An updated subgrid orographic parameterization for global atmospheric forecast models

Choi, Hyunjoo; Hong, Song‐You

doi:10.1002/2015jd024230

Cited by 52 publications

(57 citation statements)

References 25 publications

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“…With the different selections of subgrid orographic drag in WRF over the TP, the above simulations reveal that (1) the GWD/FB scheme from Choi and Hong () efficiently reduces the U10 mean bias and RMS error. (2) The TOFD scheme from Beljaars et al () is more effective in reducing the U10 bias and RMS error.…”

Section: Summary and Further Workmentioning

confidence: 99%

Evaluation of WRF Simulations With Different Selections of Subgrid Orographic Drag Over the Tibetan Plateau

et al. 2017

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Weather Research and Forecasting (WRF) simulations with different selections of subgrid orographic drag over the Tibetan Plateau have been evaluated with observation and ERA‐Interim reanalysis. Results show that the subgrid orographic drag schemes, especially the turbulent orographic form drag (TOFD) scheme, efficiently reduce the 10 m wind speed bias and RMS error with respect to station measurements. With the combination of gravity wave, flow blocking and TOFD schemes, wind speed is simulated more realistically than with the individual schemes only. Improvements are also seen in the 2 m air temperature and surface pressure. The gravity wave drag, flow blocking drag, and TOFD schemes combined have the smallest station mean bias (−2.05°C in 2 m air temperature and 1.27 hPa in surface pressure) and RMS error (3.59°C in 2 m air temperature and 2.37 hPa in surface pressure). Meanwhile, the TOFD scheme contributes more to the improvements than the gravity wave drag and flow blocking schemes. The improvements are more pronounced at low levels of the atmosphere than at high levels due to the stronger drag enhancement on the low‐level flow. The reduced near‐surface cold bias and high‐pressure bias over the Tibetan Plateau are the result of changes in the low‐level wind components associated with the geostrophic balance. The enhanced drag directly leads to weakened westerlies but also enhances the a‐geostrophic flow in this case reducing (enhancing) the northerlies (southerlies), which bring more warm air across the Himalaya Mountain ranges from South Asia (bring less cold air from the north) to the interior Tibetan Plateau.

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Section: Summary and Further Workmentioning

confidence: 99%

Evaluation of WRF Simulations With Different Selections of Subgrid Orographic Drag Over the Tibetan Plateau

et al. 2017

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“…The other is the scheme from Choi and Hong (, hereafter C15), which considers the effects of GWD together with LLWD (Hong et al, ; Kim & Arakawa, ) and FBD (Kim & Doyle, ) with improved parameters (e.g., height of the blocked layer). The wave stress due to both of the GWD and LLWD at reference level ( τ W ) is given as follows:

τ_{W} = ρ_{0} E \frac{m}{λ_{eff}} G \frac{{(||, U_{0})}^{3}}{N_{0}},

E \equiv {(OA + 2)}^{C_{E} {Fr}_{0} / {Fr}_{c}}, m \equiv {(1 + L_{x})}^{OA + 1}, G \equiv \frac{{italicFr}_{0}^{2}}{{Fr}_{0}^{2} + C_{G} {OC}^{- 1}},

where E is the enhancement factor for representing the nonlinear enhancement of drag due to the low‐level wave breaking, which is calculated by the orographic asymmetry (OA), which represents the shape and location of subgrid‐scale orography relative to the grid, and the Froude number Fr 0 (= h OD N 0 / U 0 ) normalized by its critical value ( Fr c = 1); OD is the orographic direction, which is equivalent to the horizontal aspect ratio of the orography; m is the number of subgrid‐scale orography and L x represents the subgrid‐scale mountain width in the direction of low‐level wind; λ eff is the effective grid length; G is an asymptotic function that provides a smooth transition between the nonblocking and blocking cases and includes the effect of orographic convexity (OC) corresponding to the vertical orographic aspect ratio; and C E and C G are set to 0.8 and 0.5, respectively, based on the mesoscale simulation results from Kim and Arakawa ().…”

Section: Experimental Designmentioning

confidence: 99%

Effects of Parameterized Orographic Drag on Weather Forecasting and Simulated Climatology Over East Asia During Boreal Summer

Choi

Koo

et al. 2017

JGR Atmospheres

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The impact of subgrid orographic drag on weather forecasting and simulated climatology over East Asia in boreal summer is examined using two parameterization schemes in a global forecast model. The schemes consider gravity wave drag (GWD) with and without lower‐level wave breaking drag (LLWD) and flow‐blocking drag (FBD). Simulation results from sensitivity experiments verify that the scheme with LLWD and FBD improves the intensity of a summertime continental high over the northern part of the Korean Peninsula, which is exaggerated with GWD only. This is because the enhanced lower tropospheric drag due to the effects of lower‐level wave breaking and flow blocking slows down the wind flowing out of the high‐pressure system in the lower troposphere. It is found that the decreased lower‐level divergence induces a compensating weakening of middle‐ to upper‐level convergence aloft. Extended experiments for medium‐range forecasts for July 2013 and seasonal simulations for June to August of 2013–2015 are also conducted. Statistical skill scores for medium‐range forecasting are improved not only in low‐level winds but also in surface pressure when both LLWD and FBD are considered. A simulated climatology of summertime monsoon circulation in East Asia is also realistically reproduced.

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“…Moreover, the difference in the 2 m temperature bias between GWDSBL and ST do not appear until near the end of the simulation time. This reduction in accuracy of 2 m temperature scores was found by Choi and Hong (), who attributed this to lower and more accurate wind speeds (sections 4.2 and 5.3) in model runs that resulted in less mechanical turbulence and smaller surface fluxes, when they included the GWD parametrization.…”

Section: Experimental Results For All 16 Model Runsmentioning

confidence: 94%

Small‐scale orographic gravity wave drag in stable boundary layers and its impact on synoptic systems and near‐surface meteorology

Tsiringakis

Steeneveld

Holtslag

2017

Quart J Royal Meteoro Soc

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At present atmospheric models for weather and climate use enhanced turbulent drag under stable conditions, because these empirically provide the necessary momentum drag for accurate forecast of synoptic systems. The enhanced mixing (also known as the ‘long tail’), introduces drag that cannot be physically justified and degrades the score for near‐surface temperature, wind and boundary‐layer height, and degrades fog and frost forecasting. This study hypothesizes that the insufficient representation of small‐scale orographic gravity wave drag in the stable boundary layer may explain the need for the enhanced drag formulation. Hence, we introduce a new scheme in the Weather Research and Forecasting model that accounts for this drag as a superposition on the turbulent drag induced by a so‐called short‐tail mixing function. The latter is consistent with boundary‐layer observations and large‐eddy simulations. We evaluate this scheme, against a short‐tail and a long‐tail scheme for sixteen eight‐day forecasts over the Atlantic Ocean and Europe in winter. The new scheme outperforms the short‐ and long‐tail schemes on sea‐level pressure, height of the 500 hPa field, 10 m wind and the cyclonic core pressure. Cyclonic core pressure bias is reduced by approximately 45 to 80% compared to the short‐tail scheme. Sea‐level pressure bias is reduced by up to 0.48 hPa (50%) over the whole domain compared to the short‐tail run. The new scheme has even smaller biases than the long‐tail scheme, supporting our hypothesis that small‐scale gravity wave drag may explain the need for a long‐tail function. Near‐surface wind bias is reduced by up to 40% compared to the long‐tail and up to 32% compared to the short‐tail scheme, while the 2 m temperature bias is only slightly increased (19%).

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An updated subgrid orographic parameterization for global atmospheric forecast models

Cited by 52 publications

References 25 publications

Evaluation of WRF Simulations With Different Selections of Subgrid Orographic Drag Over the Tibetan Plateau

Evaluation of WRF Simulations With Different Selections of Subgrid Orographic Drag Over the Tibetan Plateau

Effects of Parameterized Orographic Drag on Weather Forecasting and Simulated Climatology Over East Asia During Boreal Summer

Small‐scale orographic gravity wave drag in stable boundary layers and its impact on synoptic systems and near‐surface meteorology

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