Nielsen, Joakim Refslund

For nøjagtig forudsigelse of vejr og klima er det vigtig at landoverfladen, samt dens processer, er præcist repræsenteret. I en mesoskala model er landoverfladens processer beregnet i en landoverflade model (LSM). Disse processer inkluderer udvekslinger af energi, vand og impuls mellem landoverfladens komponenter, såsom vegetation og jorden, og deres interaktion med atmosfæren. Landoverfladens processer er komplekse og varierer i tid og rum. En betydelig indsats er derfor blevet udf_rt for at forbedre landoverflade modeller i de seneste årtier. Der er dog stadig behov for forbedret repræsentation af landoverfladens variabilitet samt af visse vigtige landoverflade processer. Denne afhandling undersøger to muligheder for at forbedre model forudsigelser når overfladen ved hjælp af den mesoskala vejrprognose og forskningsmodel (WRF). I den første fremgangsmåde, data fra satellitbilleder blev anvendt til at undersøge virkningen af nøjagtig repræsentation af vegetations brøkdele i Noah LSM. I den anden fremgangsmåde, undersøges avancerede landoverflade parameteriseringer inkluderet i en ny Noah LSM med multiparameteriserings indstillingsmuligheder (Noah-MP).En ny metode blev anvendt til at frembringe høj-opløst, høj-kvalitets vegetations brøkdele fra Moderate Resolution Imaging Spectro-radiometer (MODIS) vegetations-indekser. Indekserne blev omdannet til vegetations brøkdele ved hjælp af både en lineær og en kvadratisk fremgangsmåde. Den kvadratiske metode blev identificeret som overlegen overfor den linære metode til brug i WRF modellen. Desuden blev det bemærket at sæsonudsving i vegetationen i 2006 afveg signifikant fra dens klimatologi over nogle områder af Europa, muligvis på grund af perioder med varmebølger og tørke. Resultater af simuleringer over disse områder ved anvendelse af WRF modellen, ved relativ lav rumlig opløsning og ved brug af MODIS vegetations datasæt, viste forbedret forudsigelse af temperatur når den kvadratiske metode blev anvendt. Det kvadratiske MODIS vegetation datasæt og standard vegetation datasættet i WRF blev yderligere anvendt i højt-opløste simuleringer over Danmark ned til sky-opløst skala (3 km). Resultaterne fra to rumlige opløsninger blev sammenlignet for at undersøge indflydelsen af parameteriseret og opløst konvektion. Den parameteriserede konvektion viste store overestimeringer af nedbøren, mens den opløste konvektion fulgte observationerne mere nøjagtigt. Generelt var resultaterne for nedbør, temperatur og vindhastighed lige gode i sommer perioden for begge vegetations datasæt. Dog forbedrede MODIS datasættet væsentligt simuleringen af den tidslige udvikling af Bowen forholdene i sommer perioden. Arbejdet med Noah-MP LSM omfattede kobling, fejlsøgning og evaluering af dens ydeevne i rammerne af WRF modellen. Derudover blev nye indstillingsmuligheder tilføjet således at flere tusinde kombinationer af parameteriserings muligheder er tilgængelige. I denne afhandling præsenteres modellen sammen med en evaluering of dens ydeevne under en sommer simulering ved brug af anbefalede indstillingsmuligheder. Simuleringerne viser at der opnås gode resultater for temperatur og nedbør.For accurate predictions of weather and climate, it is important that the land surface and its processes are well represented. In a mesoscale model the land surface processes are calculated in a land surface model (LSM). These pro-cesses include exchanges of energy, water and momentum between the land surface components, such as vegetation and soil, and their interactions with the atmosphere. The land surface processes are complex and vary in time and space. Signi_cant e_ort by the land surface community has therefore been invested in improving the LSMs over the recent decades. However, improve-ments are still needed in the representation of the land surface variability and of some key land surface processes.This thesis explores two possibilities for improving the near-surface model predictions using the mesoscale Weather Research and Forecasting (WRF) model. In the _rst approach, data from satellite images were used to investi-gate the impact of accurate representation of vegetation fractions in the Noah LSM, which is the default LSM in WRF. In the second approach, advanced land surface parameterizations included in a new version of the Noah LSM with multiparameterization options (Noah-MP) were investigated. A novel method was used to derive high-resolution, high-quality vegetation information from Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices. The indices were converted into a vegetation fraction, which is a key parameter for the representation of vegetation in WRF, us-ing both a linear and a quadratic approach. The quadratic approach was identi_ed as superior over the linear approach for use in the WRF model. In addition, it was noted that vegetation seasonality during 2006 deviated sig-ni_cantly from its climatology over some regions of Europe, possibly due to the occurrence of heat wave and drought conditions. The results of the sim-ulations over these regions obtained using the WRF model, at relatively low spatial resolution and using MODIS vegetation data sets, showed improved temperature predictions when using the quadratic approach. The quadratic MODIS vegetation data and the default vegetation data in WRF were further used in high-resolution simulations over Denmark down to cloud-resolving scale (3 km). Results from two spatial resolutions were compared to investigate the inuence of parametrized and resolved convec-tion. The simulations using the parametrized convection showed large overes-timations of precipitation during the summer, while the ones using resolved convection more accurately followed the observations. In general, equally good performance for precipitation, temperature and wind speed was found using both vegetation data sets. However, the MODIS data improved the simulation of the temporal evolution of the Bowen ratio during the summer. The work on the Noah-MP LSM included coupling, debugging and eval-uation of its performance in the WRF modeling framework. In addition, new options were added, such that several thousand combinations of parameterization settings are now available. In this thesis, the model is presented together with an evaluation of its performance during a summer simulation using the current recommended options. The simulations show that good results are obtained for temperature and precipitation

Online Research Database In Technology

Representing vegetation processes in hydrometeorological simulations using the WRF model

http://orbit.dtu.dk/ws/files/69208136/JoakimRefslundThesis.pdf

Representing vegetation processes in hydrometeorological simulations using the WRF model

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