Increase In Horizontal Resolution Research Articles

Performance of the WRF model to simulate the seasonal and interannual variability of hydrometeorological variables in East Africa: a case study for the Tana River basin in Kenya

This study investigates the ability of the regional climate model Weather Research and Forecasting (WRF) in simulating the seasonal and interannual variability of hydrometeorological variables in the Tana River basin (TRB) in Kenya, East Africa. The impact of two different land use classifications, i.e., the Moderate Resolution Imaging Spectroradiometer (MODIS) and the US Geological Survey (USGS) at two horizontal resolutions (50 and 25 km) is investigated. Simulated precipitation and temperature for the period 2011–2014 are compared with Tropical Rainfall Measuring Mission (TRMM), Climate Research Unit (CRU), and station data. The ability of Tropical Rainfall Measuring Mission (TRMM) and Climate Research Unit (CRU) data in reproducing in situ observation in the TRB is analyzed. All considered WRF simulations capture well the annual as well as the interannual and spatial distribution of precipitation in the TRB according to station data and the TRMM estimates. Our results demonstrate that the increase of horizontal resolution from 50 to 25 km, together with the use of the MODIS land use classification, significantly improves the precipitation results. In the case of temperature, spatial patterns and seasonal cycle are well reproduced, although there is a systematic cold bias with respect to both station and CRU data. Our results contribute to the identification of suitable and regionally adapted regional climate models (RCMs) for East Africa.

Analyzing the Grell–Freitas Convection Scheme from Hydrostatic to Nonhydrostatic Scales within a Global Model

Abstract The authors implemented the Grell–Freitas (GF) parameterization of convection in which the cloud-base mass flux varies quadratically as a function of the convective updraft fraction in the global nonhydrostatic Model for Prediction Across Scales (MPAS). They evaluated the performance of GF using quasi-uniform meshes and a variable-resolution mesh centered over South America, the resolution of which varied between hydrostatic (50 km) and nonhydrostatic (3 km) scales. Four-day forecasts using a 50-km and a 15-km quasi-uniform mesh, initialized with GFS data for 0000 UTC 10 January 2014, reveal that MPAS overestimates precipitation in the tropics relative to the Tropical Rainfall Measuring Mission Multisatellite Precipitation Analysis data. Results of 4-day forecasts using the variable-resolution mesh reveal that over the refined region of the mesh, GF performs as a precipitating shallow convective scheme, whereas over the coarse region of the mesh, GF acts as a conventional deep convective scheme. As horizontal resolution increases and subgrid-scale motions become increasingly resolved, the contribution of convective and grid-scale precipitation to the total precipitation decreases and increases, respectively. Probability density distributions of precipitation highlight a smooth transition in the partitioning between convective and grid-scale precipitation, including at gray-zone scales across the transition region between the coarsest and finest regions of the global mesh. Variable-resolution meshes spanning between hydrostatic and nonhydrostatic scales are shown to be ideal tools to evaluate the horizontal scale dependence of parameterized convective and grid-scale moist processes.

A multi-model analysis of the resolution influence on precipitation climatology in the Gulf Stream region

Using climate simulations from coupled and uncoupled general circulation models, this study investigates the influence of horizontal resolution in both atmospheric and oceanic model components on the mean precipitation over the Gulf Stream (GS) region. For this purpose, three sets of model experiments are analyzed. The first two examine the effects of increasing horizontal resolution of an atmospheric general circulation model (AGCM) gradually from 100 to 10 km under fixed oceanic settings. Specifically, the AGCM is either forced with prescribed observed sea surface temperature (SST) (the first case) or coupled to a non-eddy-resolving ocean general circulation model (OGCM) at a fixed horizontal resolution near 100 km (the second case). The third set of experiments examines the effects of the oceanic resolution with a pair of long-term simulations by another coupled ocean–atmosphere general circulation model (CGCM), in which the OGCM is run respectively at non-eddy-resolving (100 km) and eddy-resolving (10 km) resolutions, while the AGCM resolution remains fixed at 50 km for both runs. In general, all simulations qualitatively reproduce the gross features of the mean GS precipitation and its annual cycle. At similar AGCM resolutions, the uncoupled models produce a GS rain band that is more realistic in both structure and strength compared to the coupled models with non-eddy-resolving oceans. This is because the prescribed observed SST better represents the gradient near the oceanic front than the non-eddy-resolving OGCMs simulate. An increase from the baseline AGCM resolution produces enhanced climatological GS precipitation, both large-scale and convective, with the latter more tightly confined to the oceanic front. The enhancement, however, is moderate and further increases in resolution achieves diminishing results. On the other hand, an increase in oceanic resolution from non-eddy-resolving to eddy resolving scheme results in more consistent simulations with observations in both intensity and structure of the rain band. The major increase is in the convective precipitation near the much-tightened SST gradient associated with the oceanic front. Therefore, the intensity improvement caused by oceanic resolution increases is more effective than that from atmospheric resolution increases. Further analyses show that the improvement of Gulf Stream precipitation climatology due to model horizontal resolution increases can be understood in terms of the atmospheric surface pressure adjustment to the sharper SST gradient near the oceanic front, which leads to stronger atmospheric surface convergence and upper level divergence. The associated ascending motion contributes to the precipitation band located in the Gulf Stream.

2014년 특별관측 기간 동안 청미천 농경지에서의 WRF/Noah-MP 고해상도 수치모의

In this paper, the high-resolution Weather Research and Forecasting/Noah-MultiParameterization (WRF/Noah-MP) modeling system is configured for the Cheongmicheon Farmland site in Korea (CFK), and its performance in land and atmospheric simulation is evaluated using the observed data at CFK during the 2014 special observation period (21 August-10 September). In order to explore the usefulness of turning on Noah-MP dynamic vegetation in midterm simulations of surface and atmospheric variables, two numerical experiments are conducted without dynamic vegetation and with dynamic vegetation (referred to as CTL and DVG experiments, respectively). The main results are as following. 1) CTL showed a tendency of overestimating daytime net shortwave radiation, thereby surface heat fluxes and Bowen ratio. The CTL experiment showed reasonable magnitudes and timing of air temperature at 2 m and 10 m; especially the small error in simulating minimum air temperature showed high potential for predicting frost and leaf wetness duration. The CTL experiment overestimated 10-m wind and precipitation, but the beginning and ending time of precipitation were well captured. 2) When the dynamic vegetation was turned on, the WRF/Noah-MP system showed more realistic values of leaf area index (LAI), net shortwave radiation, surface heat fluxes, Bowen ratio, air temperature, wind and precipitation. The DVG experiment, where LAI is a prognostic variable, produced larger LAI than CTL, and the larger LAI showed better agreement with the observed. The simulated Bowen ratio got closer to the observed ratio, indicating reasonable surface energy partition. The DVG experiment showed patterns similar to CTL, with differences for maximum air temperature. Both experiments showed faster rising of 10-m air temperature during the morning growth hours, presumably due to the rapid growth of daytime mixed layers in the Yonsei University (YSU) boundary layer scheme. The DVG experiment decreased errors in simulating 10-m wind and precipitation. 3) As horizontal resolution increases, the models did not show practical improvement in simulation performance for surface fluxes, air temperature, wind and precipitation, and required three-dimensional observation for more agricultural land spots as well as consistency in model topography and land cover data.

Global Seasonal forecast system version 5 (GloSea5): a high‐resolution seasonal forecast system

This article describes the UK Met Office Global Seasonal forecast system version 5 (GloSea5). GloSea5 upgrades include an increase in horizontal resolution in the atmosphere (N216–0.7°) and the ocean (0.25°), and implementation of a 3D‐Var assimilation system for ocean and sea‐ice conditions. GloSea5 shows improved year‐to‐year predictions of the major modes of variability. In the Tropics, predictions of the El Niño–Southern Oscillation are improved with reduced errors in the West Pacific. In the Extratropics, GloSea5 shows unprecedented levels of forecast skill and reliability for both the North Atlantic Oscillation and the Arctic Oscillation. We also find useful levels of skill for the western North Pacific Subtropical High which largely determines summer precipitation over East Asia.

Vertical transport in the ocean due to sub-mesoscale structures: Impacts in the Kerguelen region

The summertime phytoplankton bloom near the Kerguelen Plateau is in marked contrast to the low-chlorophyll conditions typical of the Southern Ocean and is thought to arise from natural iron fertilisation. The mechanisms of iron supply to the euphotic zone in this region are poorly understood, and numerical studies of iron transport have until now omitted fine-scale (sub-mesoscale) dynamics which have been shown to significantly increase vertical transport in other parts of the ocean.We present the first sub-mesoscale-resolving study of the flow and vertical transport in this region. The modelled transport and flow structure agree well with observations. We find that an increase in horizontal resolution from mesoscale-resolving (1/20°) to 1/80° resolves sub-mesoscale filamentary frontal structures in which vertical velocities are dramatically higher and are consistent with available observations. Lagrangian tracking shows that water is advected to the surface from much greater depth in the sub-mesoscale-resolving experiment, and that vertical exchange is far more rapid and frequent. This study of sub-mesoscale vertical velocities sets the foundation for subsequent investigation of iron transport in this environment.

Energetics of lateral eddy diffusion/advection: Part II. Numerical diffusion/diffusivity and gravitational potential energy change due to isopycnal diffusion

Study of oceanic circulation and climate requires models which can simulate tracer eddy diffusion and advection accurately. It is shown that the traditional Eulerian coordinates can introduce large artificial horizontal diffusivity/viscosity due to the incorrect alignment of the axis. Therefore, such models can smear sharp fronts and introduce other numerical artifacts. For simulation with relatively low resolution, large lateral diffusion was explicitly used in models; therefore, such numerical diffusion may not be a problem. However, with the increase of horizontal resolution, the artificial diffusivity/viscosity associated with horizontal advection in the commonly used Eulerian coordinates may become one of the most challenging obstacles for modeling the ocean circulation accurately. Isopycnal eddy diffusion (mixing) has been widely used in numerical models. The common wisdom is that mixing along isopycnal is energy free. However, a careful examination reveals that this is not the case. In fact, eddy diffusion can be conceptually separated into two steps: stirring and subscale diffusion. Due to the thermobaric effect, stirring, or exchanging water masses, along isopycnal surface is associated with the change of GPE in the mean state. This is a new type of instability, called the thermobaric instability. In addition, due to cabbeling subscale diffusion of water parcels always leads to the release of GPE. The release of GPE due to isopycnal stirring and subscale diffusion may lead to the thermobaric instability.

Tropical cyclones in two atmospheric (re)analyses and their response in two oceanic reanalyses

In this paper, we first evaluate the ability of the European Centre for Medium Range Forecast operational analysis and the ERA-Interim reanalysis to capture the surface wind signature of tropical cyclones (TCs). In those products, the error on the TC position is typically ∼150km, cyclones are too big (∼250km in ERA-Interim and > 100km in the operational analysis against ∼50km in observations) and the maximum wind speed is on average underestimated by 15–27m·s−1 for strong TCs. These biases are generally reduced with the increase of horizontal resolution in the operational analysis, but remain significant at T1279 (∼16km).We then assess the TCs oceanic temperature signature in two global eddy-permitting ocean reanalyses (GLORYS1 and GLORYS2) forced by the above atmospheric products. The resulting cold wake is on average underestimated by ∼50% in the two oceanic reanalyses. This bias is largely linked to the underestimated TCs strength in the surface forcing, and the resulting underestimated vertical mixing. The overestimated TC radius also tends to overemphasize the Ekman pumping response to the cyclone. Underestimating vertical mixing without underestimating Ekman pumping results in the absence of the observed subsurface warming away from the TC tracks in the two reanalyses. Data assimilation only marginally contributes to reducing these errors, partly because cyclone signatures are not well resolved by the ocean observing system. Based on these results, we propose some assimilation and forcing strategies in order to improve the restitution of TC signatures in oceanic reanalyses.

A Sensitivity Research of Numerical Simulation about Winter Sea-Land Breeze

Using the WRF model and NCEP data, a typical winter sea-land breeze progress in China tropical island was simulated, analyzing the impact of wind field simulation with the horizontal resolution, analog range, nesting level, and analog time. The results showed: the simulate effects of the WRF model could not always keep step with the increase of horizontal resolution. Obviously there was a threshold for the model horizontal resolution. By the same token, analog range, nesting level, and analog time have its threshold. The accuracy of the simulation reduced when the threshold values were exceeded.

North-Atlantic dynamics and European temperature extremes in the IPSL model: sensitivity to atmospheric resolution

The variability of the European climate is mostly controlled by the unstable nature of the North-Atlantic dynamics, especially in wintertime. The intra-seasonal to inter-annual fluctuations of atmospheric circulations has often been described as the alternation between a limited number of preferential weather regimes. Such discrete description can be justified by the multi-modality of the latitudinal position of the jet stream. In addition, seasonal extremes in European temperatures are generally associated with an exceptional persistence into one weather regime. Here we investigate the skill of the IPSL model to both simulate North-Atlantic weather regimes and European temperature extremes, including summer heat waves and winter cold spells. We use a set of eight IPSL experiments, with six different horizontal resolutions and the two versions used in CMIP3 and CMIP5. We find that despite a substantial deficit in the simulated poleward peak of the jet stream, the IPSL model represents weather regimes fairly well. A significant improvement is found for all horizontal resolutions higher than the one used in CMIP3, while the increase in vertical resolution included in the CMIP5 version tends to improve the wintertime dynamics. In addition to a recurrent cold bias over Europe, the IPSL model generally overestimates (underestimates) the indices of winter cold spells (summer heat waves) such as frequencies or durations. We find that the increase in horizontal resolution almost always improves these statistics, while the influence of vertical resolution is less clear. Overall, the CMIP5 version of the IPSL model appears to carry promising improvements in the simulation of the European climate variability.

Annual precipitation cycle in regional climate models: the influence of horizontal resolution

The influence of increased horizontal resolution on regional climate models simulations of 1961–1990 period was investigated with a focus on precipitation. The main attention was paid to the annual cycle of precipitation described by a special characteristic, precipitation half-time. Two models (RegCM3 and ALADIN-CLIMATE/CZ), both of them in two horizontal resolutions (25 and 10 km), were used. An evaluation of model simulations with 25 km resolution on the European domain is presented as well as a more detailed evaluation of both 25 and 10 km versions on the area of the Czech Republic. Generally, the effects of increased horizontal resolution vary with climate model and evaluated characteristic. For the precipitation amount and the dependence of precipitation amount on altitude, the increase in horizontal resolution decreases the accuracy of results in both models. For the simulation of annual precipitation cycle and the precipitation half-time, RegCM3 results improved with the increased horizontal resolution, whereas ALADIN-CLIMATE/CZ results worsened.

Performance of Dynamic Downscaling for Extreme Weather Event in Eastern Mongolia: Case Study of Severe Windstorm on 26 May 2008

Global weather forecasts do not have sufficient performance to predict the local severe weather events that are accompanied with cyclones and cold fronts due to their coarse horizontal resolution. This study investigated the performance of dynamical downscaling (DD) using mesoscale model to simulate the severe windstorm in eastern Mongolia which occurred on 26-27 May 2008.Our results revealed that the DD experiments were successful in capturing the general features of the windstorm in terms of wind and temperature patterns. The timing and amplitude of drastic changes in the simulated temperature and wind speed were very similar to that observed than that obtained from the global atmospheric data, suggesting that DD is capable of predicting extreme wind storm events in Mongolia. Analyses on the nested domains indicate that the DD has crucial impact on the performance for simulating severe storm even with a moderate resolution (27 km), and further nesting (9 and 3 km) plays a role to improve it. Furthermore, the maximum wind speed approaches the observed value more closely as the horizontal resolution increases, although it still underestimates the observed wind speed even in the 3 km mesh domain. On the other hand, the abrupt temperature change is captured well even in the low-resolution domain, suggesting a difference in necessary horizontal resolution for temperature change and maximum wind speed.

Dependence of partitioning of model implicit and explicit precipitation on horizontal resolution

Model precipitation can be produced implicitly through convective parameterization schemes or explicitly through cloud microphysics schemes. These two precipitation production schemes control the spatial and temporal distribution of precipitation and consequently can yield distinct vertical profiles of heating and moistening in the atmosphere. The partition between implicit and explicit precipitation can be different as the model changes resolutions. Within the range of mesoscale resolutions (about 20 km) and cumulus scale, hybrid solutions are suggested, in which cumulus convection parameterization is acting together with the explicit form of representation. In this work, it is proposed that, as resolution increases, the convective scheme should convert less condensed water into precipitation. Part of the condensed water is made available to the cloud microphysics scheme and another part evaporates. At grid sizes smaller than 3 km, the convective scheme is still active in removing convective instability, but precipitation is produced by cloud microphysics. The Eta model version using KF cumulus parameterization was applied in this study. To evaluate the quantitative precipitation forecast, the Eta model with the KF scheme was used to simulate precipitation associated with the South Atlantic Convergence Zone (SACZ) and Cold Front (CF) events. Integrations with increasing horizontal resolutions were carried out for up to 5 days for the SACZ cases and up to 2 days for the CF cases. The precipitation partition showed that most of precipitation was generated by the implicit scheme. As the grid size decreased, the implicit precipitation increased and the explicit decreased. However, as model horizontal resolution increases, it is expected that precipitation be represented more explicitly. In the KF scheme, the fraction of liquid water or ice, generated by the scheme, which is converted into rain or snow is controlled by a parameter S 1. An additional parameter was introduced into KF scheme and the parameter acts to evaporate a fraction of liquid water or ice left in the model grid by S 1 and return moisture to the resolved scale. An F parameter was introduced to combine the effects of S 1 and S 2 parameters. The F parameter gives a measure of the conversion of cloud liquid water or ice to convective precipitation. A function dependent on the horizontal resolution was introduced into the KF scheme to influence the implicit and explicit precipitation partition. The explicit precipitation increased with model resolution. This function reduced the positive precipitation bias at all thresholds and for the studied weather systems. With increased horizontal resolution, the maximum precipitation area was better positioned and the total precipitation became closer to observations. Skill scores for all events at different forecast ranges showed precipitation forecast improvement with the inclusion of the function F.

Ensemble Held–Suarez Test with a Spectral Transform Model: Variability, Sensitivity, and Convergence

Abstract The idealized test case proposed by Held and Suarez is carried out with the atmospheric general circulation model ECHAM5 of the Max Planck Institute for Meteorology. The aim is to investigate the sensitivity of the solutions of the spectral dynamical core to spatial and temporal resolution, and to evaluate the numerical convergence of the solutions. Low-frequency fluctuations at time scales as long as thousands of days are found in ultralong integrations. To distinguish the effect of changed resolution from the fluctuations caused by the internal variability, the ensemble method is employed in experiments at resolutions ranging from T31 to T159 with 16 to 81 vertical levels. Significance of the differences between ensembles is assessed by three different statistical tests. Convergence property of the numerical solution is concisely summarized by a ratio index. Results show that the simulated climate state in the Held–Suarez test is sensitive to spatial resolution. Increase of horizontal resolution leads to slight weakening and poleward shift of the westerly jets. Significant warming is detected in high latitudes, especially near the polar tropopause, while the tropical tropopause becomes cooler. The baroclinic wave activity intensifies considerably with increased horizontal resolution. Higher vertical resolution also leads to stronger eddy variances and cooling near the tropical tropopause, but equatorward shift of the westerly jets. The solutions show an indication of convergence at T85L31 resolution according to all the three statistical tests applied. Differences between integrations with various time steps are judged to be within the noise level induced by the inherent low-frequency variability.

Dynamics of Orographically Triggered Banded Convection in Sheared Moist Orographic Flows

Abstract Shallow orographic convection embedded in an unstable cap cloud can organize into convective bands. Previous research has highlighted the important role of small-amplitude topographic variations in triggering and organizing banded convection. Here, the underlying dynamical mechanisms are systematically investigated by conducting three-dimensional simulations of moist flows past a two-dimensional mountain ridge using a cloud-resolving numerical model. Most simulations address a sheared environment to account for the observed wind profiles. Results confirm that small-amplitude topographic variations can enhance the development of embedded convection and anchor quasi-stationary convective bands to a fixed location in space. The resulting precipitation patterns exhibit tremendous spatial variability, since regions receiving heavy rainfall can be only kilometers away from regions receiving little or no rain. In addition, the presence of banded convection has important repercussions on the area-mean precipitation amounts. For the experimental setup here, the gravity wave response to small-amplitude topographic variations close to the upstream edge of the cap cloud (which is forced by the larger-scale topography) is found to be the dominant triggering mechanism. Small-scale variations in the underlying topography are found to force the location and spacing of convective bands over a wide range of scales. Further, a self-sufficient mode of unsteady banded convection is investigated that does not dependent on external perturbations and is able to propagate against the mean flow. Finally, the sensitivity of model simulations of banded convection with respect to horizontal computational resolution is investigated. Consistent with predictions from a linear stability analysis, convective bands of increasingly smaller scales are favored as the horizontal resolution is increased. However, small-amplitude topographic roughness is found to trigger banded convection and to control the spacing and location of the resulting bands. Thereby, the robustness of numerical simulations with respect to an increase in horizontal resolution is increased in the presence of topographic variations.

Mountain-Wave-Like Spurious Waves Associated with Simulated Cold Fronts due to Inconsistencies between Horizontal and Vertical Resolutions

Abstract A newly developed global nonhydrostatic model is used for life cycle experiments (LCEs) of baroclinic waves, and the resolution dependency of frontal structures is examined. LCEs are integrated for 12 days with horizontal grid intervals ranging from 223 to 3.5 km in a global domain. In general, fronts become sharper and corresponding vertical flow strengthens as horizontal resolution increases. However, if the ratio of vertical and horizontal grid intervals is sufficiently small compared to the frontal slope s, the overall frontal structure remains unchanged. In contrast, when the ratio of horizontal and vertical grid intervals exceeds 2s − 4s, spurious gravity waves are generated at the cold front. A linear model for mountain waves quantitatively explains the mechanism of the spurious waves. The distribution of the basic wind is the major factor that determines wave amplitude and propagation. The spurious waves propagate up to a critical level at which the basic wind speed normal to the front is equal to the propagation speed of the front. Results from the linear model suggest that an effective way to eliminate spurious waves is to choose a stretched grid with a smaller vertical grid interval in lower layers where strong horizontal winds exist.

Effects of Land Surface–Vegetation on the Boreal Summer Surface Climate of a GCM

Abstract A land surface model (LSM) has been included in the ECMWF Hamburg version 4 (ECHAM4) atmospheric general circulation model (AGCM). The LSM is an early version of the Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) and it replaces the simple land surface scheme previously included in ECHAM4. The purpose of this paper is to document how a more exhaustive consideration of the land surface–vegetation processes affects the simulated boreal summer surface climate. To investigate the impacts on the simulated climate, different sets of Atmospheric Model Intercomparison Project (AMIP)-type simulations have been performed with ECHAM4 alone and with the AGCM coupled with ORCHIDEE. Furthermore, to assess the effects of the increase in horizontal resolution the coupling of ECHAM4 with the LSM has been implemented at different horizontal resolutions. The analysis reveals that the LSM has large effects on the simulated boreal summer surface climate of the atmospheric model. Considerable impacts are found in the surface energy balance due to changes in the surface latent heat fluxes over tropical and midlatitude areas covered with vegetation. Rainfall and atmospheric circulation are substantially affected by these changes. In particular, increased precipitation is found over evergreen and summergreen vegetated areas. Because of the socioeconomical relevance, particular attention has been devoted to the Indian summer monsoon (ISM) region. The results of this study indicate that precipitation over the Indian subcontinent is better simulated with the coupled ECHAM4–ORCHIDEE model compared to the atmospheric model alone.

Diagnosis of the summertime warm and dry bias over the U.S. Southern Great Plains in the GFDL climate model using a weather forecasting approach

Weather forecasts started from realistic initial conditions are used to diagnose the large warm and dry bias over the United States Southern Great Plains simulated by the GFDL climate model. The forecasts exhibit biases in surface air temperature and precipitation within 3 days which appear to be similar to the climate bias. With the model simulating realistic evaporation but underestimated precipitation, a deficit in soil moisture results which amplifies the initial temperature bias through feedbacks with the land surface. The underestimate of precipitation may be associated with an inability of the model to simulate the eastward propagation of convection from the front‐range of the Rocky Mountains and is insensitive to an increase of horizontal resolution from 2° to 0.5° latitude.

Sensitivity of high-resolution operational weather forecasts to the choice of the planetary boundary layer scheme

In the frame of this paper three widely used planetary boundary layer parameterisations are evaluated for their ability to forecast near-surface temperature and wind speed. The analysis is based on the operational forecasts produced by the Pennsylvania State University — National Center for Atmospheric Research MM5 model over the Athens Area (Greece) during the warm period of 2002. The near-surface temperature and wind forecasts at a grid increment of 8 and 2 km over the highly complex terrain of the Athens Area are statistically verified against the available surface station observations. The results of the analysis showed that the increase in horizontal resolution (from 8 to 2 km) increases the forecast accuracy especially for temperature. As it concerns the forecast skill of each scheme it seems that the MRF scheme produces the best forecasts as it concerns the near-surface temperature while for the near-surface wind the ETA scheme gives the best results. Blackadar schemes present a consistently good behaviour for both temperature and wind forecasts.

Maintenance of a Mountain Valley Cold Pool: A Numerical Study

Abstract A persistent cold-air pool in the Yampa Valley of northwestern Colorado was simulated with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The observed cold-air pool, which was identified by temperature measurements along a line of surface stations ascending the eastern side of the valley, remained in place throughout the day of 10 January 2004. The baseline simulation with horizontal resolution of 1 km, which is close to the resolution of operational regional mesoscale model forecasts, neither matched the strength of the observed cold-air pool nor retained the cold pool throughout the day. Varying the PBL parameterization, increasing the vertical resolution, and increasing the model spinup time did not significantly improve the results. However, the inclusion of snow cover, increased horizontal resolution, and an improved treatment of horizontal diffusion did have a sizable effect on the forecast quality. The snow cover in the baseline simulation was essential for preventing the diurnal heating from eroding the cold pool, but was only sufficient to produce a nearly isothermal temperature structure within the valley, largely because of an increased reflection of solar radiation. The increase of horizontal resolution to 333 and 111 m resulted in a stronger cold-air pool and its retention throughout the day. In addition to improving the resolution of flow features in steep terrain, resulting in, for example, less drainage out of the valley, the increase in horizontal resolution led to a better forecast because of a reduced magnitude of horizontal diffusion calculated along the terrain-following model surfaces. Calculating horizontal diffusion along the constant height levels had a beneficial impact on the quality of the simulations, producing effects similar to those achieved by increasing the horizontal resolution, but at a fraction of the computational cost.