ALERT - Towards a Save Landing

A new LES-based system for short-range forecasting of near-surface high-impact weather at air-ports

H. Knoop, B. Maronga, Institut für Meteorologieund Klimatologie, Leibniz Universität Hannover

In Short

• Severe weather conditions like dense fog andstrong gusts impose high risks onto airport en-vironments.

• The parallelized large-eddy simulation modelPALM will be used to explicitly resolve fog andgust risks.

• An operational LES-based forecasting system forairport environments will be developed.

The quality of weather forecast and warning prod-ucts strongly relies on the quality of parametrizationof all those processes that cannot be resolved by op-erational numerical weather prediction (NWP) mod-els due to their coarse grid resolution. Particularly,turbulent processes in the atmospheric boundarylayer (ABL) are still parameterized in state-of-the-artNWP models. However, turbulent processes are thekey factor for various high-impact weather conditionsrelevant for the operation of airports. The stableboundary layer (SBL) regime for example is closelylinked to the formation of fog. Dense fog usuallyforms in the nighttime ABL and can have a strongimpact on the economy and also on personal safety.Low visibility is a major issue for the operation ofairports. The life cycle of fog, from formation aftersunset to dissipation in the morning hours, and its mi-crostructure, however, are not well understood andthe quality of operational fog forecast is still a weakpoint of NWP models. Recently, it has been shownthat small-scale surface heterogeneities and airportbuildings can significantly modify the formation timeof fog, which clearly point out that the surface proper-ties are essential for an accurate forecast of criticalatmospheric conditions at airports and that tradi-tional forecast systems are often inadequate in suchconditions. In the neutral boundary layer, severewind gusts associated with strong-wind events likewinter storms are regarded as the most expensivenatural disaster in Germany. Small spatial and tem-poral scales of gusts lead to a lack of observationalstudies and hence verification data to improve windgust estimation (WGE) models used in NWP mod-els. A comparison of different approaches for WGEin the COSMO-CLM model of the german weather

service (DWD) showed that empirical gust estima-tion models still have advantages over a physicallybased approach. However, a deeper understandingof the shape and life cycle of wind gusts is needed toimprove current WGE models. A major task for theoperation of airports is a good prediction of wind gusteffects on airplanes during final approach. Airportenvironments in Germany are commonly character-ized by relatively flat terrain, accompanied by airportbuildings of varying shape and size. Such topog-raphy elements have the potential of modifying thelocal flow conditions significantly and are thus sus-pect to be an important factor for wind gusts on therunway, particularly in strong-wind conditions.

The operation of airports during such foggy orstrong-wind conditions currently relies on operationalNWP and regional model data. These models canneither resolve the relevant near-surface processesthat lead to fog or wind gusts, nor can they accountfor small-scale surface heterogeneity, such as airportbuildings or vegetation and soil heterogeneity. Alogical approach to achieve a more reliable forecastfor airport areas would be the use of an LES model,coupled to or forced by an NWP model. This LESmodel can resolve the turbulence in the ABL as wellas the interactions with airport buildings and thesurface.

In the course of the DWD project 2015EMF-13(codename ALERT) a tested and evaluated LES-based forecasting system for airport environmentswill be developed. The system will act as a mag-nifying lens and will be suitable for the forecast ofhigh-impact weather conditions related to the turbu-lent processes in the near-surface ABL that cannotbe adequately represented in state-of-the-art NWPmodels. The system eventually will allow the op-erational forecast of critical conditions relevant foraviation and airport operation, namely the forecast ofdense fog ad severe wind gusts in stormy weather. Itwill be driven by NWP model data and shall run fastenough at operational level with reasonable (limited)computational resources.

Unlike current NWP models, the LES-based sys-tem aims at explicitly resolving the effects of theactual airport buildings and other surface proper-ties, such as vegeation, impervious surfaces andsoil on the boundary layer turbulence. This will allowto explicitly predict e.g. effects of wake turbulencegenerated by airport buildings on aircraft during fi-nal approach or the hazard of low visibility due toradiation fog at the airport.

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Figure 1: Three dimensional rendering of the Munich airport environment showing radiation fog (in white) as the fog developsthroughout the night and slowly encloses the airport. The fog delaying effect of the airport buildings and the largely sealed pavementsurface of the airfield can clearly be seen via the hole in the cloud cover close to the the middle of the rendering. The animated datawas generated with PALM [1,2]

As the project is in its third year of developmentwe have already successfully simulated completeairport environments in full detail. The forecastingsystem considers the airport vicinity including theorography and all buildings as well as highly de-tailed surface and vegetation properties. Even forestcanopies can be resolved with their heterogeneousdistribution of tree trunks and leafs.

Figure 1 shows a three-dimensional data render-ing of fog as it develops throughout the Munich air-port during the night. While the systems own radia-tion model computes the dynamic radiative coolingof all surfaces on the complex topography, the airmoist air flow above these surfaces gradually coolsand slowly reaches its dew point. At that point thealso included bulk cloud microphysics model startscomputing how much water is condensed into clouddroplets. This so called liquid water content in theair flow slowly builds up as the radiative cooling con-tinues and the fog develops. Figure 1 clearly showsthe fog delaying effect of the airport buildings andthe largely sealed pavement surface of the airfieldas the fog leaves a hole in the cloud cover closeto the middle of the airport. This hole shrinks insize as the night progresses. In the morning thesuns short-wave radiation reverses that process andstarting from the middle of the airport the fog slowlydissolves and convection starts. Equally detailedsimulations can be performed in order to provide

gust predictions for aircraft during final approach.The forecasting systems boundary conditions can

be driven by lower resolution NWP model data and,once validated and optimized, can be applied to anyairport in order to serve as a magnifying glas in theNWP model chain of the German Weather service(DWD). Currently the system undergoes rigoroustesting and validation in order to assess the reliabilityand accuracy of the produced fog and gust forecasts.

WWW

https://www.muk.uni-hannover.de

More Information

[1] B. Maronga et al. (2015): The ParallelizedLarge-Eddy Simulation Model (PALM) version4.0 for Atmospheric and Oceanic Flows: ModelFormulation, Recent Developments, and FuturePerspectives Geosci. Model Dev. 8.8 pp. 2515-2551. doi:10.5194/gmd-8-2515-2015

[2] See also: https://palm.muk.uni-hannover.de

Funding

The associated research project is funded byDeutscher Wetterdienst (DWD) under the reference2015EMF-13.

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