Regional Atmospheric Modeling Research Articles

The Precipitation Structure of the Mediterranean Tropical-Like Cyclone Numa: Analysis of GPM Observations and Numerical Weather Prediction Model Simulations

This study shows how satellite-based passive and active microwave (MW) sensors can be used in conjunction with high-resolution Numerical Weather Prediction (NWP) simulations to provide insights of the precipitation structure of the tropical-like cyclone (TLC) Numa, which occurred on 15–19 November 2017. The goal of the paper is to characterize and monitor the precipitation at the different stages of its evolution from development to TLC phase, throughout the storm transition over the Mediterranean Sea. Observations by the NASA/JAXA Global Precipitation Measurement Core Observatory (GPM-CO) and by the GPM constellation of MW radiometers are used, in conjunction with the Regional Atmospheric Modeling System (RAMS) simulations. The GPM-CO measurements are used to analyze the passive MW radiometric response to the microphysical structure of the storm, while the comparison between successive MW radiometer overpasses shows the evolution of Numa precipitation structure from its early development stage on the Ionian Sea into its TLC phase, as it persists over southern coast of Italy (Apulia region) for several hours. Measurements evidence stronger convective activity at the development phase compared to the TLC phase, when strengthening or weakening phases in the eye development, and the occurrence of warm rain processes in the areas surrounding the eye, are identified. The weak scattering and polarization signal at and above 89 GHz, the lack of scattering signal at 37 GHz, and the absence of electrical activity in correspondence of the rainbands during the TLC phase, indicate weak convection and the presence of supercooled cloud droplets at high levels. RAMS high-resolution simulations support what inferred from the observations, evidencing Numa TLC characteristics (closed circulation around a warm core, low vertical wind shear, intense surface winds, heavy precipitation), persisting for more than 24 h. Moreover, the implementation of DPR 3D reflectivity field in the RAMS data assimilation system shows a small (but non negligible) impact on the precipitation forecast over the sea up to a few hours after the DPR overpass.

Influences of Lake Malawi on the spatial and diurnal variability of local precipitation

Abstract. We investigate how the intensity and spatial distribution of precipitation vary around Lake Malawi on a diurnal timescale, which can be valuable information for water resource management in tropical south-eastern African nations. Using a state-of-the-art satellite product and regional atmospheric model, the well-defined diurnal cycle is detected around Lake Malawi with harmonic and principle component analyses: the precipitation is intense during midnight to morning over Lake Malawi and the precipitation peaks in the daytime over the surrounding area. This diurnal cycle in the precipitation around the lake is associated with the lake–land breeze circulation. Comparisons between the benchmark simulation and an idealized simulation in which Lake Malawi is removed reveal that the diurnal variations in precipitation are substantially amplified by the presence of Lake Malawi. This is most evident over the lake and surrounding coastal regions. Lake Malawi also enhances the lake–land breeze circulation; the nocturnal lakeward land breeze generates surface convergence effectively and precipitation intensifies over the lake. Conversely, the daytime landward lake breeze generates the intense divergence over the lake and precipitation is strongly depressed over the lake. The lake–land breeze and the background vapour enriched by Lake Malawi drive primarily a diurnal variation in the surface moisture flux divergence/convergence over the lake and surrounding area which contributes to the diurnal cycle of precipitation in this region.

Sensitivity of the Near‐Shore Oceanic Circulation Off Central Chile to Coastal Wind Profiles Characteristics

AbstractIn Eastern Boundary Upwelling Systems (EBUS), the upwelling favorable wind speeds decrease toward the coast in the so‐called wind drop‐off coastal strip, which has been shown to be influential on the coastal upwelling dynamics, particularly in terms of the relative contributions of Ekman drift and Ekman suction to coastal upwelling. Currently, the wind drop‐off length scale is not properly resolved by the atmospheric forcing of regional ocean models in EBUS, featuring a smoother cross‐shore wind profile that results in stronger near‐shore speeds that could partly explain the coastal cold bias often found in those model simulations. Here, as a case study for the upwelling system off Central Chile, the sensitivity of upwelling dynamics to the coastal wind reduction is investigated using a Regional Ocean Modeling System (ROMS). Coastal wind profiles at different resolutions are first generated using a regional atmospheric model, validated from altimeter data, and then used to correct the coarse atmospheric wind forcing used for sensitivity experiments with ROMS. It is shown that the wind drop‐off correction induces a reduction in the oceanic coastal jet intensity, a stronger poleward undercurrent and a coherent offshore Ekman drift. It also yields a significant reduction of the cold bias along the coast compared to the simulation with “uncorrected” winds. Such reduction cannot be solely explained by the reduced Ekman transport only partially compensated by increase in Ekman suction. The analysis of the surface heat budget reveals in fact that an important contributor to the cooling reduction along the coast in the presence of coastal wind drop‐off is the heat flux term mediated by the reduction in the mixed‐layer depth. Overall, our results illustrate the nonlinear response of the upwelling dynamics to the coastal wind profiles in this region.

Energeticheskie harakteristiki tsirkulyatsii vod Mramornogo morya v 2008 godu

Purpose. The main objectives of the study consisted in analyzing the Marmara Sea energy and in identifying the basic reasons for formation of its circulation features in 2008. Methods and Results. The numerical experiment on modeling the Marmara Sea circulation and evaluating its energy characteristics on the example of 2008 was carried out based on the eddy-resolving nonlinear hydrodynamic model of Marine Hydrophysical Institute, RAS. The horizontal resolution of the model was 1.22 × 0.83 km, 18 horizons were used over vertical and the time step was 0.5 min. Temperature, salinity and water discharge through the Bosporus and the Dardanelles were preset according to the available measurement data. The applied fields of atmospheric forcing were obtained from the calculation by the regional atmospheric model MM5. The volume-integrated and yearaveraged contributions of the terms in the equations of the kinetic and potential energy budgets were analyzed. The basic factors of change in the Marmara Sea kinetic energy consisted in the buoyancy force action, wind, and vertical and horizontal mixing. Having been analyzed, the volume-average summands in the equation of rate of the kinetic energy change have shown that the wind-induced energy influx was compensated mainly by vertical friction, and the buoyancy force action – by horizontal friction. On the average, in course of a year the potential energy changed mainly due to its horizontal transport conditioned by the effect of the currents located in front of the straits, vertical diffusion and the buoyancy force impact. The results of analysis of the energy transitions permitted to reveal that in the upper sea layer, the mesoscale vortices were formed directly under the wind influence and as a result of the currents’ baroclinic instability. Generation of the sub-mesoscale vortices in the coastal regions was affected predominantly by the processes of baroclinic instability. In the region near the Bosporus, increase of the horizontal gradients in the density field and, consequently, intensification of dynamic processes are the results of inflow of the Black Sea waters with lower density. Conclusions. Study of the energy characteristics of the Marmara Sea water circulation permitted to conclude that baroclinic instability and wind are two of the basic sources of eddy formation in this basin.

Estimation of North African dust contribution on PM10 episodes at four continental Greek cities

Abstract African dust intrusions in the atmosphere of continental Greek cities have been previously associated with health impacts. This paper aimed to estimate the participation of Saharan dust in PM 10 episodes which occurred in four cities of continental Greece: Athens, Thessaloniki, Larisa and Ioannina, during the years 2012–2017. Exceedances of the European Union (EU) daily PM 10 concentration limit at background sampling sites situated in the four cities were examined for possible dust contributions with Dust Regional Atmospheric Model (DREAM). The participation of regional background emissions and dust in PM 10 Dust Related Exceedances (DRE) was estimated by an approach based on the daily PM 10 concentrations measured 15 days before and 15 days after the Saharan dust event. The average dust contribution in DRE was higher in Athens in comparison with the other cities. In addition, backward air mass trajectories indicated that the great majority of the studied DRE was triggered by direct dust intrusions from North Africa, whereas some rare cases of dust recirculation were also detected. All the studied DRE at the four cities wouldn’t have occurred without the presence of the African dust, thus the suggested approach could be a fast and inexpensive tool in order to isolate the reduced actual number of exceedances attributed to local/regional emissions. Consequently, air quality management can be simplified through the implemented method.

Suppression of winter heavy precipitation in Southeastern China by the Kuroshio warm current

The possible effects of the Kuroshio current and associated sea surface temperature (SST) fronts on Southeastern China winter precipitation are examined using twin simulations of a regional atmospheric model. When the SST frontal features in the forcing data are artificially weakened, the simulated rainfall over Southeastern China, particularly heavy rainfall, is found to increase by a statistically significant amount, compared to the simulations with the observed SST fronts. This difference is attributed to an anomalous regional overturning circulation that interferes with the onshore moisture transport. With the presence of stronger frontal SSTs, an anomalous ascending motion is anchored along the warm current, with a descending branch to its western side, forming a circulation cell that strides across the coasts of Southern China. In days of heavy rainfall over the Southeastern China, the regional atmospheric circulation tunnels oceanic moisture onto land across the coasts, serving as the primary water vapor source for the precipitation. This process is weakened by the anomalous regional overturning circulation in that the imbalance between low-level offshore and upper-level onshore flux drains the land of moisture, and the up-lifted moisture import aloft creates unfavorable condition for land precipitation.

Evaluating the impact of atmospheric forcing and air–sea coupling on near-coastal regional ocean prediction

Abstract. Atmospheric forcing applied as ocean model boundary conditions can have a critical impact on the quality of ocean forecasts. This paper assesses the sensitivity of an eddy-resolving (1.5 km resolution) regional ocean model of the north-west European Shelf (NWS) to the choice of atmospheric forcing and atmosphere–ocean coupling. The analysis is focused on a month-long simulation experiment for July 2014 and evaluation of simulated sea surface temperature (SST) in a shallow near-coastal region to the south-west of the UK (Celtic Sea and western English Channel). Observations of the ocean and atmosphere are used to evaluate model results, with a particular focus on the L4 ocean buoy from the Western Channel Observatory as a rare example of co-located data above and below the sea surface. The impacts of differences in the atmospheric forcing are illustrated by comparing results from an ocean model run in forcing mode using operational global-scale numerical weather prediction (NWP) data with an ocean model run forced by a convective-scale regional atmosphere model. The value of dynamically representing feedbacks between the atmosphere and ocean state is assessed via the use of these model components within a fully coupled ocean–wave–atmosphere system. Simulated SSTs show considerable sensitivity to atmospheric forcing and to the impact of model coupling in near-coastal areas. A warm ocean bias relative to in situ observations in the simulation forced by global-scale NWP (0.7 K in the model domain) is shown to be reduced (to 0.4 K) via the use of the 1.5 km resolution regional atmospheric forcing. When simulated in coupled mode, this bias is further reduced (by 0.2 K). Results demonstrate much greater variability of both the surface heat budget terms and the near-surface winds in the convective-scale atmosphere model data, as might be expected. Assessment of the surface heat budget and wind forcing over the ocean is challenging due to a scarcity of observations. However, it can be demonstrated that the wind speed over the ocean simulated by the convective-scale atmosphere did not agree as well with the limited number of observations as the global-scale NWP data did. Further partially coupled experiments are discussed to better understand why the degraded wind forcing does not detrimentally impact on SST results.

Assessment of climatological tropical cyclone activity over the north Indian Ocean in the CORDEX-South Asia regional climate models

This study evaluates the ability of the regional climate models (RCMs) in the Coordinated Regional Climate Downscaling Experiment for South Asia (CORDEX-SA) to simulate the tropical cyclone activity in the north Indian Ocean. The RCMs used in the CORDEX-SA are Rossby Centre regional atmospheric model version 4 (RCA4) and Regional Climatic Model version 4 (RegCM4) forced by the ECMWF Interim Reanalysis (ERA-Interim) from 1989 to 2008. Both models have reasonable skill in capturing the observed climatological seasonal and spatial distribution of the genesis and track of the tropical cyclones. However, the two issues faced are (i) in RegCM4 model, higher number of tropical cyclones throughout the year compared to observations in addition to the unrealistic simulation of tropical cyclones during winter and (ii) in the RCA4 model, relatively higher tropical cyclone activity over the Arabian Sea compared to the Bay of Bengal. In the RCA4, the anomalous large-scale lower tropospheric cyclonic circulation observed over the southern Arabian Sea might be responsible for the overestimation in the number of tropical cyclones over there. In both models, the relationship between minimum pressure and the maximum wind is well simulated even though there is an underestimation in the intensity of the tropical cyclones. Quantitative analysis of large-scale environmental parameters that influence the tropical cyclone activity using an index called genesis potential index showed that in the RegCM4 model, the model simulates seasonal variation (enhancement/suppression) of environmental conditions fairly good as seen in the observation. However, the model has higher number of tropical cyclones in all the seasons, which is due to the enhanced large scale dynamical and thermodynamical parameters. The cold and wet bias in the middle troposphere causes the reduction in the mid-layer saturation deficit throughout the year, which in turn favours an unrealistic high number of tropical cyclone genesis. Therefore, the changes in the large scale dynamics and relatively saturated middle troposphere in the model might be responsible for the higher number of tropical cyclones.

The Relative Impact of Ice Fall Speeds and Microphysics Parameterization Complexity on Supercell Evolution

Abstract The use of bin or bulk microphysics schemes in model simulations frequently produces large changes in the simulated storm and precipitation characteristics, but it is still unclear which aspects of these schemes give rise to these changes. In this study, supercell simulations using either a bin or a double-moment bulk microphysics scheme are conducted with the Regional Atmospheric Modeling System (RAMS). The two simulations produce very different storm morphologies. An additional simulation is run for each scheme in which the diameter–fall speed relationships for ice hydrometeors are modified to be similar to those used by the other scheme. When fall speed relationships are homogenized, the two parameterization schemes simulate similar storm morphology. Therefore, despite the use of largely dissimilar approaches to parameterizing microphysics, the difference in storm morphology is found to be related to the choice of diameter–fall speed relationships for ice hydrometeors. This result is investigated further to understand why. Higher fall speeds lead to higher mixing ratios of hydrometeors at low levels and thus more melting. Consequently, stronger downdrafts and cold pools exist in the high fall speed storms, and these stronger cold pools lead to storm splitting and the intensification of a left mover. The results point to the importance of hydrometeor fall speed on the evolution of supercells. It is also suggested that caution be used when comparing the response of a cloud model to different classes of microphysics schemes since the assumptions made by the schemes may be more important than the scheme class itself.

Effects of a Cold Ocean Eddy on Local Atmospheric Boundary Layer Near the Kuroshio Extension: In Situ Observations and Model Experiments

AbstractShipboard observations captured clear modulations of the marine atmospheric boundary layer (MABL) over a 300‐km‐diameter cold eddy in the Kuroshio Extension in April 2014. Regional atmospheric model experiments, with 30‐km and 10‐km horizontal resolutions for outer and inner domains, respectively, are conducted to aid the interpretations of the in situ observations. Enhanced turbulent heat fluxes promoted a well‐mixed MABL up to 1900‐m deep and a stratocumulus layer over the warm water west of the eddy. Over the cold sea surface of the eddy, the MABL was free of cloud and shrank to 1,200‐m deep. In the transition zone from the warm water in the west to the cold eddy, the MABL deepened and convective cumulus developed under a temperature inversion at 2,800 m. Model results indicate that, via the vertical mixing and pressure adjustment mechanisms, low sea surface temperatures of the cold eddy drives anomalous easterly winds in the transition zone, causing surface convergence to deepen the MABL and clouds. The model further shows that the atmospheric asymmetric responses in surface divergence and secondary circulation are due to the combined effects of the vertical mixing and pressure adjustment.

A Layer‐Averaged Nonhydrostatic Dynamical Framework on an Unstructured Mesh for Global and Regional Atmospheric Modeling: Model Description, Baseline Evaluation, and Sensitivity Exploration

AbstractThis paper describes the development and evaluation of a new nonhydrostatic dynamical framework for global and regional atmospheric modeling, with an emphasis on the numerical performance of dry dynamics. The model is formulated in a layer‐averaged manner using a generalized hybrid sigma‐mass vertical coordinate and an unstructured mesh. The mass‐based equations allow a flexible and effective switch between the hydrostatic and nonhydrostatic solvers. The unstructured mesh treats the conventional icosahedral grid and the more general Voronoi polygon in a consistent manner, allowing a flexible switch between quasi‐uniform and variable‐resolution modeling. The horizontal discretization and vertical discretization are formulated in an explicit Eulerian approach, while those terms describing the vertically propagating fast waves are solved implicitly. The model is equipped with physically based Smagorinsky diffusion as a tuning tool. A suite of multiscale test cases from hydrostatic to nonhydrostatic regimes is used to assess the model performance. The general strategies for evaluation focus on two aspects: (i) the nonhydrostatic solver should behave similarly to its hydrostatic counterpart under the hydrostatic regime and (ii) the nonhydrostatic solver should produce unique nonhydrostatic responses under the nonhydrostatic regime. In the context of model evaluation, model sensitivity to numerical configurations is further explored to understand the impact of isolated components, helping to identify appropriate configurations for realistic modeling applications. The present framework is a prototype toward a Global‐Regional Integrated forecast SysTem (GRIST).

Observing and Modeling Ice Sheet Surface Mass Balance.

Surface mass balance (SMB) provides mass input to the surface of the Antarctic and Greenland Ice Sheets and therefore comprises an important control on ice sheet mass balance and resulting contribution to global sea level change. As ice sheet SMB varies highly across multiple scales of space (meters to hundreds of kilometers) and time (hourly to decadal), it is notoriously challenging to observe and represent in models. In addition, SMB consists of multiple components, all of which depend on complex interactions between the atmosphere and the snow/ice surface, large‐scale atmospheric circulation and ocean conditions, and ice sheet topography. In this review, we present the state‐of‐the‐art knowledge and recent advances in ice sheet SMB observations and models, highlight current shortcomings, and propose future directions. Novel observational methods allow mapping SMB across larger areas, longer time periods, and/or at very high (subdaily) temporal frequency. As a recent observational breakthrough, cosmic ray counters provide direct estimates of SMB, circumventing the need for accurate snow density observations upon which many other techniques rely. Regional atmospheric climate models have drastically improved their simulation of ice sheet SMB in the last decade, thanks to the inclusion or improved representation of essential processes (e.g., clouds, blowing snow, and snow albedo), and by enhancing horizontal resolution (5–30 km). Future modeling efforts are required in improving Earth system models to match regional atmospheric climate model performance in simulating ice sheet SMB, and in reinforcing the efforts in developing statistical and dynamic downscaling to represent smaller‐scale SMB processes.

Comparison of Regional Atmospheric Model Forecasts under Different Initial and Boundary Conditions

The results of forecasts of the WRF-ARW numerical mesoscale model with two sets of initial and boundary conditions are presented. The first set comprises the forecasts of the GFS global model (USA), and the second set, the forecasts of the SL-AV global model (Russia). The quality of the WRF-ARW forecasts is assessed by their comparison with the data of surface and upper-air meteorological observations for the European part of Russia in winter and summer. It is shown that the 72-hour forecast results are close to the results based on the GFS data if the Russian global model data are used as initial and boundary conditions.

A New 200‐Year Spatial Reconstruction of West Antarctic Surface Mass Balance

AbstractHigh‐spatial resolution surface mass balance (SMB) over the West Antarctic Ice Sheet (WAIS) spanning 1800–2010 is reconstructed by means of ice core records combined with the outputs of the European Centre for Medium‐Range Weather Forecasts “Interim” reanalysis (ERA‐Interim) and the latest polar version of the Regional Atmospheric Climate Model (RACMO2.3p2). The reconstruction reveals a significant negative trend (−1.9 ± 2.2 Gt/year·per decade) in the SMB over the entire WAIS during the nineteenth century, but a statistically significant positive trend of 5.4 ± 2.9 Gt/year·per decade between 1900 and 2010, in contrast to insignificant WAIS SMB changes during the twentieth century reported earlier. At regional scales, the Antarctic Peninsula and western WAIS show opposite SMB trends, with different signs in the nineteenth and twentieth centuries. The annual resolution reconstruction allows us to examine the relationships between SMB and large‐scale atmospheric oscillations. Although SMB over the Antarctic Peninsula and western WAIS correlates significantly with the Southern Annular Mode due to the influence of the Amundsen Sea Low, and El Niño/Southern Oscillation during 1800–2010, the significant correlations are temporally unstable, associated with the phase of Southern Annular Mode, El Niño/Southern Oscillation and the Pacific decadal oscillation. In addition, the two climate modes seem to contribute little to variability in SMB over the whole WAIS on decadal‐centennial time scales. This new reconstruction also serves to identify unreliable precipitation trends in ERA‐Interim and thus has potential for assessing the skill of other reanalyses or climate models to capture precipitation trends and variability.

Assessment of two intense dust storm characteristics over Indo – Gangetic basin and their radiative impacts: A case study

The present study is focused to examine the impacts of two intense dust storms on aerosol characteristics and their radiative impacts occurred in pre-monsoon season of 2018 (i.e. 17 May and 14 June 2018) over Kanpur (26.51° N, 80.23° E, 123 above msl). Moderate Resolution Imaging Spectroradiometer (MODIS) true colour images, trajectory pathways of dust storm along with satellite observation and AErosol RObotic NETwork (AERONET) measurements confirms that both the dust storms are either originated from or transported over the Thar Desert, causing a higher aerosol loading which spread over entire Indian-Gangetic Basin (IGB) and modifying the aerosol optical (i.e. aerosol optical depth, angstrom exponent, refractive index etc.), physical (i.e. size distribution) and radiative properties (i.e. single scattering albedo, asymmetric parameter). The space-borne Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) - retrieved aerosol measurements reveal the presence of elevated dust/polluted dust aerosol (up to 3–5 km) over IGB which is well corroborated with aerosol characteristics observed by MODIS, Ozone Monitoring Instrument (OMI) and Atmospheric Infrared Sounder (AIRS). The Dust Regional Atmospheric Model (DREAM8b) shows a good agreement with satellite retrievals with higher value of surface dust concentration in the range of 320–640 μg/m3 over Kanpur during the dust storm days. An enhancement in monthly mean outgoing longwave radiation (up to 60 Wm−2) is observed over IGB and downwind flow region during the dust storm days. The atmospheric aerosol radiative forcing is found 124 Wm−2 and 84 Wm−2 during both the dust storm days (17 May and 14 June 2018) associated with heating rate 2.69 K day−1 and 1.84 K day−1 respectively which may be significant to affect the regional atmospheric dynamics and hence the climate system also.

Observed Variations of the Atmospheric Boundary Layer and Stratocumulus over a Warm Eddy in the Kuroshio Extension

Abstract A research vessel sailing across a warm eddy in the Kuroshio Extension on 13 April 2016 captured an abrupt development of stratocumulus under synoptic high pressure. Shipboard observations and results from regional atmospheric model simulations indicate that increased surface heat flux over the ocean eddy lowered surface pressure and thereby accelerated southeasterly winds. The southeasterly winds transported moisture toward the low pressure and enhanced the air–sea interface heat flux, which in turn deepened the low pressure and promoted low-level convergence and rising motion over the warm eddy. The lifting condensation level lowered and the top of the marine atmospheric boundary layer (MABL) rose, thereby aiding the development of the stratocumulus. Further experiments showed that 6°C sea surface temperature anomalies associated with the 400-km-diameter warm eddy accounted for 80% of the total ascending motion and 95% of total cloud water mixing ratio in the marine atmospheric boundary layer during the development of stratocumulus. The synthesis of in situ soundings and modeling contributes to understanding of the mechanism by which the MABL and marine stratocumulus respond to ocean eddies.

Response of tropical cyclone Phailin (2013) in the Bay of Bengal to climate perturbations

Tropical cyclones in the Bay of Bengal (BoB) are the major source of loss of properties and lives in the adjacent coastal regions during post-monsoon (October–November) season. This study explores the impact of large-scale environmental change on tropical cyclone (TC) Phailin (2013) in the Bay of Bengal. An ensemble simulation with a regional atmospheric model using different physics and initial conditions is able to capture the observed track of Phailin. The cyclone intensity, however, is underestimated due to the coarse resolution (10 km) of the model. To explore Phailin’s response to a warming climate, a global climate model change signal is added to the cyclone environment using the pseudo-global warming approach, and the ensemble is run again. It is found that changes in the track of Phailin would be small, except around the time of landfall, when Phailin shifts southward in its track. The cyclone becomes more intense and larger, but the translation speed remains nearly unchanged. An analysis of physical processes indicates a deeper TC core due to enhanced latent heating. The vertical distribution of temperature and wind indicates that the eye of Phailin would have a greater vertical extent in the future climate. The expanded size is found to be due to an increase in environmental humidity and temperature in the lower-troposphere leading to an increase in environmental convective available potential energy. The translation speed remains nearly unchanged due to insignificant changes in the steering flow. The result suggests that Phailin, under a warmer climate, may have higher damage potential (~ 22%) compared to the present at the time of landfall.

Impact of increased atmospheric moisture on the precipitation depth caused by Hurricane Ivan (2004) over a target area

In this article, numerical experiments are performed to investigate the effects of increasing atmospheric moisture on the precipitation depth (PD) produced by Hurricane Ivan (2004) over a target area, chosen as the drainage basin of the city of Asheville, NC. Atmospheric moisture was increased indirectly by increasing the sea surface temperature (SST) in the simulation initial conditions, and by letting the regional atmospheric model adjust the atmospheric fields to the SST perturbation. The SST was increased in two ways: 1) using spatially constant increments and 2) using a climate change perturbation field obtained from a climate projection. For each SST scenario, the PD over the target area was maximized by using a physically based storm transposition method. Although the mean PD, that was obtained by averaging over all shifting increments, increased with SST, the maximum PD was obtained for the case of no SST increase. It was found that, in the case of no SST increase, the worst-case tropical cyclone track was significantly different than in the SST increase scenarios. In particular, in this case, the storm spent a longer time in the simulation inner domain, thus spawning a larger PD over the target area.

Data assimilation of GPS-ZTD into the RAMS model through 3D-Var: preliminary results at the regional scale

The knowledge of water vapour distribution is a key element in atmospheric modeling and considerable information, also at the local scale, can be derived from the GPS-ZTD (global positioning system-Zenith total delay) data.This paper shows the assimilation of GPS-ZTD data into the RAMS@ISAC (Regional Atmospheric Modeling System at Institute of Atmospheric Sciences and Climate of the National Research Council) to improve the representation of the water vapour in the meteorological model.The data assimilation system is based on 3D-Var (three-dimensional variational assimilation system) and it is applied to a network of 29 receivers located within the Lazio Region, Central Italy. All collected data are processed using the PPP (precise point positioning) method through RTKLIB, an open source program package for GNSS (Global Navigation Satellite Systems) Positioning. Among the GPS receivers, three are single frequency receivers, able to acquire L1 frequency only, so that it is necessary a preliminary reconstruction of L2 synthetic observations, which is achieved by a new original ground-based augmentation strategy. Results show remarkably that the single frequency receivers can be used the same way as geodetic receivers.The RAMS@ISAC is run at 4 km horizontal resolution over central Italy and is nested, using one-way nesting, into a 10 km horizontal resolution run of the same model. The experiment was performed along to two months, from 28 July to 28 September 2017.Results show that the GPS-ZTD data, assimilated by 3D-Var, have an important impact on the analysis of the water vapour field and the RMSE of ZTD and IWV (vertically integrated water vapour) is roughly halved for the analysis compared to the background.The impact of the GPS-ZTD data assimilation is also evaluated for the very short term (VSF) forecast (1–3 h), obtaining an improvement of the ZTD and IWV RMSE for all three hours of forecast.

Development of a dynamic dust source map for NMME-DREAM v1.0 model based on MODIS Normalized Difference Vegetation Index (NDVI) over the Arabian Peninsula

Abstract. We developed a time-dependent dust source map for the NMME Dust Regional Atmospheric Model (DREAM v1.0) based on the satellite MODIS Normalized Difference Vegetation Index (NDVI). Areas with NDVI <0.1 are classified as active dust sources. The updated modeling system is tested for dust emission capabilities over SW Asia using a mesoscale model grid increment of 0.1∘×0.1∘ for a period of 1 year (2016). Our results indicate significant deviations in simulated aerosol optical depths (AODs) compared to the static dust source approach and general increase in dust loads over the selected domain. Comparison with MODIS AOD indicates a more realistic spatial distribution of dust in the dynamic source simulations compared to the static dust sources approach. The modeled AOD bias is improved from −0.140 to 0.083 for the case of dust events (i.e., for AOD >0.25) and from −0.933 to −0.424 for dust episodes with AOD >1. This new development can be easily applied to other time periods, models, and different areas worldwide for a local fine tuning of the parameterization and assessment of its performance.