Examensarbete vid institutionen för geovetenskaper ISSN 1650-6553 Nr 101 Estimation of gusty winds in RCA Maria Nordström Abstract In this study a new wind gust estimate (WGE) method proposed by Brasseur (2001) is implemented in a limited area climate model (RCA, Rossby Centre regional Atmospheric model). The WGE method assumes that wind gusts develop when air parcels higher up in the boundary layer deflect down to the surface by turbulent eddies. The method also gives an interval of possible gusts by estimating an upper and lower bound of a bounding interval. Two separate storms (December 3-4, 1999 and January 8-9, 2005) and a three month period (November 1, 2004 - January 31, 2005) are simulated with RCA. The results are compared to direct observations and to gridded analysis (MESAN). The result is highly dependent on how well the meteorological fields are represented in RCA. Since the storm of December 1999 was not well captured by RCA, the wind gusts were consequently not correctly estimated. The storm of January 2005 was well captured by the RCA and the wind gusts relatively well described. Both the storm of January 2005 and the simulation over a three month period give rather good estimated gusts over sea areas, while over land there is an obvious overestimation of the calculated gusts. A correction to the estimated gust is necessary in order to make the parameterisation useful. Such a correction is tested in this study. It shows significant improvement over most land areas and also gives a certain underestimation in other areas. Sammanfattning av ”Beräkning av byiga vindar i RCA” En ny metod (WGE-metoden) för att bestämma byvindar har i den här studien implementerats i en regional klimatmodell (RCA, Rossby Centre regional Atmospheric model). WGE-metoden utgår från att vindbyar genereras när luftpaket högre upp i gränsskiktet förs ner till marken av stora turbulenta virvlar. Ett intervall av möjliga byvindar erhålls genom att en övre och undre gräns för detta intervall beräknas. Två stormar (3-4 december 1999 och 8-9 januari 2005) och en tremånaders period (1 november 2004 – 31 januari 2005) har simulerats, och resultaten har jämförts med mätdata och MESAN. Resultatet är till stor del beroende av hur väl de meteorologiska fälten representeras av RCA. Stormen i december 1999 simulerades dåligt av RCA, vilket medförde att byvinden inte heller simulerades korrekt. Både stormen januari 2005 och simuleringen över tre månader ger en tämligen korrekt byvind över hav, samtidigt som man över land får kraftiga överskattningar av den beräknade byvinden. För att byvind-parametriseringen ska vara användbar krävs korrigeringar för att komma till rätta med överskattningen över land. En korrigering testades i den här studien med resultatet att ett förbättrat resultat över land samtidigt leder till en viss underskattning av byvinden i andra områden. 2 Table of contents 1. Introduction................................................................................................................... 4 2. Theory ............................................................................................................................ 5 2.1 Wind gusts ................................................................................................................ 5 2.2 Wind gust estimate method....................................................................................... 5 2.2.1 Gust estimate...................................................................................................... 6 2.2.2 Upper and lower bound of the bounding interval .............................................. 7 3. Method ........................................................................................................................... 9 3.1 RCA .......................................................................................................................... 9 3.1.1 A regional climate model................................................................................... 9 3.1.2 Physical parameterisations................................................................................. 9 3.1.3 Implementation and testing of the WGE method in RCA ............................... 10 3.2 MESAN................................................................................................................... 10 3.3 Observational data .................................................................................................. 11 4. Results .......................................................................................................................... 12 4.1 The storm of December 3-4, 1999 .......................................................................... 12 4.2 The storm of January 8-9, 2005 .............................................................................. 16 4.3 Simulation over a three month period..................................................................... 24 4.4 Correction to the estimated wind gust .................................................................... 28 5. Summary and conclusions.......................................................................................... 33 Acknowledgments ........................................................................................................... 34 References........................................................................................................................ 35 Appendix.......................................................................................................................... 37 The photographer of the front page picture is Eva Melakari 3 1. Introduction Severe storms can cause extensive damage due to the high wind speeds. The strongest wind intensities are caused by wind gusts, which can give a magnitude of 10-15 m/s higher than the mean wind. One example is the storm of January 2005 in southern Sweden where 70 million cubic metres of wood was damaged. The storm caused the death of 9 people and 415,000 household were affected by power failure. During this storm the highest measured gust over land was 33 m/s (in Växjö), while the maximum mean wind speed measured at the same synoptic station was 17 m/s. Warnings may to a certain extent reduce the damages, and it is therefore important to correctly estimate severe wind gusts. Estimating wind gusts is also necessary in mapping the potential use of wind energy as well as for biological processes in the oceans, where the wind causes water mixing. Brasseur (2001) suggests a new method for estimating wind gusts. Unlike statistical or empirical methods, this method gives an explanation to the physics behind the gusts. This new method of wind gust estimate (WGE) also gives a confidence interval on the predicted value by giving a range of possible gust speeds. The WGE method assumes that wind gusts develop when air parcels higher up in the boundary layer deflects down to the surface by turbulent eddies. This requires that the turbulence kinetic energy is greater than the buoyancy forcing. The method has been applied by Goyette et al. (2003) to the Canadian regional climate model. Two storms were studied with different model resolutions. The result was fairly realistic and depended on the model resolution. The magnitude and variability of the gusts were not always exactly reproduced, however most of the strengthening and weakening phases were correctly simulated. In the Rossby Centre regional Atmospheric model (RCA) severe winds are often underestimated, and the mean wind speed over a longer period of time is also underestimated. RCA is a regional climate model developed from the Swedish operational model, HIRLAM (High Resolution Limited Area Model). In this study the WGE method is applied to RCA while studying the storms of December 3-4, 1999 and January 8-9, 2005. The results are compared to MESAN (an Operational Mesoscale Analysis System) and to measured data from synop stations in southern Sweden and one buoy outside of the Swedish west coast. A simulation over a three month period is also analysed and compared to station data and MESAN. Additionally, a test where the parameterisation is tuned to achieve better agreement between estimated wind gusts and observed wind gusts has been performed. 4 2. Theory 2.1 Wind gusts Wind gusts are defined as sudden, brief increases of the mean wind. The wind gusts have a great variability of intensity and are therefore not easily predicted. Most models use an empirical or statistical method to determine gusts. In one method a constant ratio of maximum gusts to the surface wind speed is being used. This ratio depends mainly on surface roughness, and is smaller over open sea and larger over big cities. Generally, the wind is more gusty over rough land than over open water. 2.2 Wind gust estimate method A new method for estimating wind gusts is proposed by Brasseur (2001). Unlike empirical and statistical methods this wind gust estimate method gives an explanation to the physics behind the gusts. The WGE method takes under consideration the turbulence kinetic energy, the mean wind and the stability of the boundary layer. It is assumed that turbulent eddies higher up in the boundary layer cause air parcels to deflect down to the surface as shown in Figure 2.1. When these air parcels with a higher wind speed reach the surface they become gusty winds. The deflection of air parcels is strongly connected to the stability of layers. The vertical component of the turbulent kinetic energy for a given air parcel must be