Aerodynamic Roughness Length Estimation with Lidar and Imaging Spectroscopy in a Shrub-Dominated Dryland

The aerodynamic roughness length (Z0m) serves an important role in the flux exchange between the land surface and atmosphere. In this study, airborne lidar (ALS), terrestrial lidar (TLS), and imaging spectroscopy data were integrated to develop and test two approaches to estimate Z0m over a shrub dominated dryland study area in south-central Idaho, USA. Sensitivity of the two parameterization methods to estimate Z0m was analyzed. The comparison of eddy covariance-derived Z0m and remote sensing-derived Z0m showed that the accuracy of the estimated Z0m heavily depends on the estimation model and the representation of shrub (e.g., Artemisia tridentata subsp. wyomingensis) height in the models. The geometrical method (RA1994) led to 9 percent (~0.5 cm) and 25% (~1.1 cm) errors at site 1 and site 2, respectively, which performed better than the height variability-based method (MR1994) with bias error of 20 percent and 48 percent at site 1 and site 2, respectively. The RA1994 model resulted in a larger range of Z0m than the MR1994 method. We also found that the mean, median and 75th percentiles of heights (H75) from ALS provides the best Z0 m estimates in the MR1994 model, while the mean, median, and MLD (Median Absolute Deviation from Median Height), as well as AAD (Mean Absolute Deviation from Mean Height) heights from ALS provides the best Z0m estimates in the RA1994 model. In addition, the fractional cover of shrub and grass, distinguished with ALS and imaging spectroscopy data, provided the opportunity to estimate the frontal area index at the pixel-level to assess the influence of grass and shrub on Z0m estimates in the RA1994 method. Results indicate that grass had little effect on Z0m in the RA1994 method. The Z0m estimations were tightly coupled with vegetation height and its local variance for the shrubs. Overall, the results demonstrate that the use of height and fractional cover from remote sensing data are promising for estimating Z0m, and thus refining land surface models at regional scales in semiarid shrublands

Estimativa de fluxo de calor sensível com suporte de cintilometria – estudo de caso : Cidade Universitária - Ilha do Fundão/RJ

The large aperture scintillometer (CinLA) is an optical device used to estimate averaged sensible heat fluxes (H) from fluctuations in the refractive index of the turbulent atmosphere in a light beam spread between a transmitter and a receiver. The present dissertation proposes to study the behavior of the sensible heat production during the day in the University City of the Federal University of Rio de Janeiro (UFRJ), campus of the Fundão Island / RJ, through the use of a CinLA. In particular, the sensitivity of the refractive index structure parameter (Cn2) to changes in the collection time scale and to changes in humidity and air temperature are analyzed. The data were collected, uninterruptedly, between September 23 and October 3, 2016, with the scintillometer installed on the terrace of two buildings on the main campus of UFRJ. In addition, meteorological data provided by the IGEO / UFRJ hydrometeorological station, images of the GOES satellite and synoptic charts were used to characterize the local atmosphere. Preliminary evaluations indicated the option of taking measurements at 5-minute intervals, obtained from subsamples collected every 30 seconds. Sensitivity analysis showed that variations of up to 10 ° C in temperature can alter H values up to 8,8%, while changes of ±10 hPa in atmospheric pressure can produce changes of 1,4% in H. In the experiment conducted, the highest value of H found was 443,98 W.m-2, and the mean value was approximately 64,38 W.m-2.Os cintilômetros de larga abertura (CinLA) são aparelhos ópticos que estimam o fluxo de calor sensível (H) a partir de flutuações registradas na intensidade de luz de um feixe que se propaga entre um transmissor e um receptor. A presente dissertação propõe-se a estudar o comportamento da produção de calor sensível ao longo do dia na Cidade Universitária da Universidade Federal do Rio de Janeiro (UFRJ), campus da Ilha do Fundão/RJ, mediante o emprego de um CinLA. Em especial, analisa-se a sensibilidade do parâmetro de estrutura do índice refrativo ( Cn 2) a variações na escala temporal de coleta e no que concerne a alterações na umidade e na temperatura do ar. Os dados foram coletados, ininterruptamente, no período entre 23 de setembro e 03 de outubro de 2016, com o cintilômetro instalado sobre o terraço de dois prédios no campus principal da UFRJ. Complementarmente, dados meteorológicos, fornecidos pela estação hidrometeorológica do IGEO/UFRJ, imagens do satélite GOES e cartas sinóticas foram utilizados para caracterizar a atmosfera local. Avaliações preliminares indicaram a opção por medições em intervalos de 5 minutos, obtidos a partir de subamostras coletadas a cada 30 segundos. Análise de sensibilidade mostrou que variações de até 10º C na temperatura podem alterar em até 8,8% os valores de H, enquanto que variações de ±10 hPa na pressão atmosférica provocaram alterações de 1,4% em H. No experimento conduzido, o maior valor de H encontrado foi 443,98 W.m-2 , e o valor médio foi de aproximadamente 64,38 W.m-2