The Global Weather Research and Forecasting (GWRF) Model: Model Evaluation, Sensitivity Study, and Future Year Simulation

Yang Zhang, Joshua Hemperly, Nicholas Meskhidze, William C. Skamarock

Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, USA.
Mesoscale and Microscale Meteorology Division, NCAR, Boulder, USA.

DOI: 10.4236/acs.2012.23024                

Abstract

Global WRF (GWRF) is an extension of the mesoscale Weather Research and Forecasting (WRF) model that was developed for global weather research and forecasting applications. GWRF is being expanded to simulate atmospheric chemistry and its interactions with meteorology on a global scale. In this work, the ability of GWRF to reproduce major boundary layer meteorological variables that affect the fate and transport of air pollutants is assessed using observations from surface networks and satellites. The model evaluation shows an overall good performance in simulating global shortwave and longwave radiation, temperature, and specific humidity, despite large biases at high latitudes and over-Arctic and Antarctic areas. Larger biases exist in wind speed and precipitation predictions. These results are generally consistent with the performance of most current general circulation models where accuracies are often limited by a coarse grid resolution and inadequacies in sub-filter-scale parameterizations and errors in the specification of external forcings. The sensitivity simulations show that a coarse grid resolution leads to worse predictions of surface temperature and precipitation. The combinations of schemes that include the Dudhia shortwave radiation scheme or the Purdue Lin microphysics module, or the Grell-Devenyi cumulus parameterization lead to a worse performance for predictions of downward shortwave radiation flux, temperature, and specific humidity, as compared with those with respective alternative schemes. The physical option with the Purdue Lin microphysics module leads to a worse performance for precipitation predictions. The projected climate in 2050 indicates a warmer and drier climate, which may have important impacts on the fate and lifetime of air pollutants.

Keywords

Global Weather Simulation; Physical Options; Horizontal Grid Resolution

1. Introduction

The Weather Research and Forecasting (WRF) model has been developed by the National Center for Atmospheric Research (NCAR) to improve weaknesses of the Mesoscale Meteorological Model, Version 5 (MM5) and provide a flexible and portable open-source community model for both atmospheric research and operational forecasting [1,2]. The WRF system allows users to interchange various cores and physics packages, which is useful for inter-model evaluations and module sensitivity studies [3]. WRF has been utilized by thousands of users around the world in many different areas of atmospheric research including large-eddy simulations (e.g. [4]), realtime numerical weather predictions (NWP) (e.g. [5]), data assimilation [6,7], regional climate simulations (e.g. [3,8-12], air quality modeling [e.g. 13-18], air quality forecasting (e.g. [19-22]), and atmosphere-ocean coupling (e.g. [23,24]).

A global version of WRF (GWRF) released in 2008 is an extension of mesoscale WRF and a variant of planet WRF, which was initially designed to study the atmospheres and climate systems of other planets such as Mars, Titan, and Venus [25,26]. Four major modifications to the mesoscale WRF were made for application to the planetary atmosphere as the planet WRF: modification of the projection from an isotropic to a non-isotropic grid (i.e. to accommodate a latitude-longitude mesh), the addition of polar Fourier filters to remove model instabilities near the poles, adaptation of planetary constants and timing parameters, and parameterizations of sub-grid scale physical processes associated with specific planets [25]. To adapt the planetary model to the Earth, as opposed to other planets, certain Earth-specific planetary constants (e.g. acceleration due to gravity, reference pressure, and ideal gas constants) and necessary timing conventions (e.g. orbital parameters) have been incorporated into GWRF. GWRF enables modeling of global atmospheric circulation and the coupling between weather systems on global and regional scales with the same basic dynamics and physics [25,26]. Its initial evaluation showed an overall good performance in terms of the global zonal mean climatology for Earth, Mars, Titan, and Venus [26,27]. Compared to traditional general circulation models (GCMs) that have been developed since 1950s, GWRF enables a unified framework for the modeling of atmospheric processes and their interactions across scales spanning from global to local scales through 1- way or 2-way nesting. For example, GWRF can be used to provide initial conditions (ICs) and BCs for mesoscale WRF in nested simulations, which reduces inaccuracies arising from the use of different models with inconsistent model dynamics and physics.

While the mesoscale WRF model has been extensively evaluated using observations, there has been little evaluation of GWRF. In this work, GWRF version 3.0 is evaluated using available observations and reanalysis data. A number of sensitivity simulations are conducted to identify the most appropriate physical parameterizetions in GWRF that produce the highest accuracy for global atmosphere. The main objectives of this study are to evaluate the capability of GWRF in reproducing global boundary layer meteorological variables that are most influential to air pollutants and to examine the sensitivity of the model predictions to various physical parameterizations. Such an evaluation is a critical step toward the extension of GWRF to include emissions and chem.- istry needed to simulate/forecast the global transport and fate of air pollutants, the impact of emissions on global air quality and radiative forcing, as well as forecasting the future climate change and its impact on air quality and vice versa.

2. Model Description and Simulation Design

GWRF provides a number of options for physical schemes or parameterizations [2]. The model radiation and physical schemes/parameterizations selected for the baseline and sensitivity simulations as well as a future year simulation are summarized in Table 1. The set of physics configurations in these simulations is selected with consideration of their suitability for long-term simulation over a global domain. For longwave radiation received at the surface (LW), the Community Atmosphere Model version 3 (CAM3.0) LW radiation scheme of Collins et al. [28] is used in the baseline simulation and the rapid radiative transfer model (RRTM) [29] is used in the sensitivity simulation. CAM3 LW is a spectral-band scheme with 2 bands used for climate simulations adapted from the NCAR CAM 3.0. It can handle water vapor, O₃, and CO₂, and interacts with model-resolved clouds and cloud fractions. RRTM LW is a spectral-band scheme with 16 bands using the correlated-k method, which calculates radiative transfer with k referring to the absorption coefficient. RRTM LW accounts for cloud optical depth and uses lookup tables to represent outgoing LW radiations caused by water vapor, O₃, CO₂, and trace gases. For shortwave radiation received at the surface (SW), the Goddard shortwave radiation scheme of Chou and Suarez [30] and Chou et al. [31] is used in the baseline simulation and the CAM3 shortwave radiation of Collins et al. [28] and the Dudhia scheme [32] are used in the sensitivity simulations. The Goddard SW scheme is a spectral band scheme with 11 bands. This scheme accounts for diffuse and direct solar radiation components in a two-stream approach, including the effects of water vapor, O₃, and CO₂ on radiation. The CAM3 SW is a spectral band that can handle several aerosol types and trace gases and their interactions with clouds. The CAM3 SW scheme is especially suited for

Table 1. Model configurations in the baseline and sensitivity simulations.

regional climate simulations [2]. The Dudhia SW scheme has a simple downward integration of solar flux and accounts for clear-air scattering, water vapor absorption, and cloud albedo and absorption.

For cloud microphysics parameterization, the WRF Single Moment 3-Class (WSM3) scheme is used in the baseline simulation and WSM 6-Class (WSM6) and Purdue Lin (PL) scheme [33,34] are used in the sensitivity simulations. WSM3 is a simple-ice scheme with three categories of hydrometers (i.e. water vapor, cloud water/ ice, and rain/snow). It is computationally efficient, but does not treat super cooled water and gradual melting rates. WSM6 extends WSM3 to explicitly include water vapor, rain, snow, cloud ice, cloud water, and graupel. Among all WSM options in WRF, WSM6 is the most comprehensive option. It has been recently extended to a double moment warm rain microphysics (i.e. WDM6) [35]. The PL scheme treats six classes of hydrometeors: water vapor, cloud water, rain, cloud ice, snow, and graupel. For land-surface model (LSM), the National Center for Environmental Prediction (NCEP), Oregon State University, Air Force, and Hydrologic Research Lab (NOAH) model of Chen and Dudhia [36,37] and Ek et al. [38] is used in the baseline simulation and the simple thermal diffusion (Slab) scheme [39] is used in the sensitivity simulation. NOAH is a 4-layer soil temperature and moisture model with canopy moisture and snow cover prediction. The 4-layer thicknesses are 10, 30, 60 and 100 cm from the surface down. The NOAH scheme includes root zone, evapotranspiration, soil drainage, and runoff and accounts for vegetation categories, monthly vegetation fraction, soil texture, soil ice, fractional snow cover effects, surface emissivity properties, and improved urban treatment. The scheme provides sensible and latent heat fluxes to the boundary-layer scheme. It is used for both research and operational applications. The Slab LSM is based on the MM5 soil temperature model with 5 layers of 1, 2, 4, 8, and 16 cm thickness. The Slab LSM includes energy budget calculations accounting for radiation, sensible, latent heat flux, and a crude snow treatment with a constant snow cover. The soil moisture is fixed with a land-use and season-dependent constant value and vegetation effects are not explicitly considered. For the cumulus parameterization, the Kain-Fritsch II (KF II) [40-42] is used in the baseline simulation and the Grell-Devenyi (GD) ensemble [43] is used in the sensitivity simulation. The KF II scheme is a simple cloud model with moist updrafts and downdrafts and includes the effects of detrainment, entrainment, and simple microphysics. The GD scheme is an ensemble cumulus scheme which averages the results from multiple cumulus schemes with an equal weight. These cumulus schemes are all mass-flux type schemes, but with different updraft and downdraft entrainment and detrainment parameters, and different precipitation efficiencies. In both baseline and sensitivity simulations, the Yonsei University (YSU) PBL scheme of Hong and Dudhia [44] and Hong et al. [45,46] and the Monin-Obukhov scheme adapted from MM5 [47] are used. The YSU scheme is a non-local scheme that includes an explicit treatment of the entrainment layer at the PBL top and provides a well-mixed boundary-layer profile. It also includes an enhanced stable boundary-layer diffusion algorithm that allows deeper mixing in windier conditions. The surface-layer scheme is based on the Monin-Obukhov similarity theory with Carslon-Boland viscous sub-layer and standard similarity functions from look-up tables. It includes four stability regimes as described in Zhang and Anthes [48] and uses stability functions from Paulson [49], Dyer and Hicks [50], and Webb [51] to compute surface exchange coefficients for heat, moisture, and momentum.

Table 2 summarizes the configurations used in all simulations. GWRF is initialized using the WRF Preprocessing System version 3.0 (WPS3). The data used as input into WPS3 is from the NCEP Final Global Data Assimilation System (FNL), which has a horizontal grid resolution of 1˚ latitude × 1˚ longitude and is available every six hours since July 1999. NCEP FNL is used to initialize the Global Forecasting System (GFS) model. To evaluate the performance of GWRF, simulations of the year 2001 are performed and analyzed. The simulations are initialized using the NCEP FNL analysis. The simulations are constrained by a monthly reinitialization of the WRF prognostic variables, including the soil temperature and skin temperature. Sea surface temperature (SST) and sea-ice fraction are reinitialized from weekly analyses, as recommended for simulations longer than one week [2]. A GWRF baseline simulation is conducted for the entire year of 2001 at a horizontal grid resolution of 1˚ × 1˚, with 27 eta layers from 0 to 50 h Pain the vertical domain. A number of sensitivity simulations are carried out to evaluate the impacts of physics parameterizations and grid resolutions on model performance. These sensitivity simulations include seven sets of simulations at 1˚ × 1˚ (i.e. three for different combinations of longwave and shortwave radiation schemes (RAD1 for Dudhia SW and CAM3 LW, RAD2 for CAM3 SW and RRTM LW, and RAD3 for CAM3 SW and CAM3 LW), two for different cloud microphysics (CMP1 for WSM6 and CMP2 for PL), one for LSM (i.e. Slab), and one for cumulus parameterization (CCP) (i.e. GD)) and one with the same radiation and physics configurations as the baseline simulation but at a coarser horizontal grid resolution of 4˚ latitude × 5˚ longitude.

To evaluate the capability of GWRF in simulating climate changes in future years, a simulation is completed for the year 2050 with the baseline physics configurations at a horizontal grid resolution of 4˚ latitude × 5˚

Table 2. Model configurations for GWRF baseline, sensitivity, and future year simulations.

longitude. GWRF model outputs for 2050 are compared with results from the 2001 baseline simulation to analyze the variation trends of major meteorological variables. Since GWRF is not yet a climate model, it is initialized using atmospheric, land, and ocean/sea-ice output from the NCAR Community Climate System Model 3.0 (CCSM3) simulation for 2050 as part of the IPCC Special Report on Emission Scenario (SRES) B1 experiment. CCSM3 is a coupled atmosphere-ocean model at a spatial resolution of 257 × 129 grid points (1.4˚ latitude × 1.4˚ longitude). The SRES B1 represents a low greenhouse gas concentration scenario with a CO₂ level of 550 ppm. Each month of the 2050 GWRF simulations were initialized using the corresponding monthly mean CCSM outputs. The 3-D variables used to initialize GWRF from the atmospheric component model of CCSM (i.e. Community Atmospheric Model) include vertical profiles of temperature (T), relative humidity (RH), geopotential height (Z3), the zonal (U) and meridional (V) components of wind speed, and surface pressure (PS). CAM outputs are vertically interpolated to map with the vertical structure of GWRF. Additional initialization data include 3-Dsoil temperature (TSOI) and moisture (H₂OSOI) predictions from the CCSM Community Land Model (CLM) and 2-D sea-ice fraction (ICEFRAC) from the CCSM Community Sea Ice Model (CSIM).

3. Evaluation Datasets and Methodology for 2001

3.1. Datasets for Model Evaluation

GWRF predictions are evaluated against surface observational networks and gridded reanalysis data which combine data from surface and satellite observations with other model outputs. A summary of the datasets is shown in Table 3. The baseline surface radiation network (BSRN) is a global surface-based observational network established by the World Radiation Monitoring Center since 1992 for surface radiation fluxes at the Earth’s surface for climate research [52]. It consists of 59 sites worldwide as of 2011, but only 28 sites have observations in 2001. The SW downward radiation flux measured by pyranometers and LW downward radiation flux measured by pyrgeometers every minute are used to compute hourly averaged observations for model evaluation. Meteorological parameters from the National Climactic Data Center (NCDC) are from the Global Climate Observing System (GSN) Surface Network, Monthly (GSNMON), with over 900 sites worldwide. The observed mean monthly temperature (˚C) and total monthly precipitation (mm) for ~200 sites in 2001 are used for model evaluation. The Global Precipitation Climatology Project (GPCP) is a part of the Global Energy and Water Cycle Experiment (GEWEX) of the World Climate Research program (WCRP). The monthly mean precipitation data are produced from an analysis designed by the Global Precipitation Climatology Centre by merging precipitation estimates from microwave, infrared, and sounder data from international precipitation-related satellites, as well as from precipitation gauges over 6000 stations on a 2.5˚ × 2.5˚ latitude-longitude grid from 1979 to present [53]. The version of GPCP used in this study is the 2.5-degree version 2. The NCEP and the National Center for Atmospheric Research (NCAR) Reanalysis (NNR) dataset is widely used to validate global simulations in the atmospheric modeling community. Kalnay et al. [54] published a 40-year record (1957-1996) of global atmospheric parameters from reanalysis. The reanalysis uses a combination of assimilated observations (remotely-sensed and surface based) at a horizontal grid

Table 3. Summary of datasets used to evaluate GWRF.

resolution of 2.5˚ × 2.5˚ latitude/longitude for each year and model regression to produce global analyses of atmospheric fields.

3.2. Evaluation Protocol

The model evaluation focuses on major boundary layer meteorological variables including a combination of nonconvective and convective weekly accumulated precipitation (RAINC + RAINNC), 2-meter temperature (T2) and specific humidity (Q2), and 10-meter wind velocities and their zonal (U10) and meridional (V10) components, as well as radiation variables such as SW and LW radiation. The overall performance of GWRF is evaluated in terms of spatial distribution, seasonal and temporal variations, and statistics over the global domain, the Northern and Southern Hemispheres, and the six populated continents. The six circulations cells are also used as subdomains for model evaluation, they include: the Polar (60˚N - 90˚N and 60˚S - 90˚S), Ferrel (30˚N - 60˚N and 30˚S - 60˚S), and Hadley (0˚N - 30˚N and 0˚S - 30˚S) Cells in the Northern and Southern hemispheres. The statistical measures include normalized mean bias (NMB), normalized mean error (NME), mean bias (MB), root mean square error (RMSE), and correlation coefficient (Corr) over the entire domain, sub-domains, and continental domains. The formulas used to calculate these statistical metrics are taken from Zhang et al. [55].

4. Evaluation of Baseline Results

Table 4 summarizes the overall performance statistics for all meteorological variables. Figures 1 and 2 show simulated downward SW and LW radiation fluxes overlaid with observations from BSRN during winter and summer. GWRF reproduces the radiation observations reasonably well in both seasons with moderate overpredictions (MBs of 38.5 - 51.5 W·m^–2 and NMBs of 24% - 27%) for SW radiation and underproductions (MBs of –32.1 to –30.0 W·m^–2 and NMBs of –10.3% to –9.2%) for LW radiation. As shown in Table 4 and Figure 3, similar overpredictions of SW and underpredictions of LW radiation fluxes against the NNR data also occur on a global scale but to a lesser extent than against the BSRN data. Such overpredictions of SW fluxes and underpredictions of LW radiation occur mainly between 30˚N and 30˚S and dominate the trends of the annual predictions with MBs of 16.6 W·m⁻² for SW and –16.6 W·m⁻² for LW radiation against the NNR data. These results are overall consistent with other GCMs. Wild et al. [56] reported that a tendency common to all GCMs which overestimate SW by an average of 10 - 15 W·m⁻², due likely to an underestimation of atmospheric absorption. The performance of ECHAM3 in terms of SW radiation also showed a latitudinal dependence, with overpredictions by up to 40 W·m⁻² over the low latitudes and underestimations by up to 20 W·m⁻² over the high latitudes due to an underestimation of cloud cover in the annual mean. Annual performance statistics for GWRF compares well with these trends, exhibiting the largest overpredictions for SW radiation fluxes over the Northern (MB of 41.1 W·m⁻² and NMB of 16.8%) and Southern (42.9 W·m⁻² and NMB of 17.9%) Hadley Cells, and the largest underpredictions over the North Pole and North Ferrel cells which have MBs of –9.0 W·m⁻² and –3.3 W·m⁻² and NMBs of –6.8% and –1.7 W·m⁻², respectively (see Figure 3 for NMBs). Consistently, GWRF also shows large overpredictions of SW in major populated continents over the low latitudes such as South America, Australia, and Africa and underpredictions for the continent over the high latitude such as Europe in terms of winter, summer, and annual mean values (see Figure 3). Wild et al. [56] indicated a tendency of GCMs to underestimate LW by an average of 10 - 15 W·m⁻² (corresponding to the global mean observed and simulated values of 293 W·m⁻² and 278 W·m⁻², respectively), due to an underestimation of low-level clouds. For comparison, GWRF underestimates LW radiation with a global annual MB value of –16.6 W·m⁻² (corresponding to mean observed and simulated values of 295 W·m⁻² and 278 W·m⁻², respectively). It also shows large underpredictions of LW fluxes in all six major populated

Table 4. Performance statistics of GWRF at a horizontal grid resolution of 1˚ × 1˚ on a global scale.

Figure 1. 2001 winter (top) and summer (bottom) downward shortwave fluxes at the surface simulated at the horizontal grid resolution of 1˚ × 1˚ overlaid with observations from BSRN (denoted by circles).

Figure 2. 2001 winter (top) and summer (bottom) downward longwave fluxes at the surface simulated at the horizontal grid resolution of 1˚ × 1˚ overlaid with observations from BSRN (denoted by circles).

Figure 3. 2001 winter (top), summer (middle), and annual (bottom) mean normalized mean bias (NMB) (%) of T2, Q2, SW, and LW against the NNR data and Precip against the GPCP data at a horizontal grid resolution of 1˚ × 1˚ for the six continental domains (left column) and the six circulation cell domains (right column).

continents and circulation cells in seasonal and annual mean values (see Figure 3).

Figure 4 shows simulated T2 overlaid with observations from NCDC during winter and summer. GWRF generally reproduces well the spatial distribution of observed T2 in both seasons with mean cold bias of –2.0˚C in winter and –0.5˚C in summer (NMBs of –17.2% and –2.4%, respectively). The largest cold biases occur in the North Pole in winter and in the South Ferrell cell in summer. A similar trend for T2 is found against the NNR data (see Figure 3). Over the North Ferrell cell, the mean NRR and simulated values are –0.59˚C and 0.13˚C during summer, respectively, resulting in an MB of 0.72˚C and NMB of 121.8%, as shown in Figure 3. Such an overprediction is caused by overestimations in SW radiation fluxes over the Northern mid-latitudes. For major continents, an overprediction of SW radiation fluxes correlates well with an overprediction of T2 (except for Europe). The anti-correlation exists between T2 and SW over Europe in the summer, winter, and annual means and over the North Ferrel cell in the annual means. Although SW is underpredicted over Europe during winter, the simulated T2 of –1.0˚C is warmer than the NNR value of –2.7˚C, leading to a high NMB of 62.6% in winter and an annual NMB of 8.2%. Similarly, during winter the simulated T2 of 0.13˚C is much warmer than the observed T2 of –0.59˚C, leading to a very high winter NMB of 121.8% and an annual NMB of 7.3% over the Northern Ferrel cell. Large errors associated with surface temperature exist over the high latitudes and areas of sharp terrain gradients, which are consistent with most GCMs.

Conflicts of Interest

The authors declare no conflicts of interest.

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