of more than 200 independent soundings. The lidar observations were found to have a positive impact on the analyses by reducing the dry bias in the first 500 m above ground level and by diminishing the root-mean-square error by roughly 15% in the first km. Then the impact of the lidar observations was assessed by comparing the precipitation forecasts (obtained with and without the lidar observations for the period 15 July�2 August) with the gridded precipitation observations provided by the Vienna Enhanced Resolution Analysis. In general, the impact was found to be positive but not significant for the 24 h precipitation and positive and significant for the 6 h precipitation, with an improvement lasting up to 24 h. Some selected case studies show that the improvement was obtained through a better depiction of convection initiation or through a more accurate positioning of the precipitation systems

Evaluation of a mesoscale coupled ocean‐atmosphere configuration for tropical cyclone forecasting in the South West Indian Ocean basin

The performance in term of tropical cyclone track and intensity prediction of the new coupled ocean-atmosphere system based on the operational atmospheric model AROME-Indian Ocean and the ocean model NEMO is assessed against that of the current operational configuration in the case of seven recent tropical cyclones. Five different configurations of the forecast system are evaluated: two with the coupled system, two with an ocean mixed layer parameterization and one with a constant sea surface temperature. For each ocean-atmosphere coupling option, one is initialized directly with the MERCATOR-Ocean PSY4 product as in the current operational configuration and the other with the ocean state that is cycled in the AROME-NEMO coupled suite since a few days before the cyclone intensification. The results show that the coupling with NEMO generally improves the intensity of cyclones in AROME-IO, reducing the mean intensity bias of the 72 h forecast of about 10 hPa. However, the impact is especially significant when the TCs encounter a slow propagation phase. For short-term forecasts (less than 36 hours), the presence of a cooling in the initial state that has been triggered by the AROME high-resolution cyclonic winds in a previous coupled forecast already improves the tropical cyclone intensity bias of 2-3 hPa for both coupled or uncoupled configurations. Key Points AROME/NEMO improves the forecast of tropical cyclone compared to the operational configuration with a 1D ocean mixed layer parameterization The improvement mostly comes from quasi-stationary or very slow moving intense cyclones The tropical cyclone forecasts are sensitive to the ocean initial conditions Plain Language Summary The ocean provides a large part of the energy for the intensification of tropical cyclones through warm sea surface temperature and sea-air heat and moisture exchanges. However, the ocean-atmosphere interactions also trigger processes which cools the sea surface temperature beneath the tropical cyclone and thus generates a negative feedback on the TC intensification. The numerical forecasts of the regional numerical weather prediction model AROME-IO are valuable guidance for the Regional Specialized Meteorological Centre for Tropical Cyclones, La Réunion. The objective of our study is to evaluate the possibility of replacing the current ocean mixed layer parameterization by a more realistic ocean able to represent more complex processes such as the explicit transport by the currents. Overall, we found that the new coupling improves the cyclone intensity in AROME-IO both in terms of bias and standard deviation. These improvements come almost entirely from tropical cyclones that encounter a slow propagation phase. For short-term forecasts, the presence of a cooling that is triggered by AROME high-resolution cyclonic winds in the initial state of the ocean already improves the TC intensity forecast, even when the ocean mixed layer parameterization is used

Multiscale Modeling of Convection and Pollutant Transport Associated with Volcanic Eruption and Lava Flow: Application to the April 2007 Eruption of the Piton de la Fournaise (Reunion Island)

International audienceVolcanic eruptions can cause damage to land and people living nearby, generate high concentrations of toxic gases, and also create large plumes that limit observations and the performance of forecasting models that rely on these observations. This study investigates the use of micro- to meso-scale simulation to represent and predict the convection, transport, and deposit of volcanic pollutants. The case under study is the 2007 eruption of the Piton de la Fournaise, simulated using a high-resolution, coupled lava/atmospheric approach (derived from wildfire/atmosphere coupled code) to account for the strong, localized heat and gaseous fluxes occurring near the vent, over the lava flow, and at the lava–sea interface. Higher resolution requires fluxes over the lava flow to be explicitly simulated to account for the induced convection over the flow, local mixing, and dilution. Comparisons with air quality values at local stations show that the simulation is in good agreement with observations in terms of sulfur concentration and dynamics, and performs better than lower resolution simulation with parameterized surface fluxes. In particular, the explicit representation of the thermal flows associated with lava allows the associated thermal breezes to be represented. This local modification of the wind flow strongly impacts the organization of the volcanic convection (injection height) and the regional transport of the sulfur dioxide emitted at the vent. These results show that explicitly solving volcanic activity/atmosphere complex interactions provides realistic forecasts of induced pollution

Evaluation of a Mesoscale Coupled Ocean‐Atmosphere Configuration for Tropical Cyclone Forecasting in the South West Indian Ocean Basin

Abstract The performance in term of tropical cyclone track and intensity prediction of the new coupled ocean‐atmosphere system based on the operational atmospheric model AROME‐Indian Ocean and the ocean model NEMO is assessed against that of the current operational configuration in the case of seven recent tropical cyclones. Five different configurations of the forecast system are evaluated: two with the coupled system, two with an ocean mixed layer parameterization and one with a constant sea surface temperature. For each ocean‐atmosphere coupling option, one is initialized directly with the MERCATOR‐Ocean PSY4 product as in the current operational configuration and the other with the ocean state that is cycled in the AROME‐NEMO coupled suite since a few days before the cyclone intensification. The results show that the coupling with NEMO generally improves the intensity of cyclones in AROME‐IO, reducing the mean intensity bias of the 72 hr forecast of about 10 hPa. However, the impact is especially significant when the TCs encounter a slow propagation phase. For short‐term forecasts (less than 36 hr), the presence of a cooling in the initial state that has been triggered by the AROME high‐resolution cyclonic winds in a previous coupled forecast already improves the tropical cyclone intensity bias of 2–3 hPa for both coupled or uncoupled configurations

The Effect of Atmosphere–Ocean Coupling on the Structure and Intensity of Tropical Cyclone Bejisa in the Southwest Indian Ocean

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