Meteorology and Atmospheric Physics

The northeastern depressed region of Bangladesh is highly susceptible to recurrent flash flooding due to excessive rainfall over these areas and in the upstream hilly regions. Two such severe pre-monsoon flash flood events occurred in 2016 and 2017. This research attempts to forecast both flash flood events using a coupled atmospheric-hydrological numerical weather prediction (NWP) model, namely the weather research and forecasting (WRF) model. The ARW (Advanced Research WRF) model is able to predict the rainfall over these areas with a lead time of 91 h. However, the discharge and water level are overestimated by the WRF-Hydro model. The model predicts a flash flood with a lag of approximately 12 h with respect to the highest amount of rainfall. The overall performances of the models were satisfactory. The two parameters, rainfall and subsequent discharge, which are required for delineation of lag time, were almost precisely simulated. Simulated values also had fewer errors, justified by the root mean squared error (RMSE) and mean absolute error values. The Nash–Sutcliffe efficiency criterion scores for model-derived discharge were close to 1.0, and the RMSE-observation standard deviation ratio scores were less than 0.5. This finding proves that the NWP models could be considered for forecasting flash flood events over selected areas of Bangladesh.

A heavy rainfall event during the period from 30th of March to 2nd of April 2009 has been studied using upper air and surface data as well as NOAA HYSPLIT model. This observational study attempts to determine factors responsible for the occurrence of heavy rainfall over Iran induced by Mediterranean cyclone, a western severe sub-tropical storm that made rainfall on most regions of the country. On the surface chart, cyclones, anticyclones and weather fronts were identified. The positions of the cold and warm fronts, which extended from a two-core low pressure center, were quite in good agreements with directions of winds i.e., westerly, southerly and easterly flows as well as the regions of precipitation. The heavy rain event occurred due to a Mediterranean cyclone’s activity over the study area, while other conditions were also responsible for this event such as an unstable atmosphere condition with abundant low-level moisture, which the warm and moist air parcels were brought by the southwesterly low-level jet into the country from Persian Gulf, Oman Sea, Indian Ocean and Caspian Sea at lower levels as well as Mediterranean Sea, Red Sea and Persian Gulf at upper levels over the examined period. A strong low-level convergence zone was observed along the wind-shift line between the southwesterly flow because of the low-level jet and the northeasterly flow due to the Russian high pressure. The amount of precipitable water varied between 20 and 24 kg m−2, surface moisture convergence exceeded 2.5 g kg−1 s−1 and the highest CAPE value in the sounding profiles was observed in Birjand site with 921 J kg−1 during the study period. The HYSPLIT model outputs confirmed the observed synoptic features for the examined system over the country.

The investigation of the impacts of climate change on the total runoff volume in two different watersheds named Melen and Munzur in Turkey is the main purpose of this study. The dynamically downscaled outputs of GFDL-ESM2M, HadGEM2-ES, and MPI-ESM-MR general circulation models (GCMs) under RCP4.5 and RCP8.5 scenarios were used as climatic forces to drive a hydrological model called WASMOD-D. The parameters of WASMOD-D model were optimized by using the multi-objective genetic algorithm (GA). In order to wane the influences of uncertainties which are rooted in GCMs, in addition to using various models, the biases in climatic parameters (precipitation and temperature) were corrected using the quantile-mapping method, with respect to the observed data during the reference period (1971–2000). Future projections were developed by taking two 30-years periods into the account: (1) mid-period (2041–2070) and (2) late future (2071–2100). The results of this study show that the total water volume will decrease in accord with precipitation diminution and temperature increase during the mid-time and late future in both watersheds. The percentage of the decline in runoff volume by the end of the 21st century were found as 15.42 and 26.65 on average for Melen and Munzur rivers, respectively. However, the monthly distribution of the runoff was found to be not changed during the current century.

Three models, MM5, COAMPS, and WRF, have been applied for the warm season in 2003 and the cool season in 2003–2004 to evaluate their performances. All models run over the same domain area covering the north Gulf Mexico and southeastern United States (US) region with the same spatial resolution of 27 km. It was found that the temporal variations of the mean error distribution and strength at 24 and 36 h were rather weak for surface temperature, sea level pressure, and surface wind speed for all models. A warm bias in surface temperature forecasts dominated over land during the warm season, whereas a cool bias existed during the cool season. The MM5 and WRF produced negative biases of sea level pressure during the warm season and positive biases during the cool season while the COAMPS yielded a similar distribution of sea level pressure biases during both seasons. During both seasons, similar surface wind speed biases produced by each model included a high wind speed forecast over most areas by MM5 while the COAMPS and WRF yielded weak surface winds over the western Plains and stronger surface winds over the eastern Plains. Root-mean-squared errors revealed that the forecast of surface temperature, sea level pressure, and surface wind speed were degraded with the increase of forecast time. For rainfall evaluation, it was found that the MM5 underpredicted seasonal precipitation while the COAMPS and WRF overpredicted. The bias scores revealed that the MM5 yielded an underprediction of the coverage of precipitation areas, especially for heavier rainfall events. The MM5 presented the lower threat score at lighter rainfall events compared to the COAMPS and WRF. For moderate and heavier thresholds, all models lacked forecast accuracy. The WRF accuracy in predicting precipitation was heavily dependent upon the performance of the selected cumulus parameterization scheme. Use of the Grell–Devenyi and Bette–Miller–Janjic schemes helps suppress precipitation overprediction.

This study researched the precipitation variability across 53 meteorological stations in Mauritius and different subregions of the island, over a 30-year study period (1981–2010). Time series was investigated for each 5-year interval and also for the whole study period. Non-parametric Mann–Kendall and Spearman’s rho statistical tests were used to detect trends in annual precipitation. A mix of positive (increasing) and negative (decreasing) trends was highlighted for the 5-year interval analysis. The statistical tests nevertheless agreed on the overall trend for Mauritius and the subregions. Most regions showed a decrease in precipitation during the period 1996–2000. This is attributed to the 1998–2000 drought period which was brought about by a moderate La Niña event. In general, an increase in precipitation levels was observed across the country during the study period. This increase is the result of an increase in extreme precipitation events in the region. On the other hand, two subregions, both located in the highlands, experienced a decline in precipitation levels. Since most of the reservoirs in Mauritius are located in these two subregions, this implies serious consequences for water availability in the country if existing storage capacities are kept.

Stored aerosols belonging to the Aitken nucleus size range are investigated both experimentally and theoretically. Using the exhaustion method of diffusion, it is found that they tend to be composed of particles of different size ranges. The processes of sedimentation, diffusion and coagulation of aerosols in closed containers are discussed in the light of the results. It is considered that turbulence affects the results obtained.

During winters, the absence of solar radiation combined with clear skies and weak synoptic forcing enables cold pooling in the complex topographic basins of interior Alaska. Under these conditions, shallow, small-scale cold flows originating within, or flowing from, north-facing semi-enclosed basins are able to penetrate the frigid atmospheric boundary layer (ABL) of the open south-facing basins. This paper introduces the Winter Boundary Layer Experiment carried out during three consecutive periods in Fairbanks (2009–2011) and examines observational results illustrating the changes in the mean and turbulent state of the ABL during the occurrence of shallow flows. Observations introduced here demonstrate that during flow penetration, surface layer stratification is destroyed allowing mixing and thermal stabilization of the basin cooling regime. Evidence of upper level ABL thermal turbulence related to shear driven flow is introduced and discussed. Basin-scale turbulent heat fluxes are shown to reach −20 Wm−2 during flow occurrence.