Precipitation simulation of synoptic scale systems over western Himalayan region using Advanced Regional Prediction System (ARPS) model

Operational weather prediction over western Himalayan region is a challenging job due to scarcity of data and complex topography that interacts with approaching weather system. Accurate prediction of complex weather phenomena requires dense data network which is difficult to establish in mountain due to complex terrain and hostile weather conditions over Himalaya. The alternate method to overcome this problem is by ingesting three-dimensional meteorological variables from global model’s analysis and forecast values as initial and lateral boundary conditions in meso-scale numerical models. Simultaneously, data assimilation is a potential tool in which non-conventional [satellite, radar and Automatic Weather Station (AWS)] and conventional (surface and upper air observations) data are ingested in the numerical models to generate high resolution and accurate initial fields for the initialization of the mesoscale model. In the present study, Advanced Regional Prediction System (ARPS) model has been used for the prediction of synoptic weather system known as Western Disturbance (WD) that affects the weather of western and central Himalaya during winter period (November – April).The ARPS model has been selected for this study because the model has its own objective analysis and quality control system. It has the capacity to ingest the satellite, Doppler weather radar data and other types of observations. Its assimilation system can also be used to overcome the problem of data scarcity in Himalayan region. In this study, initial and lateral boundary fields are taken from the T-80 spectral global model operationally used at National Centre for Medium Range Prediction (NCMRWF), Noida (UP), India. The global model’s analysis was taken as the initial condition and 24 hour’s interval forecasts as lateral boundary conditions. The model has been used for the simulation of few WDs for 96 hours (Four days). The comparison of ARPS simulation with T-80 forecast shows that the ARPS model could produce better results in respect of the circulation of WDs and hence it can be utilized for the operational weather prediction over the Indian region.

Sensitivity of Tropical Cyclone Idai Simulations to Cumulus Parametrization Schemes

Weather simulations are sensitive to subgrid processes that are parameterized in numerical weather prediction (NWP) models. In this study, we investigated the response of tropical cyclone Idai simulations to different cumulus parameterization schemes using the Weather Research and Forecasting (WRF) model with a 6 km grid length. Seventy-two-hour (00 UTC 13 March to 00 UTC 16 March) simulations were conducted with the New Tiedtke (Tiedtke), New Simplified Arakawa–Schubert (NewSAS), Multi-Scale Kain–Fritsch (MSKF), Grell–Freitas, and the Betts–Miller–Janjic (BMJ) schemes. A simulation for the same event was also conducted with the convection scheme switched off. The twenty-four-hour accumulated rainfall during all three simulated days was generally similar across all six experiments. Larger differences in simulations were found for rainfall events away from the tropical cyclone. When the resolved and convective rainfall are partitioned, it is found that the scale-aware schemes (i.e., Grell–Freitas and MSKF) allow the model to resolve most of the rainfall, while they are less active. Regarding the maximum wind speed, and minimum sea level pressure (MSLP), the scale aware schemes simulate a higher intensity that is similar to the Joint Typhoon Warning Center (JTWC) dataset, however, the timing is more aligned with the Global Forecast System (GFS), which is the model providing initial conditions and time-dependent lateral boundary conditions. Simulations with the convection scheme off were found to be similar to those with the scale-aware schemes. It was found that Tiedtke simulates the location to be farther southwest compared to other schemes, while BMJ simulates the path to be more to the north after landfall. All of the schemes as well as GFS failed to simulate the movement of Idai into Zimbabwe, showing the potential impact of shortcomings on the forcing model. Our study shows that the use of scale aware schemes allows the model to resolve most of the dynamics, resulting in higher weather system intensity in the grey zone. The wrong timing of the peak shows a need to use better performing global models to provide lateral boundary conditions for downscalers.

Influence of different domain configurations on WRF solar irradiance estimation at Badajoz (Spain)

<p>Over the last decades, numerical prediction models, such as the Weather Research and Forecasting (WRF), have emerged as one of the most powerful tools for solar radiation exploitation as renewable energy. A reliable forecast of solar radiation is an effective method to account for its variability and facilitate its integration into the grid. This study analyzes the influence of different domain configurations and spatial resolutions on the WRF solar radiation estimation. To this aim, different domain configurations centered on the city of Badajoz (Spain) have been tested. Thus, three different combinations of two nested domains (D01; higher domain; D02; inner domain) defined on a Lambert Conformal projection have been analyzed. Configurations C1 and C2 use the same domains but differ in the resolution of the nested domain (D02): 9 km for C1 and 3 km for C2. C3 has been defined to perform simulations at a higher resolution, consisting of two nested domains of 9 km for D01 and 1 km for D02. Due to WRF’s requirements on grid ratio between nested domains and computational efficiency criteria, this third configuration uses the same D02 dimensions as C1 and C2, but notably smaller D01 dimensions. All these configurations have employed the same WRF parameterizations. The initial and lateral boundary conditions for the meteorological fields are obtained from the reanalysis ERA5. Finally, the estimated solar radiation for the inner domains at 9, 3 and 1 km has been compared with ground-based solar radiation measurements. The results show a good performance of all the analyzed configurations, with an average relative MABE value of 14.95% and mean relative RMSE of 23.7%. Linear regression analysis between simulated and reference ground measurements have reported a slope of 0.83 for C1, 0.80 for C2 and 0.77 for C3. C3 tends to overestimate the reference measurements, while C1 and C2 tend to underestimate them. This underestimation is more remarkable for C2, likely due to the higher grid ratio in this configuration, 1:9 versus 1:3 in C1. Additionally, the analysis of differences between WRF simulation and reference data with respect to geometrical factors and sky conditions have reported differences between configurations. All these results reveal that different aspects related to the domain configuration, and not only final resolution, can influence the solar radiation forecasting and point out the need to find the most suitable configuration for each specific problem. <em>Acknowledgments.</em> This work is partially funded by FEDER/Ministerio de Ciencia, Innovación y Universidades-Agencia Estatal de Investigación of Spain through project RTI 2018-097332-B-C22, and by Junta de Extremadura-FEDER through project GR18097.</p>

The Response of Extratropical Transition of Tropical Cyclones to Climate Change: Quasi-Idealized Numerical Experiments

This study uses small ensembles of convection-allowing, quasi-idealized simulations to examine the response of North Atlantic tropical cyclones (TCs) undergoing extratropical transition (ET) to climate change. Using HURDAT2 and ERA5 data over a 40-yr period from 1979 to 2018, we developed storm-relative composite fields for past North Atlantic recurving, oceanic ET events. The quasi-idealized present-day simulations are initialized from these composites and run in an aquaplanet domain. A pseudo–global warming approach is used for future simulations: Thermodynamic changes between late twenty-first century and twentieth century, derived from an ensemble of 20 CMIP5 GCMs under the RCP8.5 scenario, are added to the present-day initial and lateral boundary conditions. The composite-initialized present-day simulations exhibit realistic ET characteristics. Future simulations show greater intensity, heavier precipitation, and stronger downstream midlatitude wave train development relative to the present-day case. Specifically, the future ET event is substantially stronger before ET completion, though the system undergoes less reintensification after ET completion. Reductions in lower-tropospheric baroclinicity associated with Arctic amplification could contribute to this result. The future simulation exhibits 3-hourly ensemble-mean precipitation rate increases ranging from ~23% to ~50%, depending on ET phase and averaging radius. In addition, larger eddy kinetic energy accompanies the future storm, partly created by increased baroclinic conversion, resulting in stronger amplification of downstream energy maxima via intensified ageostrophic geopotential flux convergence and divergence. These results suggest that future TCs undergoing ET could have greater potential to cause high-impact weather in western Europe through both direct and remote processes.

A Progress Report on the Development of the High-Resolution Rapid Refresh Ensemble

The High-Resolution Rapid Refresh Ensemble (HRRRE) is a 36-member ensemble analysis system with nine forecast members that utilizes the Advanced Research Weather Research and Forecasting (ARW-WRF) dynamic core and the physics suite from the operational Rapid Refresh/High-Resolution Rapid Refresh deterministic modeling system. A goal of HRRRE development is a system with sufficient spread amongst members, comparable in magnitude to the random error in the ensemble mean, to represent the range of possible future atmospheric states. HRRRE member diversity has traditionally been obtained by perturbing the initial and lateral boundary conditions of each member, but recent development has focused on implementing stochastic approaches in HRRRE to generate additional spread. These techniques were tested in retrospective experiments and in the May 2019 Hazardous Weather Testbed Spring Experiment (HWT-SE). Results show a 6–25% increase in the ensemble spread in 2-m temperature, 2-m mixing ratio, and 10-m wind speed when stochastic parameter perturbations are used in HRRRE (HRRRE-SPP). Case studies from HWT-SE demonstrate that HRRRE-SPP performed similar to or better than the operational High-Resolution Ensemble Forecast system version 2 (HREFv2) and the non-stochastic HRRRE. However, subjective evaluations provided by HWT-SE forecasters indicated that overall, HRRRE-SPP predicted lower probabilities of severe weather (using updraft helicity as a proxy) compared to HREFv2. A statistical analysis of the performance of HRRRE-SPP and HREFv2 from the 2019 summer convective season supports this claim, but also demonstrates that the two systems have similar reliability for prediction of severe weather using updraft helicity.

Effect of boundary conditions on adjoint-based forecast sensitivity observation impact in a regional model

In this study, the effect of boundary condition configurations in the regional Weather Research and Forecasting (WRF) model on the adjoint-based forecast sensitivity observation impact (FSOI) for 24 h forecast error reduction was evaluated. The FSOI has been used to diagnose the impact of observations on the forecast performance in several global and regional models. Different from the global model, in the regional model, the lateral boundaries affect forecasts and FSOI results. Several experiments with different lateral boundary conditions were conducted. The experimental period was from 1 to 14 June 2015. With or without data assimilation, the larger the buffer size in lateral boundary conditions, the smaller the forecast error. The nonlinear and linear forecast error reduction (i.e., observation impact) decreased as the buffer size increased, implying larger impact of lateral boundaries and smaller observation impact on the forecast error. In all experiments, in terms of observation types (variables), upper-air radiosonde observations (brightness temperature) exhibited the greatest observation impact. The ranking of observation impacts was consistent for observation types and variables among experiments with a constraint in the response function at the upper boundary. The fractions of beneficial observations were approximately 60%, and did not considerably vary depending on the boundary conditions specified when calculating the FSOI in the regional modeling framework.

Evaluation of the new high resolution unstructured grid Marmara Sea model

<p>Marmara Sea including Bosphorus and Dardanelles Straits (i.e. Turkish Strait Systems, TSS) is the connection between the Black Sea and the Mediterranean. The exchange flow that occurs in the Straits is crucial to set the deep water properties in the Black Sea and the surface water conditions in the Northern Aegean Sea. We have developed a new high-resolution unstructured grid model (U-TSS) for the Marmara Sea including the Bosporus and Dardanelles Straits using the System of HydrodYnamic Finite Element Modules (SHYFEM). Using an unstructured grid in the horizontal better resolves geometry of the Turkish Straits. The new model has a resolution between 500 meter in the deep to 50 meter in the shallow areas, and 93 geopotential coordinate levels in the vertical. We conducted a 4 year hindcast simulation between 2016 and 2019 using lateral boundary conditions from CMEMS (https://marine.copernicus.eu/) analysis, in particular Black Sea Forecasting System (BS-FS) for the northern boundary and Mediterranean Sea Forecasting System (MS-FS) for the southern boundary. Atmospheric boundary conditions fare from the ECMWF dataset.</p><p>Mean averaged surface circulation shows that there is a cyclonic gyre in the middle of the basin due to Bosphorus jet flowing to the south and turning to west after reaching the southern Marmara coast. The U-TSS model has been validated against the seasonal in situ observations obtained from four different cruises between 2017 and 2018. The maximum bias occurs at around halocline depth between 20 to 30 meters.  We also found that root mean square error field is higher in the mixed layer interface. We conclude that capturing shallow mixed layer depth is very in the Marmara Sea due to the interplay of air-sea fluxes and mixing parametrizations uncertainties. Maximum salinity bias and rms in the new U-TSS model are around 3 psu which is a significant improvement with respect to previous studies. This new model will be used as an operational forecasting system and will provide lateral boundary conditions for the BS-FS and MS-FS models in the future.</p>

Miniature tropics and bi-diurnal oscillations

<p>Convective self-aggregation is a well-studied atmospheric state, obtained in typically multi-week idealized numerical experiments, where boundary conditions are constant and spatially homogeneous. As radiative convective equilibrium is approached, the atmosphere develops a heavily precipitating moist patch, which is surrounded by subsiding, cloud-free regions. It was recently shown that a homogeneous, but temporally oscillating surface temperature can quickly lead to the emergence of so-called mesoscale convective systems (MCS, diameters of >100 km) - on temporal scales of only a few days. Furthermore, the patterns formed by these MCS remind of checkerboards, and alternate from day to day [1]. </p><p>We here extend this finding further, to add realism to the otherwise preserved idealization: Mimicking a form of “miniature tropics” we retain a laterally periodic domain (Lx, Ly), but impose spatial variation in mean surface temperature along one dimension - reminiscent of a meridional reduction in mean surface temperature, when moving poleward from the equator. By making the wavelength of spatial variation commensurate with domain size, we retail double-periodic lateral boundary conditions. When the diurnal cycle is set to zero, the system quickly organizes to a forcefully aggregated caricature of the actual tropics - with heavy convection near the equator and pronounced subsidence and enhanced long-wave cooling in the subtropics. When the diurnal cycle is increased, bi-diurnal temporal oscillations appear, which lead to a single precipitation peak centered on the equator on one day, but a bimodal meridional pattern with precipitation away from the equator on the next.</p><p>Our findings, obtained for a still idealized numerical experiment, may have implications for “edge intensifications” suggested from observations and numerical modeling of tropical precipitation patterns near the ITCZ [2,3].</p><p>[1] Haerter, J.O., Meyer, B. & Nissen, S.B. Diurnal self-aggregation. <em>npj Clim Atmos Sci</em> <strong>3, </strong>30 (2020). https://doi.org/10.1038/s41612-020-00132-z</p><p>[2] Mapes, B. E., E.-S. Chung, W. M. Hannah, H. Masunaga, A. J. Wimmers and C. S. Velden, 2018: The meandering margin of the meteorological moist Tropics, <em>Geophys. Res. Lett.</em>, <strong>45</strong>, 1177-1184. doi:10.1002/2017GL076440</p><p>[3] Windmiller, J. M., & Hohenegger, C. 2019: Convection on the edge. <em>J. Adv. Model. Earth Syst.</em>, <strong>11</strong>, 3959-3972, 10.1029/2019MS001820</p>