Nonhydrostatic Mesoscale Model Research Articles

A study of the microphysical processes in a numerically simulated heavy snowfall event in North China: the sensitivity of different snow intercept parameters

We performed a modeling study of the cloud processes in a heavy snowfall event occurring in North China on 20–22 December 2004. The nonhydrostatic Mesoscale Model (MM5) was used to carry out experiments with the Reisner-2 explicit microphysical parameterizations in four nested domains to test the sensitivity of simulated heavy snowfall to different snow intercept parameters. Results show that while the different intercept parameters do not significantly affect the accumulated snowfall amounts at the surface in either total amount or location, some microphysical characteristics of the modeled heavy snowfall event are impacted. The budget of cloud microphysics is analyzed to determine the dominant cloud processes. In the control experiment (CTL) with the snow intercept (Nos) specified as a function of temperature, the primary simulated hydrometeor is snow, and its mixing ratio is an order of magnitude larger than that of the other cloud species. Relative to CTL, the experiment with a fixed intercept (CON3E6) produced lower snow mixing ratios, more cloud water and graupel mixing ratios. Among the two experiments, while snowfall is slightly smaller in CON3E6, other processes like the rate of graupel fall, condensation and evaporation of cloud water, deposition and sublimation of graupel are all larger in CON3E6 than in CTL. Among CTL, CON3E6, and two more experiments (CON2E7: with a smaller fixed intercept; and NOSQS: Nos a function of snow mass mixing ratio), the budget shows that CON3E6 produces the smallest deposition and sublimation of snow, the largest deposition of cloud ice, and the largest conversion from cloud ice to snow.

The effect of typhoon-induced SST cooling on typhoon intensity: The case of Typhoon Chanchu (2006)

In order to investigate air-sea interactions during the life cycle of typhoons and the quantificational effects of typhoon-induced SST cooling on typhoon intensity, a mesoscale coupled air-sea model is developed based on the non-hydrostatic mesoscale model MM5 and the regional ocean model POM, which is used to simulate the life cycle of Typhoon Chanchu (2006) from a tropical depression to a typhoon followed by a steady weakening. The results show that improved intensity prediction is achieved after considering typhoon-induced SST cooling; the trend of the typhoon intensity change simulated by the coupled model is consistent with observations. The weakening stage of Typhoon Chanchu from 1200 UTC 15 May to 1800 UTC 16 May can be well reproduced, and it is the typhoon-induced SST cooling that makes Chanchu weaken during this period. Analysis reveals that the typhoon-induced SST cooling reduces the sensible and latent heat fluxes from the ocean to the typhoon’s vortex, especially in the inner-core region. In this study, the average total heat flux in the inner-core region of the typhoon decrease by 57.2%, whereas typhoon intensity weakens by 46%. It is shown that incorporation of the typhoon-induced cooling, with an average value of 2.17°C, causes a 46-hPa weakening of the typhoon, which is about 20 hPa per 1°C change in SST.

Enhancement of land surface information and its impact on atmospheric modeling in the Heihe River Basin, northwest China

With increasing computational resources, atmospheric/environmental models continue to run at finer‐grid spacing that can resolve land surface characteristics, such as topography, land use/land cover, and soil texture. This paper assesses the improvement in land surface information data sets and its impact on atmospheric modeling. The study focuses on the Heihe River Basin (HRB) in northwestern China. Fine‐scale, remotely sensed, and in situ land surface data in HRB are derived and compared with the global data sets used in most mesoscale models. The incorporation of these fine‐scale land surface data, compared to those currently used in MM5, yields substantially improved HRB land surface data sets. HRB local and regional data sets and the global land data set are used in a nonhydrostatic mesoscale model (MM5) to investigate the influences of land surface uncertainty on meteorological modeling in the lower atmosphere. Main results suggest the following: (1) enhanced land data sets have a stronger impact on atmospheric water vapor fields in the lower boundary layer than other meteorological fields. Soil texture data greatly impacts the local precipitation simulation and landuse data improves the air temperature simulation in the lower atmosphere; (2) generally, the average land surface temperature biases are reduced using the enhanced land surface information, but the low bias in zones with higher elevation and high bias in zones with lower elevation still persist; (3) the wet bias over rugged terrain and dry bias in the simulated water vapor in the flat plains are both reduced. Area mean bias of simulated accumulated monthly precipitation is greatly reduced using the enhanced soil data. Convective available potential energy was larger in the HRB mountain regions using the default land data, while it was decreased using the enhanced ones; (4) analysis of the correlation coefficient between simulation bias and the geographic features shows that there are some patterns in the simulation bias distribution. Generally, larger bias still exists in the foothills of the mountains.

Sensitivity Studies of the Role of Aerosols in Warm-Phase Orographic Precipitation in Different Dynamical Flow Regimes

Abstract Aerosols serve as a source of cloud condensation nuclei (CCN) and influence the microphysical properties of clouds. In the case of orographic clouds, it is suspected that aerosol–cloud interactions reduce the amount of precipitation on the upslope side of the mountain and enhance the precipitation on the downslope side when the number of aerosols is increased. The net effect may lead to a shift of the precipitation distribution toward the leeward side of mountain ranges, which affects the hydrological cycle on the local scale. In this study aerosol–cloud interactions in warm-phase clouds and the possible impact on the orographic precipitation distribution are investigated. Herein, simulations of moist orographic flow over topography are conducted and the influence of anthropogenic aerosols on the orographic precipitation formation is analyzed. The degree of aerosol pollution is prescribed by different aerosol spectra that are representative for central Switzerland. The simulations are performed with the Consortium for Small-Scale Modeling’s mesoscale nonhydrostatic limited-area weather prediction model (COSMO) with a horizontal grid spacing of 2 km and a fully coupled aerosol–cloud parameterization. It is found that an increase in the aerosol load leads to a downstream shift of the orographic precipitation distribution and to an increase in the spillover factor. A reduction of warm-phase orographic precipitation is observed at the upslope side of the mountain. The downslope precipitation enhancement depends critically on the width of the mountain and on the flow dynamics. In the case of orographic precipitation induced by stably stratified unblocked flow, the loss in upslope precipitation is not compensated by leeward precipitation enhancement. In contrast, flow blocking may lead to leeward precipitation enhancement and eventually to a compensation of the upslope precipitation loss. The simulations also indicate that latent heat effects induced by aerosol–cloud–precipitation interactions may considerably affect the orographic flow dynamics and consequently feed back on the orographic precipitation development.

Rotation of sea/land breezes along the northeastern Adriatic coast

Abstract. The diurnal evolution of the sea/land breeze hodographs over the northeastern Adriatic coast has been examined for several episodes, each lasting a few days. A limited set of observations, as well as the results of a three-dimensional nonhydrostatic mesoscale model are studied by applying a rotary-component method. This revealed a spatial distribution of the clockwise versus anti-clockwise rotation of the wind vectors for the sea/land breeze events. An application of the rotary-component method to the results of additional numerical sensitivity tests showed that the coastal topography height considerably influences both the shape of the wind hodographs and the sense of wind rotation. The method also displayed which areas are under a strong influence of the 24-h topographic forcing and which areas show a large wind consistency. A further analysis of the 24-h modeled wind hodographs at several selected stations has been performed, thus revealing the hodograph shapes and the reasons for them.

A Midlatitude Precipitating Cloud Database Validated with Satellite Observations

Abstract The simulations of five midlatitude precipitating events by the nonhydrostatic mesoscale model Méso-NH are analyzed. These cases cover contrasted precipitation situations from 30° to 60°N, which are typical of midlatitudes. They include a frontal case with light precipitation over the Rhine River area (10 February 2000), a long-lasting precipitation event at Hoek van Holland, Netherlands (19 September 2001), a moderate rain case over the Elbe (12 August 2002), an intense rain case over Algiers (10 November 2001), and the “millennium storm” in the United Kingdom (30 October 2000). The physically consistent hydrometeor and thermodynamic outputs are used to generate a database for cloud and precipitation retrievals. The hydrometeor vertical profiles that were generated vary mostly with the 0°C isotherm, located between 1 and 3 km in height depending on the case. The characteristics of this midlatitude database are complementary to the GPROF database, which mostly concentrates on tropical situations. The realism of the simulations is evaluated against satellite observations by comparing synthetic brightness temperatures (BTs) with Advanced Microwave Sounding Unit (AMSU), Special Sensor Microwave Imager (SSM/I), and Meteosat observations. The good reproduction of the BT distributions by the model is exploited by calculating categorical scores for verification purposes. The comparison with 3-hourly Meteosat observations demonstrates the ability of the model to forecast the time evolution of the cloud cover, the latter being better predicted for the stratiform cases than for others. The comparison with AMSU-B measurements shows the skill of the model to predict rainfall at the correct location.

Preliminary study of an intense rainfall episode in Corsica, 14 September 2006

Abstract. A preliminary study of the 14 September 2006 heavy rainfall episode in Corsica is presented. Two sets of initial and coupling fields were used to run the French non-hydrostatic mesoscale model, MESO-NH. Three interactive 2-way nested domains were used, having 150×120, 150×150 and 125×200 grid points and 50 km, 10 km and 2 km mesh sizes. 50 vertical levels were defined. From ground level to 2000 m, the vertical resolution was 60 m, whereas it was 600 m above. Convection was parameterized for the coarser and intermediate domains and assumed to be explicitly resolved for the 2 km grid. At large scale, the situation is well represented by both simulations but, at the fine scale, they present significant differences. Comparison with the observations did not allow the two simulations to be discriminated. Even though the main high-level dynamical ingredients usually associated with rain storms were present in both simulations, their fine scale evolution was not reproduced well. Near the surface, there were significant differences between the two simulations. Vertical motion fields associated with convective phenomena were different in the innermost domain. A cold pool acting as a warm front off the east coast was associated with the main rainfall area over the sea in one simulation whereas in the second one, it seemed that the Corsican mountains acted as a barrier, leading to inland rainfall. This preliminary study gives perspectives for future sensitivity tests. For example, an investigation is planned of the influence of the high level dynamics, the SST and a better description of the orography.

The VOL‐CALPUFF model for atmospheric ash dispersal: 2. Application to the weak Mount Etna plume of July 2001

Here the application of the VOL‐CALPUFF model by Barsotti et al. to a weak plume erupted from Mount Etna in July 2001 is presented and discussed. The reconstruction of the explosive event was obtained by using high‐resolution weather forecasts, produced by a mesoscale nonhydrostatic model, and volcanic source data coming from observations and analytical studies. The plume rise and atmospheric dispersal models were investigated over 5 d of eruption mostly in terms of column height, aerial ash concentration, and ground deposition. Modeling results are shown as a function of various source conditions and compared to independent observations derived from satellite images and deposit mapping. The application of VOL‐CALPUFF clearly highlights the crucial role played by meteorological conditions in determining dispersal dynamics. Some of the most important effects described by the model are (1) the large wind field influence on the plume height determination and tilting, (2) the contrasting dispersal patterns of ash particles of different sizes, (3) the complex and somehow nonintuitive distribution of ash on the ground resulting in preferential directions of dispersal and quite irregular deposit patterns, and (4) the impossibility to reproduce both the column height and the deposit accumulation pattern by adopting a steady state vent mass flow rate over the investigated 4‐d period due to observed temporal changes in eruption dynamics. Modeling results also suggest the need for further integration of simulation outcomes with remote sensing and field reconstructions on ash dispersal processes in future.

Cluster Analysis for Object-Oriented Verification of Fields: A Variation

Abstract In a recent paper, a statistical method referred to as cluster analysis was employed to identify clusters in forecast and observed fields. Further criteria were also proposed for matching the identified clusters in one field with those in the other. As such, the proposed methodology was designed to perform an automated form of what has been called object-oriented verification. Herein, a variation of that methodology is proposed that effectively avoids (or simplifies) the criteria for matching the objects. The basic idea is to perform cluster analysis on the combined set of observations and forecasts, rather than on the individual fields separately. This method will be referred to as combinative cluster analysis (CCA). CCA naturally lends itself to the computation of false alarms, hits, and misses, and therefore, to the critical success index (CSI). A desirable feature of the previous method—the ability to assess performance on different spatial scales—is maintained. The method is demonstrated on reflectivity data and corresponding forecasts for three dates using three mesoscale numerical weather prediction model formulations—the NCEP/NWS Nonhydrostatic Mesoscale Model (NMM) at 4-km resolution (nmm4), the University of Oklahoma’s Center for Analysis and Prediction of Storms (CAPS) Weather Research and Forecasting Model (WRF) at 2-km resolution (arw2), and the NCAR WRF at 4-km resolution (arw4). In the small demonstration sample herein, model forecast quality is efficiently differentiated when performance is assessed in terms of the CSI. In this sample, arw2 appears to outperform the other two model formulations across all scales when the cluster analysis is performed in the space of spatial coordinates and reflectivity. However, when the analysis is performed only on spatial data (i.e., when only the spatial placement of the reflectivity is assessed), the difference is not significant. This result has been verified both visually and using a standard gridpoint verification, and seems to provide a reasonable assessment of model performance. This demonstration of CCA indicates promise in quickly evaluating mesoscale model performance while avoiding the subjectivity and labor intensiveness of human evaluation or the pitfalls of non-object-oriented automated verification.

The Effect of Changes of Urban Surfaces on Rainfall Phenomenon as Determined by a Non-Hydrostatic Mesoscale Model

A non-hydrostatic, mesoscale model, LOCALS, is used to investigate the effect of land surface changes on precipitation in an urban area. The horizontal mesh used in the model is approximately 5 km and the domain includes Tokyo and central Japan. Land surfaces identified as pavement or urban are transformed into forested land and model simulations are performed for both the current urban land surface and the hypothetical forested land surface. Detailed analysis is performed for two sample cases. On 5 August 2003, increased heating by the urban surfaces increases the height of the urban boundary layer resulting in much more rainfall. On 25 July 2001, air that is advected over Tokyo but forms a precipitating cloud away from the urban core carries less moisture and therefore produces less rainfall. These case studies suggest that changes to land surface types in urban areas may increase or decrease rainfall in the urban area and further research is necessary before it is possible to generalize these results.

Investigation of the atmospheric boundary layer dynamics during the ESCOMPTE campaign

Abstract. This paper presents some results about the behavior of the atmospheric boundary layer observed during the ESCOMPTE experiment. This campaign, which took place in south-eastern France during summer 2001, was aimed at improving our understanding of pollution episodes in relation to the dynamics of the lower troposphere. Using a large data set, as well as a simulation from the mesoscale non-hydrostatic model Meso-NH, we describe and analyze the atmospheric boundary layer (ABL) development during two specific meteorological conditions of the second Intensive Observation Period (IOP). The first situation (IOP2a, from 22 June to 23 June) corresponds to moderate, dry and cold northerly winds (end of Mistral event), coupled with a sea-breeze in the lower layer, whereas sea-breeze events with weak southerly winds occurred during the second part of the period (IOP2b, from 24 June to 26 June). In this study, we first focus on the validation of the model outputs with a thorough comparison of the Meso-NH simulations with fields measurements on three days of the IOP: 22 June, 23 June and 25 June. We also investigate the structure of the boundary layer on IOP2a when the Mistral is superimposed on a sea breeze. Then, we describe the spatial and diurnal variability of the ABL depths over the ESCOMPTE domain during the whole IOP. This step is essential if one wants to know the depth of the layer where the pollutants can be diluted or accumulated. Eventually, this study intends to describe the ABL variability in relation to local or mesoscale dynamics and/or induced topographic effects, in order to explain pollution transport processes in the low troposphere.

Simulation of a supercellular storm using a three‐dimensional mesoscale model with an explicit lightning flash scheme

A complete lightning flash scheme is implemented in the three‐dimensional (3‐D) nonhydrostatic mesoscale model Méso‐NH of the French community. The scheme, which is part of the electrical scheme, follows a new approach with two steps. First, lightning flashes are modeled as bidirectional leaders to mimic the vertical propagation of the initial discharge channels along the electric field. Then, a probabilistic branching algorithm is adapted from the dielectric breakdown concept to reinforce the flash propagation toward distant regions of high charge density but immersed in a weak electric field. This results in a high increase of the total length of the lightning flash channel and also in a better capture of the morphology of intracloud lightning flashes. The electrification and lightning schemes are tested for an ideal case of a supercellular storm. The model succeeds in reproducing the general features of a storm and the electric charge cycle. Sensitivity analyses show that the implementation of a branching stage is necessary and efficient enough to relax the growth of the electric field. The intracloud discharges generated by the model look realistic with a two‐layer horizontal structure extending over tens of kilometers from the triggering area. The lightning flash length and the quantity of charge neutralized are ten times more important when the branching algorithm is taken into account. The main conclusion drawn from this study is the feasibility and the benefit of an advanced treatment of lightning flashes in 3‐D numerical simulations with an electrification scheme.

Impact of cloud microphysical processes on hurricane intensity, part 2: Sensitivity experiments

Seven different microphysical sensitivity experiments were designed with an objective to evaluate their respective impacts in modulating hurricane intensity forecasts using mesoscale model MM5. Microphysical processes such as melting of graupel, snow and cloud ice hydrometeors, suppression of evaporation of falling rain, the intercept parameter and fall speed of snow and graupel hydrometeors are modified in the existing NASA Goddard Space Flight Center (GSFC) microphysical parameterization scheme. We studied the impacts of cloud microphysical processes by means of track, intensity, precipitation, propagation speed, kinematic and thermodynamic vertical structural characteristics of hurricane inner core. These results suggest that the set of experiments where (a) melting of snow, graupel and cloud ice were suppressed (b) melting of snow and graupel were suppressed and (c) where the evaporation of rain water was suppressed all produced most intense storms. The major findings of this study are the interconversion processes such as melting and evaporation among hydrometeors and associated feedback mechanism are significantly modulate the intensity of the hurricane. In particular an experiment where the melting of graupel, snow and cloud ice hydrometeors was eliminated from the model parameterization scheme produced the most explosively intensified storm. In the experiment where rain water evaporation was eliminated from the model, it produced a stronger storm as compared to the control run but it was not as strong as the storms produced from absence of melting processes. The impact on intensity due to variations made in intercept parameters of the hydrometeors (i.e., snow and graupel) were not that evident compared to other experiments. The weakest storm was noted in the experiment where the fall speeds of the snow hydrometeors were increased two fold. This study has isolated some of the factors that contributed to a stronger hurricane and concludes with a motivation that the findings from this study will help in further improvement in the design of sophisticated explicit microphysical parameterization for the mesoscale non-hydrostatic model for realistic hurricane intensity forecasts.

Nonhydrostatic Atmospheric Models and Operational Development at JMA

This paper reviews nonhydrostatic atmospheric models for research and NWP. Classification of nonhydrostatic atmospheric models and numerical methods to treat sound waves are described with their relative advantages. The current operational nonhydrostatic NWP models at various forecast centers and community nonhydrostatic models for research are reviewed. Brief history and development of the JMA nonhydrostatic model, a community mesoscale model for research and NWP in Japan, is introduced. Current status and near future plans of the operational nonhydrostatic mesoscale model at JMA are presented.

A study on kinematic trajectory calculations with different schemes using wind field of varying temporal frequency

Computation of kinematic trajectories has got immense importance in assessing transport pathways of tracers and evaluating airflow patterns within meteorological systems. In a Lagrangian description of the transport of air pollutants, the kinematic trajectory of an air parcel is estimated using observed wind data or the output of a Numerical Weather Prediction (NWP) model. In this study, a non-hydrostatic mesoscale model is used to generate the meteorological data for a period, during which an active Western Disturbance (WD) crossed the Indian region. The 'true atmospheric trajectories (forward)' are constructed using model-simulated wind with 12 minutes time interval. The true trajectories are then compared to the trajectories that were constructed using simulated wind data with degraded temporal frequency of 36 min. and 1, 2, 3 and 6 hour. Five schemes including the well-known Petterssen scheme are used to calculate 72-hour forward trajectories. Comparison of the results using different schemes show that in terms of accuracy and computational efficiency, the midpoint method of integration (scheme-4) is comparatively better and the simplified Innocentini's scheme, which accounts for the spatial dependence of the wind field all along the particle path, gives results similar to the Petterssen scheme. Further, it is shown that when wind data are of low temporal frequency, use of smaller time steps will not improve the results.

Explicit simulation on the track and intensity of tropical cyclone Winnie (1997)

An explicit simulation of the tropical cyclone Winnie (1997) was conducted by use of a triply-nested 3-D nonhydrostatic mesoscale model MM5, before and after its landfall. The simulated results show that the model can well reproduce the track of Winnie (1997) with a BOGUS storm-like vortex inserted in the large-scale first-guess field at the initial model time. The mean track error in a 6 h interval is 54.5 km, which is much less than that obtained with the operational prediction. Comparisons between simulation and observation near the landfall place show that the main temporal variation features of sea level pressure and surface wind speed can also be successfully captured. The characteristics of the eyewall, the spiral cloud band and the corresponding precipitation were disclosed, and the applications of the model outputs were also discussed.

A New Visibility Parameterization for Warm-Fog Applications in Numerical Weather Prediction Models

Abstract The objective of this work is to suggest a new warm-fog visibility parameterization scheme for numerical weather prediction (NWP) models. In situ observations collected during the Radiation and Aerosol Cloud Experiment, representing boundary layer low-level clouds, were used to develop a parameterization scheme between visibility and a combined parameter as a function of both droplet number concentration Nd and liquid water content (LWC). The current NWP models usually use relationships between extinction coefficient and LWC. A newly developed parameterization scheme for visibility, Vis = f (LWC, Nd), is applied to the NOAA Nonhydrostatic Mesoscale Model. In this model, the microphysics of fog was adapted from the 1D Parameterized Fog (PAFOG) model and then was used in the lower 1.5 km of the atmosphere. Simulations for testing the new parameterization scheme are performed in a 50-km innermost-nested simulation domain using a horizontal grid spacing of 1 km centered on Zurich Unique Airport in Switzerland. The simulations over a 10-h time period showed that visibility differences between old and new parameterization schemes can be more than 50%. It is concluded that accurate visibility estimates require skillful LWC as well as Nd estimates from forecasts. Therefore, the current models can significantly over-/underestimate Vis (with more than 50% uncertainty) depending on environmental conditions. Inclusion of Nd as a prognostic (or parameterized) variable in parameterizations would significantly improve the operational forecast models.

Numerical experiments investigating the mechanisms of a heavy rainfall event over northeastern Taiwan and a mesovortex during TAMEX

This study analyzes the mechanisms of the development of a heavy rainfall event (17 June 1987) over the lee side of the Central Mountain Range (CMR) in northeastern Taiwan during the southwesterly monsoon. This heavy rainfall event was examined using gridded data from the European Centre for Medium-Range Weather Forecasts, surface rainfall data and numerical model results, employing a non-hydrostatic fifth-generation mesoscale model (MM5) developed by the National Center for Atmospheric Research and Pennsylvania State University. A tropical depression was simulated over the northern South China Sea on 16 June. Convergence, resulting from the southeasterly winds associated with the circulation from the tropical depression, and northeasterly winds over the Taiwan Strait, occurred over the northern Bashi Channel at 850 hPa. The convergence amplified planetary vorticity and the vorticity associated with the intensifying tropical depression. Consequently, a mesovortex with low pressure over the northeastern edge of the tropical depression near southern Taiwan was produced. Additional convergence over the ocean adjacent to southern Taiwan caused by the interaction between the northeasterly flow, which was deflected over the southeastern slope of the CMR, and the southeasterly flow of the tropical depression, also affected the intensity of the mesovortex. When the mesovortex moved northward and reached southern Taiwan, the southeasterly flow associated with it interacted with an east-southeasterly flow, which was related to the tropical depression, to form a mesoscale convective system (MCS) over the ocean adjacent to southeastern Taiwan. As the mesovortex moved northward, the MCS, which was embedded in the southeasterly flow, also drifted inland toward northeastern Taiwan. The orographic lifting and the ascending motion associated with the deceleration of the easterly flow near the CMR enhanced the MCS over northeastern Taiwan and produced heavy rainfall.

Shallow mesoscale flows at the north‐eastern Adriatic coast

AbstractA summertime anticyclonic situation over the northern part of the eastern Adriatic coast is studied using a three‐dimensional nonhydrostatic mesoscale model. The aim is to evaluate small‐scale variability, such as mesoscale eddies, sea‐breeze convergence zones (CZs) and the coastal jet‐like flow in the thermally driven mesoscale circulations over very complex terrain. The results show that the model reproduces main known features of the diurnal flow with certain local specific characteristics, in addition to some new ones, previously only somewhat qualitatively known. After comparisons with measurements, further analyses are performed by sensitivity tests in which topography and synoptic input are varied.The simulations show a development of two diurnal intensive mesoscale eddies inside Kvarner Bay (∼20 km×20 km). The night‐time deeper eddies exhibit anticlockwise rotation, while the late afternoon shallow one shows the opposite rotation. Their appearance and dynamics are connected primarily to the coastline shape and topography; meanwhile, the large‐scale flow in the anticyclonic weather type, as addressed here, has small influence. Moreover, a CZ develops over Istria and the island of Krk as a result of merged sea breezes. The intensity, speed, position and configuration of the sea‐breeze cells are examined. The results also show that the topography is more important for the formation of the CZ over the island of Krk and for the coastal jet‐like flow between the island and mainland than for the Istrian CZ. Copyright © 2006 Royal Meteorological Society

Comparison of Impacts of WRF Dynamic Core, Physics Package, and Initial Conditions on Warm Season Rainfall Forecasts

Abstract A series of simulations for 15 events occurring during August 2002 were performed using the Weather Research and Forecasting (WRF) model over a domain encompassing most of the central United States to compare the sensitivity of warm season rainfall forecasts with changes in model physics, dynamics, and initial conditions. Most simulations were run with 8-km grid spacing. The Advanced Research WRF (ARW) and the nonhydrostatic mesoscale model (NMM) dynamic cores were used. One physics package (denoted NCEP) used the Betts–Miller–Janjic convective scheme with the Mellor–Yamada–Janjic planetary boundary layer (PBL) scheme and GFDL radiation package; the other package (denoted NCAR) used the Kain–Fritsch convective scheme with the Yonsei University PBL scheme and the Dudhia rapid radiative transfer model radiation. Other physical schemes were the same (e.g., the Noah land surface model, Ferrier et al. microphysics) in all runs. Simulations suggest that the sensitivity of the model to changes in physics is a function of which the dynamic core is used, and the sensitivity to the dynamic core is a function of the physics used. The greatest sensitivity in general is associated with a change in physics packages when the NMM core is used. Sensitivity to a change in physics when the ARW core is used is noticeably less. For light rainfall, the spread in the rainfall forecasts when physics are changed under the ARW core is actually less at most times than when the dynamic core is changed while NCAR physics are used. For light rainfall, the WRF model using NCAR physics is much more sensitive to a change in dynamic core than the WRF model using NCEP physics. For heavier rainfall, the opposite is true with a greater sensitivity occurring when NCEP physics is used. Sensitivity to initial conditions (Eta versus the Rapid Update Cycle with an accompanying small change in grid spacing) is generally less substantial than the sensitivity to changes in dynamic core or physics, except in the first 6–12 h of the forecast when it is comparable. As might be expected for warm season rainfall, the finescale structure of rainfall forecasts is more affected by the physics used than the dynamic core used. Surprisingly, however, the overall areal coverage and rain volume within the domain may be more influenced by the dynamic core choice than the physics used.