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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Processes important for forecasting of clouds over snow

Hagman, Martin January 2020 (has links)
The Swedish Armed Forces setup of the Weather Research and Forecasting Model (WRF) has problems to forecast low clouds in stably stratified conditions when the ground is covered by snow. The aim of this thesis is to understand what causes this deficit. Simulations during January and February 2018 are here compared with observations from Sodankylä in northern Finland. It is revealed that neither type of planetary boundary layer parameterization chosen nor vertical or horizontal interpolation are responsible for the deficiency. Instead, our experiments show that, to first order, poor initialization of Stratocumulus (Sc) clouds from the host model, Atmospheric Model High Resolution (HRES), of the Integrated Forecast System (IFS) is the missing link. In situations when Sc clouds are missing in the IFS analysis, although they exist in reality, we use information from vertical soundings from Sodankylä. In the initialization process we used the fact that liquid potential temperature is constant in a well-mixed cloud. Initializing cloud water and cloud ice from IFS HRES and from soundings with different methods improves the model performance and the formation of very low artificial clouds at the first model level is prohibited.
2

IMPLEMENTAÇÃO DE UM MODELO DE COLUNA SIMPLES PARA A CAMADA LIMITE ESTÁVEL UTILIZANDO DIFERENTES FORMULAÇÕES DE TURBULÊNCIA / IMPLEMENTATION OF A SINGLE COLUMN MODEL FOR THE STABLE BOUNDARY LAYER USING DIFFERENT TURBULENCE FORMULATIONS

Schmengler, Moacir 01 October 2012 (has links)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work a single column model is implemented, aiming primarily to simulate the Stable Boundary Layer (SBL). The model is composed of prognostic equations for the potential temperature, specific humidity and the wind components. A prognostic equation for the turbulent kinetic energy (TKE) is also used. The turbulent fluxes are approximated from diffusion coefficients that are directly proportional to TKE and to the turbulent mixing length, with a adjustable proportionality factor. To estimate the turbulent mixing lengths, four different parametrizations from the literature are considered. First, a stable case in the Arctic is simulated, then the model is adjusted to reproduce previously published Large Eddy Simulations (LES) results for the same case. After the calibration, the model is validated through a comparison with observed nocturnal data, from tethered balloon soundings performed at a deforested area in the Amazon region, in the months of July and October 2001 and November 2003. The temporal evolution of potential temperature and specific humidity vertical profiles are simulated and compared to the observations from 13 different nights. The performance of the different formulations is evaluated through a statistic analysis for all nights. Potential temperature profiles were better represented by the model than specific humidity ones. In spite being able to reproduce different stability conditions, the model represented the most stable nights better. Finally, the model was coupled to a surface energy budget scheme and for these simulations the initial conditions from the night of 28 July 2001 were used. The scheme was used to evaluate how the simulated atmospheric variables depend on the wind intensity at the top of the vertical domain for the different formulations and with or without the energy budget scheme being added. / Neste trabalho é implementado um Modelo de Coluna Simples, com o intuito de simular a Camada Limite Estável (CLE). O modelo é composto por equações prognósticas para a temperatura potencial, umidade específica e para as componentes horizontais do vento, além de uma equação prognóstica para a energia cinética turbulenta (ECT). Os fluxos turbulentos são aproximados por coeficientes de difusão que são diretamente proporcionais à ECT e ao comprimento de mistura turbulenta, de forma que o coeficiente de proporcionalidade é um fator ajustável. Para estimar os comprimentos de mistura turbulentos são utilizadas quatro diferentes parametrizações encontradas na literatura. Primeiramente, é simulado um caso estável no Ártico, sendo o modelo ajustado de forma a produzir resultados próximos aos da Simulação dos Grandes Turbilhões (LES), obtidos por outros autores para o mesmo caso. Realizados os ajustes, o modelo é validado através da comparação com dados noturnos, obtidos por sondagens de balão cativo realizadas em um sítio experimental localizado numa área desmatada da região Amazônica, nos meses de julho e outubro de 2001 e de novembro de 2003. São simulados os perfis verticais de temperatura potencial e de umidade específica e comparados com as observações de 13 diferentes noites. A avaliação do desempenho das diferentes formulações utilizadas é feita através de uma análise estatística, utilizando dados de todas as noites. Observou-se, que os perfis verticais de temperatura potencial foram melhor representados do que os de umidade específica. Embora o modelo tenha sido capaz de reproduzir as diferentes condições de estabilidade, as noites mais estáveis foram melhor simuladas. Finalmente, é acrescentado um esquema que resolve o balanço de energia em superfície ao modelo, sendo utilizadas para este fim as condições iniciais da noite de 28 de julho de 2001. Este esquema foi utilizado para avaliar como as variáveis atmosféricas simuladas dependem da intensidade do vento no topo do domínio vertical, para as diferentes formulações analisadas e com ou sem a utilização do balanço de superfície.

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