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This study presents a detailed analysis of the 2019 record-breaking seasonal (June-September) monsoon rainfall, since 1901, over large areas of Western India, which evolved in the backdrop of an intense positive Indian Ocean Dipole (p-IOD) event. Analysis of multi-source observational datasets (rain gauge, GPM, NOAA, ERA5) reveals that the 2019 unusually excessive and flood-producing monsoonal rains over Western India markedly came from three intense rain episodes (IREs) viz., 1) 28th June - 12th July 2) 24th July -11th August 3) 2nd September – 13th September and was characterised by large-scale bands of organised convection. We observe a preponderance of stratiform precipitating mesoscale convective systems embedded within the organised rainband. Regression analyses of various atmospheric fields were performed to understand the dominant spatial structures during the IRE days. Our findings indicate that the continual top-heavy stratiform heating forced a Rossby-wave pattern with regional stretching of mid-tropospheric potential vorticity across South and Southeast Asia. Results show that the pIOD-induced enhanced cross-equatorial moisture transport fostered deep convection and heavy precipitation in this region. This study highlights the potential ramifications of intense pIOD manifestations on IREs and their hydrological consequences over the monsoon-centric areas of South and Southeast Asia in the 21st century., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The article of record as published may be located at http://dx.doi.org/10.1175/WAF1029.1 A numerical study using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was performed to assess the impact of initial and boundary conditions, the parameterization of turbulence transfer and its coupling with cloud-driven radiation, and cloud microphysical processes on the accuracy of mesoscale predictions and forecasts of the cloud-capped marine boundary layer. Aircraft, buoy, and satellite data and the large eddy simulation (LES) results during the Dynamics and Chemistry of Marine Stratocumulus field experiment (DYCOMS II) in July 2001 were used in the assessment. Three of the tested input fields (Eta, NCEP, and ECMWF) show deficiencies, mainly in the thermodynamic structure of the lowest 1500 m of the marine atmosphere. On a positive note, the simulated marine-layer depth showed good agreement with aircraft observations using the Eta fields, while using the NCEP and ECMWF datasets underestimated the marine-layer depth by about 20%–30%. The predicted turbulence kinetic energy (inversion strength) was about 50% of that obtained from the LES results (aircraft observed). As a consequence of moisture overprediction, the predicted liquid water path was twice the observed by 1–2 g kg 1. The sensitivity tests have shown that the selections of turbulence and cloud microphysical schemes significantly influence the turbulence estimates and cloud parameters. Two of the tested turbulence schemes (Eta PBL and Burk–Thompson) did not exhibit the coupling with radiation. The significant differences in the simulated turbulence estimates appear to be a consequence of the use of water-conserving potential temperature variables. The microphysical parameterization, which uses the number concentration of cloud drops in the autoconversion process, simulates a realistic evolution of precipitable hydrometeors in the cloudy marine layer on the positive side, but on the other hand enhances the decoupling in the turbulence structure. This study can provide guidance to operational forecasters concerning accuracy issues of the commonly used large-scale analyses for model initialization, and optimal selection of model parameterizations in order to simulate and forecast the cloudy atmospheric boundary layer over the ocean.

Satellite-tracked drifter data and meteorological wind measurements indicated summer bora event of 22 - 25 June 1995 as interesting from both meteorological and oceanographic point of view. The studied summer situation was controlled by a mesoscale atmospheric disturbance that imparts strong cyclonic vorticity to the wind field, with the orographic effect being of secondary importance. During the bora event currents of 70 cm s-1 were measured by drifter movements tracked along the western Adriatic coast. Drifter trajectories for the selected period were simulated by an oceanographic model forced by the outputs of two mesoscale meteorological models, ALADIN and MM5, and by river discharges. The atmospheric models provided realistic space- and time-varying atmospheric fields (wind, air temperature and humidity) with ca 10 km horizontal resolution by ALADIN and with 9 and 3 km by MM5. Several numerical experiments with different combinations of the three forcing agents – wind stress, river discharges and surface heat fluxes – were performed. Proper formulation of the drag coefficient was crucial for the correct simulations of the drifter trajectories, as its increased values do reproduce well the effects of the sea surface roughness and atmospheric stability to the surface ocean layer with respect to the observed drifter movements. Numerical experiments revealed that strong current along the western Adriatic coast resulted from a combined influence of the wind stress and river discharges. The importance of the baroclinic, buoyancy driven contribution to the current field was particularly pronounced in the periods with weak winds. At the same time, drifter movements in the open sea area in front of the eastern Istrian coast were the result of the wind stress acting solely.