Orographic Drag Research Articles

Improving the Near-Surface Wind and Turbulence at the Edge of the Orographic Drag Gray Zone by Tuning the Roughness Length

Abstract In this paper, we present the implementation and evaluate the impact of the new roughness length configuration in the ALARO canonical model configuration of the ALADIN system at the edge of the orographic gravity wave drag gray zone. As an essential input for turbulence parameterization, the roughness length affects the near-surface turbulent fluxes and the screen-level interpolation of meteorological parameters. We utilize GMTED2010 and ECOCLIMAP-II databases to derive orographic and vegetation components of the effective roughness length and introduce tuning parameters enabling us to optimize predicted near-surface turbulent momentum fluxes and 10-m wind speed. Based on sensitivity tests, we (i) prove the necessity of tuning the roughness length fields, (ii) considerably reduce the RMSE of near-surface turbulent momentum fluxes (6%–7%) and 10-m wind speed for different groups of stations (3%–10%), and (iii) identify the tree height as the most influential input parameter in our computational domain. The RMSE decomposition indicates that the improvement of 10-m wind speed mostly comes from a decrease in the random error and bias of the mean. The variability is slightly underestimated, thus reducing the model accuracy for wind speeds above the 95th percentile but at an acceptable level. We explain that roughness length tuning also compensates for the missing roughness sublayer correction in our system. Finally, we show that, although the impact of the orographic gravity wave drag scheme at a horizontal mesh size of 1.8 km is generally small, it is still beneficial for capturing some finer features observed in atmospheric soundings. Significance Statement Aiming to improve the 10-m wind speed forecast without sacrificing the accuracy of turbulent momentum fluxes in the kilometric resolution numerical weather prediction model, we derived new roughness length fields from high-resolution physiography databases. Therein, we proved the importance of tuning the input orography and vegetation fields and, depending on the time of day and year, reduced the root-mean-square error of 10-m wind speed by 3%–10%. Further, we demonstrated that orographic gravity wave drag parameterization is still needed to predict finer details seen in wind profiles from atmospheric soundings. Finally, we discussed the related simplifications in our model and their implications and proposed steps toward a more consistent and complete treatment of the near-surface turbulence.

Reducing Winter Precipitation Biases Over the Western Tibetan Plateau in the Model for Prediction Across Scales (MPAS) With a Revised Parameterization of Orographic Gravity Wave Drag

AbstractClimate models often overestimate the precipitation in western Tibetan Plateau (TP) during winter, due to their poor ability in representing the orographic drag of unresolved complex terrain. In this study, the parameterization scheme of orographic gravity wave drag (OGWD) in the Model for Prediction Across Scales (MPAS) is revised to account for the nonhydrostatic effects (NHE) on the surface momentum flux of vertically‐propagating orographic gravity waves (OGWs). The effect of revised OGWD scheme on the simulation of winter precipitation over the TP is examined using parallel numerical experiments with the original and revised schemes, respectively. The results show that the revised scheme can effectively alleviate the precipitation biases in western TP by improving the atmospheric circulation and water vapor transport (WVT). The NHE reduces the surface wave momentum flux of OGWs which results in weaker zonal OGWD in the mid‐low troposphere and thus stronger westerlies over the TP, reducing the easterly biases in the experiment with the original OGWD scheme. The weakened zonal OGWD promotes a plateau‐scale cyclonic circulation to the north of the TP, which suppresses the northern branch of the bifurcated westerlies detouring the TP. According to quantitative analysis, the WVT into the western TP and surrounding areas by upstream westerlies is notably reduced by the northeasterlies of the cyclonic circulation, which eventually leads to the decrease of precipitation in this region.

Explicit Representation of Orographic Anisotropy for All Directions Improves Nanling Mountain Rainfall Simulation

AbstractClimate models exhibit significant rainfall bias in mountainous regions. One reason is the insufficient representation of orographic anisotropy in these models. In this study, we implement the orographic drag scheme with 3‐D orographic anisotropy (all flow directions (AFD)) into a general circulation model and investigate its impact on Nanling rainfall simulation where models have systematic dry bias in the summer. It is shown that the AFD alleviated the Nanling mountain rainfall bias by over 60%. This is through an anomalous “lower‐convergence‐higher‐divergence” deceleration pattern of the flow windward of the Nanling Mountains that enhanced vertical motion and low‐level moisture convergence. The results suggest the importance of explicit orographic anisotropy representation in rainfall simulation in mountainous regions.

Impact of Sub‐Grid Turbulent Orographic Form Drag on the Summer Monsoon Precipitation Simulation Over China

AbstractTerrain has multi‐scale dynamic impacts on the climate. The Regional Climate Model version 4 (RegCM4) has simulation uncertainty partly due to insufficient description of sub‐grid orographic drag (SOD). In this study, the sub‐grid turbulent orographic form drag (TOFD) scheme has been adopted into the RegCM4 to better describe SOD. Results show that the overestimated near surface wind over rugged areas leads to non‐negligible biases of the summer monsoon precipitation simulation over China in the original RegCM4, whose skill in simulating the summer precipitation over the monsoonal regions of China can be remarkably improved by adopting the sub‐grid TOFD scheme. Compared to the original RegCM4, the Taylor scores (root mean square error) of the precipitation simulated by the RegCM4 with the sub‐grid TOFD scheme are increased (decreased) by 4%, 13%, 11%, and 7% (8%, 5%, 30%, and 16%) over the southeastern China, the mid‐lower Yangtze River Basin, the Huang‐Huai Basin, and the northeastern China, respectively. The improved precipitation simulation mainly comes from the atmospheric circulation simulation improved by adopting the sub‐grid TOFD scheme, which leads to cyclonic (anti‐cyclonic) wind differences over the southeastern (northeastern) China from the boundary layer to the mid‐troposphere and thereafter vertical motion differences in the middle troposphere, further resulting in the variation of the simulated precipitation intensity (heavy to torrential) and the convective precipitation and consequential improvement of summer monsoon precipitation simulation. This study indicates that the dynamic impact of micro‐scale terrain plays an important role in the summer monsoon precipitation simulation.

Impact of Parameterized Topographic Drag on a Simulated Northeast China Cold Vortex

AbstractNortheast China cold vortex (NECV) is the major influencing weather system in northern China. Yet the impacts of complex terrain on the evolution of NECV remains poorly understood. This work studies the influence of subgrid orographic drag (SOD) on a heavy‐rain‐producing NECV occurred in July 2011 using the Weather Research and Forecasting model. A series of numerical experiments are conducted with different parameterizations of SOD including turbulent orographic form drag (TOFD), flow blocking drag (FBD), and mountain wave drag (MWD). Results show that the NECV intensity is overestimated in the absence of SOD parameterization, accompanied with too‐low geopotential height (GPH) and too‐strong horizontal winds. The parameterization of TOFD can significantly decelerate the 10‐m winds, whereas the FBD and MWD play a minor role. However, the influence of TOFD is overwhelmed by FBD and MWD in the troposphere, especially the latter. This implies that the breaking of mountain waves play a more important role in weakening the NECV than the low‐level flow blocking. The lower‐tropospheric MWD directly weakens the convergence and ascent motion of the NECV, producing an anti‐cyclonic circulation that uplifts the GPH under the constraint of quasi‐geostrophic vertical vorticity. This MWD‐induced circulation indirectly weakens the NECV in the mid‐upper troposphere by producing a warm advection that counters the low‐level cold advection to the southwest of the NECV. The findings provide useful insights into the impacts of complex terrain on the NECV and highlight the importance of topographic drag parameterization in the simulation and short‐range forecast of NECV.

Implementation of an Orographic Drag Scheme Considering Orographic Anisotropy in All Flow Directions in the Earth System Model CAS‐ESM 2.0

AbstractA reasonable representation of orographic anisotropy in earth system models is vital for improving weather and climate modeling. In this study, we implement the orographic drag scheme, including 3‐D orographic anisotropy (3D‐AFD), into the Chinese Academy of Sciences Earth System Model version 2 (CAS‐ESM 2.0). Three groups of simulations named sensitivity run, medium‐range forecast, and seasonal forecast respectively are conducted using the updated CAS‐ESM model and validated against station observation and reanalysis data. These simulations are run using the three schemes—3D‐AFD scheme, the 3D orographic anisotropy scheme for the eight‐direction (3D‐8x), and the 2D isotropic scheme (2D)—to compare their performance in CAS‐ESM 2.0. Sensitivity runs show that the 3D‐AFD provide more continuous transition of calculated drag as function of wind direction than the 3D‐8x, while the drag calculated using the 2D scheme show no variation with change of wind direction. Enhanced drag in the medium range and seasonal forecast using the updated CAS‐ESM alleviates part of the winter wind speed bias over the Tibetan Plateau (TP) and the cold bias over TP and the Siberian polar region. It is shown that the 3D‐AFD scheme alleviates more bias than that of the 3D‐8x scheme (by wind speed reduction of 1 ∼ 2 m/s and temperature of 1 ∼ 2 K) especially in the seasonal forecast. The results suggest that reasonable representation of the orographic anisotropy is important in climate modeling.

Revisiting the Role of Mountains in the Northern Hemisphere Winter Atmospheric Circulation

AbstractThe impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, version 6 (WACCM6). A suite of experiments explores the roles of both resolved orography and the parameterized effects of unresolved orographic drag (hereafter parameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extratropical tropospheric and stratospheric zonal mean zonal wind by up to 80%; this is substantially greater than previous estimates. Ultimately, parameterized orography accounts for 60%–80% of this reduction; however, away from the surface most of the forcing of by parameterized orography is accomplished by resolved planetary waves. We propose that a catalytic wave–mean-flow positive feedback in the stratosphere makes the stratospheric flow particularly sensitive to parameterized orography. Orography and land–sea contrast contribute approximately equally to the strength of the midlatitude stationary waves in the free troposphere, although orography is the dominant cause of the strength of the Siberian high and Aleutian low at the surface and of the position of the Icelandic low. We argue that precisely quantifying the role of orography on the observed stationary waves is an almost intractable problem, and in particular should not be approached with linear stationary wave models in which is prescribed. We show that orography has less impact on stationary waves, and therefore on , on a backward-rotating Earth. Last, we show that atmospheric meridional heat transport shows remarkable constancy across our simulations, despite vastly different climates and stationary wave strengths.

Accounting for moist processes in a sub‐grid orographic drag scheme

AbstractA method for estimating the effective moist stability within a sub‐grid orographic drag scheme is described. It accounts for latent heat effects during low‐level saturated sub‐grid orographic ascent, with the lifting condensation level determining how much of the assumed sub‐grid ascents are saturated. The scheme has been verified for two cases with moist south to southwesterly low‐level flow impinging on mountain ranges of the USA and India, within both low‐resolution global simulations and high‐resolution limited‐area model simulations with degraded orography. It produces significant improvements to the model wind fields in both regions. In particular, the amount of low‐level orographic blocking predicted by the orographic drag scheme is reduced, particularly at low resolution, as are the associated large negative near‐surface wind biases over the mountains. Precipitation is shifted slightly downstream. A suite of 24 five‐day global NWP forecasts at N320 resolution was used to demonstrate that these results are robust, and that similar effects are seen over a number of mountain ranges. Over India, the convection scheme is very active in the global simulations, and has a larger effect on the winds at all levels than the orographic drag scheme. However the two schemes interact. Reducing the sub‐grid low‐level orographic blocking drag resulted in a reduction in the convergence occurring as the westerly flow decelerates towards the mountains. This decreased the amount of convection occurring over the sea upstream and along the coast, reducing the associated negative low‐level wind bias there, as well as reducing the negative wind biases over land.

Improved Circulation in the Northern Hemisphere by Adjusting Gravity Wave Drag Parameterizations in Seasonal Experiments With ICON‐NWP

AbstractThe stratosphere is one of the main potential sources for subseasonal to seasonal predictability in midlatitudes in winter. The ability of an atmospheric model to realistically simulate the stratospheric dynamics is essential in order to move forward in the field of seasonal predictions in midlatitudes. Earlier studies with the ICOsahedral Nonhydrostatic atmospheric model (ICON) point out that stratospheric westerlies in ICON are underestimated. This is the first extensive study on the evaluation of Northern Hemisphere stratospheric winter circulation with ICON in numerical weather prediction (NWP) mode. Seasonal experiments with the default setup are able to reproduce the basic climatology of the stratospheric polar vortex. However, westerlies are too weak and major stratospheric warmings too frequent in ICON. Both a reduction of the nonorographic, and a reduction of the orographic gravity wave and wake drag lead to a strengthening of the stratospheric vortex and a bias reduction, in particular in January. However, the effect of the nonorographic gravity wave drag scheme on the stratosphere is stronger. Stratosphere‐troposphere coupling is intensified and more realistic due to a reduced gravity wave drag. Furthermore, an adjustment of the subgrid‐scale orographic drag parameterization leads to a significant error reduction in the mean sea level pressure. As a result of these findings, we present our current suggested improved setup for seasonal experiments with ICON‐NWP.

Evaluation of the near-surface climate of the recent global atmospheric reanalysis for Qilian Mountains, Qinghai-Tibet Plateau

In this study, temperature, air pressure, wind speed and precipitation from 19 basic stations over Qilian Mountains are used to evaluate the applicability for reanalysis datasets (ERA5, JRA-55, ERAI and HAR) during 1980–2018. The four global reanalysis products describe the state of the near-surface climate with different performances. The results show that (1) The four reanalysis products perform pretty satisfactorily for temperature and air pressure while the precipitation with the lowest quality; (2) ERA5 outperforms for most variables in correlation coefficients, especially for wind speed, but not significantly improved than EARI of other variables; (3) Overestimate of wind speed issues in HAR were also identified as the sub-grid orographic drag scheme is not included in WRF; (4) Despite some discrepancy, a warming trend is showed with all these four products; in addition, the decade from 1990 to 1999 has the fastest warming rate.

COnstraining ORographic Drag Effects (COORDE): A Model Comparison of Resolved and Parametrized Orographic Drag

AbstractThe parametrization of orographic drag processes is a major source of circulation uncertainty in models. The COnstraining ORographic Drag Effects (COORDE) project makes a coordinated effort to narrow this uncertainty by bringing together the modeling community to: explore the variety of orographic drag parametrizations employed in current operational models; assess the resolution sensitivity of resolved and parametrized orographic drag across models; and to validate the parametrized orographic drag in low‐resolution simulations using explicitly resolved orographic drag from high‐resolution simulations. Eleven models from eight major modeling centers are used to estimate resolved orographic drag from high‐resolution (km‐scale) simulations and parametrized orographic drag from low‐resolution simulations, typically used for seasonal forecasting (∼40 km) and climate projections (∼100 km). In most models, at both seasonal and climate resolutions, the total (resolved plus parametrized) orographic gravity wave drag over land is shown to be underestimated by a considerable amount (up to 50%) over the Northern and Southern Hemisphere and by more than 60% over the Middle East region, with respect to the resolved gravity wave drag estimated from km‐scale simulations. The km‐scale simulations also provide evidence that the parametrized surface stress and the parametrized low‐level orographic drag throughout the troposphere are overestimated in most models over the Middle East region, particularly at climate resolutions. Through this process‐based evaluation, COORDE provides model developers new valuable information on the current representation of orographic drag at seasonal and climate resolutions and the vertical partitioning of orographic low‐level and gravity wave drag.

Performance Evaluation of Sub-Grid Orographic Parameterization in the WRF Model over Complex Terrain in Central Asia

The terrain of Central Asia is complex and rugged over mountains. Consequently, wind speed is overestimated over mountains and plains when using the Weather Research Forecast (WRF) model in winter. To solve this problem, three different simulations (named as control simulation (CRTL), gravity waves (GWD), and flow-blocking drag (FBD), respectively) were designed to investigate the impact of sub-grid orography (gravity waves and flow-blocking drag) on wind forecasts. The results illustrated that near-surface wind-speed overestimations were alleviated when sub-grid orographic drag was used in GWD, though the upper-level wind fields at 500 hPa were excessively reduced compared to CRTL. Thus, we propose eliminating the gravity wave breaking at the upper level to improve upper-level wind underestimations and surface wind speeds at the same time. The sub-grid orographic drag stress of the vertical profile over mountains was reduced when only the flow-blocking drag was retained in FBD. This alleviated underestimations of the upper-level wind speed and near-surface wind, which both have the same positive effects as the gravity wave and flow-blocking total. The mean bias and root mean squared error reduced by 32.76% and 9.39%, respectively, compared to CRTL. Moreover, the temperature and specific humidity in the lower troposphere were indirectly improved. The results of the study demonstrate that it is better to remove sub-grid orographic gravity wave drag when using the gravity wave drag scheme of the WRF model.

Impacts of Subgrid Orographic Drag on the Summer Monsoon Circulation and Precipitation in East Asia

AbstractThis work is an investigation of the impacts of three different components of parameterized subgrid orographic drag (SOD) on the summer monsoon circulation and precipitation in East Asia, that is, flow‐blocking drag (FBD), low‐level gravity wave drag (GWD), and upper‐level GWD. Using the global Weather Research and Forecasting model, a series of numerical experiments are conducted with different configurations of SOD parameterization. In the case of no SOD, the simulated lower tropospheric winds are too strong in an elongated region extending from East China to the east of Japan, accompanied with excessive rainfall. The intensity of the western Pacific subtropical high is overestimated, thus suppressing the precipitation over the subtropical ocean. With the parameterization of SOD, the circulation and precipitation biases are alleviated, depending on the geographical and vertical distribution of SOD. The FBD is very weak and only contributes to the circulation in eastern East Asia. Contrastingly, there is remarkable low‐level GWD in the lower latitudes which helps reduce the low‐level wind biases in South China. The upper‐level GWD only occurs in the midlatitudes, preferentially above the subtropical jet. It can not only decelerate the winds in the midlatitudes but also reduce the southwesterlies in East China through the interaction with the upper‐level jet. Alleviation of precipitation bias over the land of East Asia is primarily due to the low‐level GWD which weakens the water vapor flux. In contrast, the upper‐level GWD has the greatest contribution to the enhancement of precipitation over the subtropical ocean by increasing the low‐level lifting.

Impact of boundary layer simulation on predicting radioactive pollutant dispersion: A case study for HANARO research reactor using the WRF-MMIF-CALPUFF modeling system

Wind plays an important role in cases of unexpected radioactive pollutant dispersion, deciding distribution and concentration of the leaked substance. The accurate prediction of wind has been challenging in numerical weather prediction models, especially near the surface because of the complex interaction between turbulent flow and topographic effect. In this study, we investigated the characteristics of atmospheric dispersion of radioactive material (i.e. 137Cs) according to the simulated boundary layer around the HANARO research nuclear reactor in Korea using the Weather Research and Forecasting (WRF)-Mesoscale Model Interface (MMIF)-California Puff (CALPUFF) model system. We examined the impacts of orographic drag on wind field, stability calculation methods, and planetary boundary layer parameterizations on the dispersion of radioactive material under a radioactive leaking scenario. We found that inclusion of the orographic drag effect in the WRF model improved the wind prediction most significantly over the complex terrain area, leading the model system to estimate the radioactive concentration near the reactor more conservatively. We also emphasized the importance of the stability calculation method and employing the skillful boundary layer parameterization to ensure more accurate low atmospheric conditions, in order to simulate more feasible spatial distribution of the radioactive dispersion in leaking scenarios.

An Orographic‐Drag Parametrization Scheme Including Orographic Anisotropy for All Flow Directions

AbstractOrographic drag is an essential process for numerical weather predictions in complex terrain regions, which depends on the inflow direction. In this study, we define the orographic asymmetry vector (OAV) for a coarse grid as the normalized vector between the grid's center point and its center of mass, and the orographic asymmetry in a flow direction—which describes the inclination direction and the extent of the subgrid terrain—is calculated as the projection of OAV on this direction. Calculations of the effective orographic length (OL) and the model grid length λ are extended to all flow directions. A new orographic drag scheme, which considers the effect of orographic anisotropy in all directions, is then developed based on the OAV projection and the extended OL and λ for any given direction. Sensitivity tests of the orographic drag under the new scheme are conducted using a 5 m/s vertically uniform wind along different directions for four coarse grid points in typical mountain regions. The new scheme is shown to provide a more continuous transition of the orographic parameters and the resulting stress as a function of flow direction than piecewise transition of schemes with only eight directions. The predicted momentum flux profile of the new scheme is compared with mountain‐wave simulations obtained from the integrated modeling system IAP‐WRF (Institute of Atmospheric Physics‐Weather Research and Forecasting Model) for the Rocky Mountain. The new scheme is shown to predict an overall narrower stress scatter about the reference simulation than the old scheme.

Synergy of orographic drag parameterization and high resolution greatly reduces biases of WRF-simulated precipitation in central Himalaya

Current climate models often have significant wet biases in the Tibetan Plateau and encounter particular difficulties in representing the climatic effect of the Central Himalaya Mountain (CHM), where the gradient of elevation is extremely steep and the terrain is complex. Yet, there were few studies dealing with the issue in the high altitudes of this region. In order to improve climate modeling in this region, a network consisting of 14 rain gauges was set up at elevations > 2800 m above sea level along a CHM valley. Numerical experiments with Weather Research and Forecasting model were conducted to investigate the effects of meso- and micro-scale terrain on water vapor transport and precipitation. The control case uses a high horizontal resolution (0.03°) and a Turbulent Orographic Form Drag (TOFD) scheme to resolve the mesoscale terrain and to represent sub-grid microscale terrain effect. The effects of the horizontal resolution and the TOFD scheme were then analyzed through comparisons with sensitivity cases that either use a low horizontal resolution (0.09°) or switch off the TOFD scheme. The results show that the simulations with high horizontal resolution, even without the TOFD scheme, can not only increase the spatial consistency (correlation coefficient 0.84–0.92) between the observed and simulated precipitation, but also considerably reduce the wet bias by more than 250%. Adding the TOFD scheme further reduces the precipitation bias by 50% or so at almost all stations in the CHM. The TOFD scheme reduces precipitation intensity, especially heavy precipitation (> 10 mm h−1) over high altitudes of the CHM. Both high horizontal resolution and TOFD enhance the orographic drag to slow down wind; as a result, less water vapor is transported from lowland to the high altitudes of CHM, causing more precipitation at lowland area of the CHM and less at high altitudes of CHM. Therefore, in this highly terrain-complex region, it is crucial to use a high horizontal resolution to depict mesoscale complex terrain and a TOFD scheme to parameterize the drag caused by microscale complex terrain.

What can we learn about orographic drag parametrisation from high‐resolution models? A case study over the Rocky Mountains

AbstractComprehensive high‐resolution numerical weather prediction models provide a virtual laboratory for modelling the atmospheric flow over complex mountain ranges. In this study, global and regional simulations with horizontal grid spacing ranging from 2 to 32 km, focused over the northern Rocky Mountains, are used to assess the orographic blocking and gravity wave drag parametrisations employed in the Met Office Unified Model (UM) and the European Centre for Medium‐Range Weather Forecasts Integrated Forecasting System (IFS). The total, resolved and parametrised drag components in coarse‐resolution simulations are compared with those in high‐resolution simulations, in which the orographic drag processes are better resolved. The total surface stresses and gravity wave momentum fluxes in the free atmosphere of the global 16 km UM and IFS simulations are shown to compare well with 2 km regional simulations in terms of variability and mean. While the total gravity wave momentum flux is somewhat underestimated by both global models, its vertical distribution is well captured. The “seamlessness” of the parametrisation scheme is then assessed by comparing the total orographic stress – and its components – across several horizontal resolutions of the UM. The surface stress remains relatively constant across resolutions, such that the reduction in resolved orographic stress at coarser resolutions is compensated for by an almost equivalent increase in parametrised orographic stress. However, the parametrised orographic gravity wave momentum flux in the free atmosphere remains almost constant with resolution, failing to compensate for the lack of resolved flux at coarse resolutions. This leads to an underestimation of the total gravity wave drag at coarser resolutions. Further analysis suggests that this underestimation is due to the monochromatic wave assumption made by the gravity wave drag parametrisation scheme.

Uncertainty in the Representation of Orography in Weather and Climate Models and Implications for Parameterized Drag

AbstractThe representation of orographic drag remains a major source of uncertainty for numerical weather prediction (NWP) and climate models. Its accuracy depends on contributions from both the model grid‐scale orography and the subgrid‐scale orography (SSO). Different models use different source orography data sets and different methodologies to derive these orography fields. This study presents the first comparison of orography fields across several operational global NWP models. It also investigates the sensitivity of an orographic drag parameterization to the intermodel spread in SSO fields and the resulting implications for representing the Northern Hemisphere winter circulation in a NWP model. The intermodel spread in both the grid‐scale orography and the SSO fields is found to be considerable. This is due to differences in the underlying source data set employed and in the manner in which this data set is processed (in particular how it is smoothed and interpolated) to generate the model fields. The sensitivity of parameterized orographic drag to the intermodel variability in SSO fields is shown to be considerable and dominated by the influence of two SSO fields: the standard deviation and the mean gradient of the SSO. NWP model sensitivity experiments demonstrate that the intermodel spread in these fields is of first‐order importance to the intermodel spread in parameterized surface stress, and to current known systematic model biases. The revealed importance of the SSO fields supports careful reconsideration of how these fields are generated, guiding future development of orographic drag parameterizations and reevaluation of the resolved impacts of orography on the flow.

Dependence on initial conditions versus model formulations for medium‐range forecast error variations

Understanding the root causes of forecast errors and occasional very poor forecasts is essential but difficult. In this paper we investigate the relative importance of initial conditions and model formulation for medium‐range errors in 500 hPa geopotential height. The question is addressed by comparing forecasts produced with ECMWF‐IFS and NCEP‐GFS forecasting systems, and with the GFDL‐fvGFS model initialized with the ECMWF and NCEP initial conditions. This gives two pairs of configurations that use the same initial conditions but different models, and one pair with the same model but different initial conditions. The first conclusion is that the initial conditions play the major role in differences between the configurations in terms of the average root‐mean‐square error for both Northern and Southern Hemispheres as well as Europe and the contiguous US (CONUS), while the model dominates the systematic errors. A similar conclusion is also found by verifying precipitation over low latitudes and the CONUS. The day‐to‐day variations of 500 hPa geopotential height scores are exemplified by one case of a forecast bust over Europe, where the error is found to be dominated by initial errors. The results are generalized by calculating correlations between errors integrated over Europe, CONUS and a region in the southeastern Pacific from the different configurations. For Europe and southeast Pacific, the correlations in the medium range are highest between the pairs that use the same initial conditions, while over CONUS they are highest for the pair with the same model. This suggests different mechanisms behind the day‐to‐day variability of the score for these regions. Over CONUS the link is made to the propagation of troughs over the Rockies, and the result suggests that the large differences in parametrizations of orographic drag between the models play a role.

Dynamical impact of parameterized turbulent orographic form drag on the simulation of winter precipitation over the western Tibetan Plateau

Sub-grid orographic drag directly acts on wind and impacts the regional water cycle through control of atmospheric water vapor (AWV) transport. The effect of turbulent orographic form drag (TOFD) on wind and precipitation is investigated in this study using the WRF model for a winter month over the western Tibetan Plateau (TP), where solid precipitation supplies large amounts of water resources. The diurnal cycle of wind components and atmospheric circulation simulated with TOFD are consistent with observations and ERA-Interim data, whereas stronger westerlies exist in the simulation without the TOFD scheme. The latter results in more zonal AWV transport from the west and more precipitation over the western TP and surroundings. The implementation of the TOFD scheme leads to reduced biases, when evaluated with two observation-based precipitation products. It is therefore concluded that this scheme has a clear dynamical control on the regional atmospheric water recharge and thus the parameterization of the small-scale orographic drag in the model helps to improve the prediction of wintertime precipitation in the western TP region.