Initial-Condition Sensitivities and the Predictability of Downslope Winds

Reinecke, P. Alexander; Durran, Dale R.

doi:10.1175/2009jas3023.1

Cited by 66 publications

(48 citation statements)

References 39 publications

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“…slopes in rather homogeneous terrain (Mahrt, 1999;Aubinet, 2008), while gravity waves are usually caused by topographic changes or irregularities of the canopy top (Lee et al, 1997). In complex terrain, all of these processes may occur, and a large effort has been made within projects like T-REX (T-REX stands for Terrain-induced Rotor EXperiment; Grubisic et al, 2008) and VTMX 2000 (Vertical Transport and MiXing; Doran et al, 2002) to understand the physics behind such (nocturnal) flows and how they influence transport of matter and energy (Cooper et al, 2006;Pinto et al, 2006;Reinecke and Durran, 2009;Choukulkar et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer

El-Madany¹,

Duarte

Durden

et al. 2014

Biogeosciences

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Abstract. Sodar (SOund Detection And Ranging), eddycovariance, and tower profile measurements of wind speed and carbon dioxide were performed during 17 consecutive nights in complex terrain in northern Taiwan. The scope of the study was to identify the causes for intermittent turbulence events and to analyze their importance in nocturnal atmosphere-biosphere exchange as quantified with eddycovariance measurements. If intermittency occurs frequently at a measurement site, then this process needs to be quantified in order to achieve reliable values for ecosystem characteristics such as net ecosystem exchange or net primary production.Fourteen events of intermittent turbulence were identified and classified into above-canopy drainage flows (ACDFs) and low-level jets (LLJs) according to the height of the wind speed maximum. Intermittent turbulence periods lasted between 30 and 110 min. Towards the end of LLJ or ACDF events, positive vertical wind velocities and, in some cases, upslope flows occurred, counteracting the general flow regime at nighttime. The observations suggest that the LLJs and ACDFs penetrate deep into the cold air pool in the valley, where they experience strong buoyancy due to density differences, resulting in either upslope flows or upward vertical winds.Turbulence was found to be stronger and better developed during LLJs and ACDFs, with eddy-covariance data presenting higher quality. This was particularly indicated by spectral analysis of the vertical wind velocity and the steady-state test for the time series of the vertical wind velocity in combination with the horizontal wind component, the temperature, and carbon dioxide.Significantly higher fluxes of sensible heat, latent heat, and shear stress occurred during these periods. During LLJs and ACDFs, fluxes of sensible heat, latent heat, and CO 2 were mostly one-directional. For example, exclusively negative sensible heat fluxes occurred while intermittent turbulence was present. Latent heat fluxes were mostly positive during LLJs and ACDFs, with a median value of 34 W m −2 , while outside these periods the median was 2 W m −2 . In conclusion, intermittent turbulence periods exhibit a strong impact on nocturnal energy and mass fluxes.

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Section: Introductionmentioning

confidence: 99%

Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer

El-Madany¹,

Duarte

Durden

et al. 2014

Biogeosciences

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“…The ensemble based study of Reinecke and Durran (2009) showed a strong dependence of the predictability to small-scale features in the synoptic flow. Here, merely changing a parameterization in the atmospheric model is decisive for a successful forecast.…”

Section: Discussionmentioning

confidence: 99%

Downslope windstorm in Iceland – WRF/MM5 model comparison

et al. 2011

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Abstract.A severe windstorm downstream of Mt. Öraefa-jökull in Southeast Iceland is simulated on a grid of 1 km horizontal resolution by using the PSU/NCAR MM5 model and the Advanced Research WRF model. Both models are run with a new, two equation planetary boundary layer (PBL) scheme as well as the ETA/MYJ PBL schemes. The storm is also simulated using six different micro-physics schemes in combination with the MYJ PBL scheme in WRF, as well as one "dry" run. Output from a 3 km MM5 domain simulation is used to initialise and drive both the 1 km MM5 and WRF simulations. Both models capture gravity-wave breaking over Mt. Öraefajökull, while the vertical structure of the lee wave differs between the two models and the PBL schemes. The WRF simulated downslope winds, using both the MYJ and 2EQ PBL schemes, are in good agreement with the strength of the observed downslope windstorm. The MM5 simulated surface winds, with the new two equation model, are in better agreement to observations than when using the ETA scheme. Micro-physics processes are shown to play an important role in the formation of downslope windstorms and a correctly simulated moisture distribution is decisive for a successful windstorm prediction. Of the microphysics schemes tested, only the Thompson scheme captures the downslope windstorm.

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“…The lee-side development may furthermore be strongly affected by non-linear wave dynamics and variability in the impinging flow Durran, 1997, 1998), possibly leading to large errors. Studies of ensembles of simulations have in fact shown that downslope windstorms can be highly sensitive to small-scale features in the initial conditions (Reinecke and Durran, 2009). It is however not clear what aspects of the large-scale flow have the greatest impact on the lee-side development or what flow 'regimes' are most sensitive, but it may in fact be model dependent (Doyle et al, 2011).…”

Section: Discussionmentioning

confidence: 99%

“…This data often originates from global atmospheric re-analysis, operational analysis or forecasts at relatively coarse resolutions, typically 15Á80 km in the horizontal, with a temporal resolution of 1Á6 hours. Frequently, the accuracy and resolution of this data is not adequate for high-resolution simulations of local and small-scale features which may be dependent on small deviations in the large-scale flow, as pointed out by Reinecke and Durran (2009) in their study of gravity wave activity above complex orography. In spite of the global coverage of the remote sensing data, which has significantly improved atmospheric analyses, such analyses may suffer in otherwise data-sparse and mountainous regions of the world.…”

Section: Introductionmentioning

confidence: 98%

The impact of assimilating data from a remotely piloted aircraft on simulations of weak-wind orographic flow

Ágústsson

Ólafsson

Jonassen

et al. 2014

Tellus A: Dynamic Meteorology and Oceanography

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A B S T R A C T Orographic winds near a 914 m high mountain in Southwest-Iceland are explored using unique observations made aloft with a small remotely piloted aircraft, as well as with traditional observations and high-resolution atmospheric simulations. There was an inversion well above mountain top level at about 2 km with weak winds below. Observed winds in the lee of the mountain were indicative of flow locally enhanced by wave activity aloft. Winds descended along the lee slope with a prevailing direction away from the mountain. They were relatively strong and gusty at the surface close to the mountain, with a maximum at low levels, and weakening and becoming more diffuse a short distance further downstream. The winds weakened further aloft, with a minimum on average near mountain top level. This situation is reproduced in a high-resolution atmospheric simulation forced with atmospheric analysis as well as with the observed lee-side profiles of wind and temperature below 1.4 km. Without the additional observations consisting of the lee-side profiles, the model fails to reproduce the winds aloft as well as at the surface in a region in the lee of the mountain, as was also the case for the operational numerical models at that time. A sensitivity simulation indicates that this poor performance is a result of the poorly captured strength and sharpness of the inversion aloft. The study illustrates, firstly, that even at very low wind speed, in a close to neutral low-level flow, gravity waves may still be a dominating feature of the flow. Secondly, the study presents an example of the usefulness of lee-side atmospheric profiles, retrieved by simple model aircraft, for improving numerical simulations and short-term weather forecasting in the vicinity of mountains. Thirdly, the study confirms the sensitivity of downslope flow to only moderate change in the sharpness of an upstream inversion.

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Initial-Condition Sensitivities and the Predictability of Downslope Winds

Cited by 66 publications

References 39 publications

Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer

Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer

Downslope windstorm in Iceland – WRF/MM5 model comparison

The impact of assimilating data from a remotely piloted aircraft on simulations of weak-wind orographic flow

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