Repositório RCAAP

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Conselho Nacional de Desenvolvimento Científico e Tecnológico

The planetary boundary layer (PBL) is a physical system presenting a variety of complex states characterized by the turbulence phenomenon. An understanding of the turbulence patterns and its structural details is of fundamental importance in large and small meteorological scales and atmospheric dispersion. From the numerical point of view, the PBL turbulence has been investigated employing LES models. In LES, only the energy-containing eddies of the degrees of freedom of the turbulent field are explicitly resolved and the effect of the smaller, more isotropic eddies, needs to be parameterized. Modeling these residual turbulent motions, which are also termed subfilter-scale motions, is in large part a phenomenological procedure based on heuristic arguments. These movements are called subfiltro or subgrade and the description of its effects is a major problem in the formulation models of large eddy simulation. Therefore, the derivation of subfilter viscosities expressed in terms of a cutoff wave number is a fundamental procedure in LES methodology. The purpose of the present study is to derive a new turbulent subfilter viscosity based on the energy at cutoff from the Taylor statistical diffusion theory. As an additional purpose, this new viscosity is used in a LES model to obtain a new profile of the vertical velocity skewness in the convective boundary layer (CBL). A comparison between the results of the skewness generated by the LES simulation with the observed skewness values in surface shows that the calculated numerically values are consistent with those observed. Vertical profiles of the simulated skewness by the LES is employed in a Lagrangian stochastic diffusion model to reproduce the contaminants concentrations measured during the Prairie Grass experiment. This new profile simulated from the LES can be used in diffusion models to reproduce the observed contaminants concentrations.

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Carbon dioxide and Radon nocturnal vertical accumulations over a pasture site at the Amazon region are investigated. The site is characterized by a strong thermal stability along the night, which is responsible for suppressing a good portion of the local turbulent mixing. Therefore, it is problematic to use techniques such as the eddy covariance for estimating nocturnal turbulent fluxes. The analysis considers the seasonal variation of the averaged nocturnal profiles between the wet season, from January to June and the dry season, from July to December. Radon profiles are analyzed with the purpose of helping to understand the physical processes affecting CO2 profiles, by comparing them to an independent and inert scalar. The analysis also considers the prevailing nocturnal wind direction. Results indicate that there is a large difference between CO2 fluxes and vertical gradients from one season to the other. Nocturnal CO2 vertical gradients between the two uppermost levels at the micrometeorological tower are often positive, indicating that the concentration increases with height. Such inversions are rarely observed on the Radon profiles, which follow the canonical form of a monotonical decrease with height. At the dry season, gradient inversions are more frequent and are, in general, associated to a direction inversion of the turbulent vertical CO2 flux, indicating that there is a downward vertical transport of this scalar at the measurement level. It causes flux convergence and an intense accumulation just above the surface. A cospectral analysis shows that such downward transport happens on the largest temporal scales of the motion, suggesting that it is associated to exchange between the surrounding forest and the pasture, in the form of mesoscale fluxes. Seasonally averaged behavior is well defined and uniform along the night, but case studies show that the processes are dominated by specific intense events. Radon fluxes measured during experimental field campaigns are used to estimate CO2 fluxes from the relationship between the vertical gradients of both scalars on six nights, three on each season. The comparison to the eddy covariance fluxes is satisfactory during the wet season, but poor during the dry season and the motives for that are discussed.

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Turbulent exchange of scalars is analyzed for a year of turbulence data, above and within an Araucaria Forest in southern Brazil. Sensible and latent heat fluxes, as well as CO2 fluxes are calculated by the eddy covariance method and analyzed in terms of the seasonal variation of their daily cycles. Diurnal and nocturnal vertical velocity spectra, sensible and latent heat flux cospectra andCO2 flux cospectra are also calculated using the multiresolution decomposition in both levels. Seasonal variations in spectra and cospectra are analyzed, as well as nocturnal and diurnal differences and the differences between the levels. Emphasis is given to the mechanisms controlling the connection between the two levels and its implications on scalar flux quantification. All scalars show a clear seasonality along the year. CO2 fluxes are negative during the day and positive at night above the canopy. In the lower level, CO2 fluxes are mostly positive along the day, with the same magnitude observed in the upper level during the night, indicating that nocturnal emission is dominated by lower levels of the forest, originated predominantly close to the floor. This fact is attributed to periods with low turbulence intensity when CO2 respired accumulates near the surface and gets transported horizontally, being captured by the lower system but missed by the upper one. Sensible heat fluxes show opposite signal in both levels, being more intense above the canopy, with a small contribution from the lower level to the total flux due the low penetration of radiation within the canopy. Latent heat fluxes are mostly positive in both levels, with the larger values above the canopy, indicating that the canopy is responsible for most of the forest evapotranspiration. Analysis of multiresolution spectra and cospectra shows that, above the canopy, the nocturnal time scale of turbulent exchange is smaller than during the day. Between the two levels, the opposite occurs, and the time scale within the canopy shifts towards the larger scales. There is a connection between the two levels, and not only in the highly turbulent nights, but also intermittently in more stable conditions. During these events of connection, the turbulence time scales above and within the canopy become very similar, and the differences in scalar concentration vanish. When calm periods precede highly turbulent ones, a good portion of scalars that accumulated at the canopy during the earlier calm period gets transferred by intermittent events. We show that this process is systematic enough to affect the overall fluxes when the turbulence history is taken into account, and the effect of this process is more intense for CO2 fluxes than for energy fluxes.

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Understanding the interactions between ocean and atmosphere in regions of oceanographic fronts is of vital importance for the improvement of numerical models for weather and climate forecasting. In the South Atlantic Ocean (SAO) the meeting between the warm waters of the Brazil Current (BC) and the cold waters of the Malvinas (Falkland) Current (MC) in the region known as the Brazil-Malvinas Confluence (BMC), results in intense mesoscale oceanic activity and, for this reason, this region is considered one of the most energetic of the Global Ocean. The interactions resulting from the thermal contrast in regions oceanographic fronts of the OAS are investigated in this work through estimates of heat fluxes based on data collected in situ and by satellite. The results of this study show that the response to the thermal contrasts found in the ocean is in the form of heat fluxes and these fluxes are critical in modulating the atmospheric boundary layer (ABL). Estimation based on data collected in situ show that in the warm side (north) of the oceanographic front the fluxes are more intense (latent heat: 62 W/m² and sensible heat: 0.6 W/m²) than in the cold side (south) (latent heat: 5.8 W/m² and sensible heat: -13.8 W/m²). In the South Atlantic Current (SAC) along the 30° S parallel, heat fluxes are directly related to the meandering characteristic of the current. The data collected in situ, in addition to allow heat flux estimates at a better spatial resolution, were used to develop a new method for estimating the heat energy exchanged between the atmosphere and the ocean caused by the presence of mesoscale oceanic structures. This methodology consists in the comparison of a radiosonde profile taken over waters of the structure of interest and another taken over waters which do not belong to this structure. The methodology was used to estimate the heat energy transfer between the atmosphere and the ocean over the top of three structures sampled in the OAS. The estimation of the heat energy transferred by a warm eddy detached from the BC points to an energy in the latent (sensible) form of 1.6 1017 J (-2.8 1016 J) which corresponds to approximately 0.011 % of the total heat energy of the eddy transferred to the atmosphere during the field experiment and 0.78 % transferred during the supposed lifetime of the eddy (3 months). Along the CSA two oceanic structures were studied: (i) a cold meander that receives from the atmosphere energy in the latent (sensible) form of 1.4 106 J/m2 (5.4 105 J/m2), and (ii) warmer waters associated with a detached eddy from the Agulhas Current (AC) that transfer to the atmosphe heat energy of approximately 4 106 J/m2 an 5.7 106 J/m2 in the latent and sensible forms, respectively. The estimation of heat energy transfer on top of mesoscale oceanic structures clearly demonstrate the importance of these structures for the heat exchanges between the ocean and the atmosphere and must be taken into account in future works about this subject in the SAO.

Conselho Nacional de Desenvolvimento Científico e Tecnológico

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The goal of this work is to define the atmospheric patterns associated with the extreme precipitation events in the Rio Grande do Sul (RS) state in spring. The data used were the rainfall series provided by the Agência Nacional das Águas (ANA, in portugues) and reanalysis data of high resolution of the Climate Forecast System Reanalysis (CFSR) of 00 and 12 UTC. From the data of the ANA, on the RS, six homogeneous regions of precipitation were obtained through of Hierarchical Cluster Analysis technique, which are: the East and North Coast (R1), Southern and South Coast (R2), the West and the border with Uruguay (R3), Northern (R4), Midwest (R5) and Northwest (R6). With the averages series for each homogeneous regions, utilizing the technique of quantis, has located the days that precipitation over each region was equal or superior to the quantile 99%. With data of the CFSR was generated a matrix (NxM) being N the dates of extreme events and M are meteorological fields normalized in grid points, selected in a grid 21x19 on the RS. The fields chosen are: mean sea level pressure, thickness between 500 and 1000hPa, geopotential height at 500, zonal component of wind at 850hPa, meridional component of wind at 850hPa, zonal component of wind at 200hPa, meridional component of wind at 200hPa and specific humidity at 850hPa. Using the Non-Hierarchical Cluster Analysis, over the array, were obtained 5 atmospheric clusters, defined as the patterns. For the Cluster 1 the principal events of extreme precipitation had locating in the regions R2, R3, R5 and R6. In Cluster 2, the events of extreme of precipitation was, principally, over the region R1. In Cluster 3, the events of extreme precipitation ocurred in the coastal regions (R1 and R2), border with Uruguay (R3) and regions wit high topography (R4 and R5). In Cluster 4, the rainfall extremes is widespread over all regions. And in Cluster 5, extreme precipitation ocurred principally in the R2, R3 and R4 regions. The Cluster 3 showed strong features of a front incursion, while the characteristics of a Mesoscale Convective System (MCS) are present in Cluters 1, 4 and 5. Thus, the three major weather systems that cause extreme precipitation in RS, the MCS are the most important and only the Upper-level cut-off low not has been clearly identified, but the Cluster 2 presents some features that may indicate their presence, which are for example, the trough axis on the RS and the formation of a surface cyclone on the coast.

The Weather Research and Forecasting model (WRF) was executed with high resolution and three nested grids of 12 km, 3 km and 1 km in the South Shetland and particularly in Deception Island s bay, during the period from 5 to 23 February 2011 at Operation Antarctica 29. Data were obtained in situ by Polar Ship Almirante Maximiano from Brazilian Navy, with a frequency of 1 h. The measured variables are pressure, temperature, dew point temperature, wind speed and direction. The model lateral and boundary conditions were data from NCEP global model, with a horizontal resolution of 1o and updated every 6 hours. Two domain sizes have been tested for the innermost grid, one with 127 and the other with 187 points in each horizontal direction. Besides, two physical configurations have been tested, with the main differences being snow microphysics and soil thermal diffusion. Pressure was the variable best represented by the model, as it is the most dependent on large scale meteorological patterns having only small dependence on the microscale. The wind direction and speed at 10 meters from the surface were also accurate, except when there were sudden fluctuations in wind speed and direction influenced by the topography, in which case there has been an underestimation of the speed. Thermodynamic variables have been very poorly represented by the small inner domain, but improved appreciably when it was expanded. The simulated temperature has an average daily cycle less intense than observed and the underestimation increases in warmer temperatures, probably caused by snow packets in the border areas. The dew point temperature daily average cycle followed the observations. Some observed and simulated patterns in circulation are briefly discussed. Finally, a simulation for a 13-day period during which there were in situ measurements at the Deception Island are presented, showing a good performance when compared to the observations.

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This study discusses the influence of tropopause folding in ozone total column during the passage of frontal systems. This research aims to analyze, from the relations between the upper level jet and the ozone total column, tropopause folding in southern South America. The aim is also to quantify the cold fronts that have acted throughout 2008, as well as identify them in tropopause folding events and action of the jet stream. Finally, it verifies the occurrence of stratospheric air into the troposphere at selected events. This research is characterized by being predominantly qualitative in relation to the collection and analysis of data obtained from Brewer Spectrophotometers, installed at the Southern Space Observatory, and the Ozone Monitoring Instrument (OMI). It is noteworthy that were initially selected all events of cold fronts that occurred in 2008. However, only events of greater magnitude were analyzed and the chosen were the ones that evidenced a strong tropopause folding. Of the results analyzed, we can conclude that the ozone total column is influenced by the passage of fronts and may increase when there is intrusion of stratospheric air in the troposphere through the tropopause folding, caused by the action of the jet stream that comes with the front.

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