Vagner Anabor

Serial Upstream-Propagating Mesoscale Convective System Events over Southeastern South America

Abstract Serial mesoscale convective system (MCS) events with lifetimes over 18 h and up to nearly 70 h are routinely observed over southeastern South America from infrared satellite imagery during the spring and summer. These events begin over the southern La Plata River basin, with individual convective systems generally moving eastward with the cloud-layer-mean wind. However, an important and common subset of these serial MCS events shows individual MCSs moving to the east or southeast, yet the region of convective development as a whole shifts upstream to the north or northwest. Analyses of the composite mean environments from 10 of these upstream-propagating serial MCS events using NCEP–NCAR reanalysis data events indicates that the synoptic conditions resemble those found in mesoscale convective complex environments over the United States. The serial MCS events form within an environment of strong low-level warm advection and strong moisture advection between the surface and 700 hPa from the Amazon ...

Surface-to-Atmosphere Exchange in a River Valley Environment

Abstract Observations of the turbulent exchange between a river surface and the atmosphere in a mountainous area in southern Brazil are presented and discussed. A micrometeorological tower was installed directly above the surface of a 60-m-wide river. This paper describes the observed turbulent fluxes over 12 days of observations at this site. Eddy correlation sensible and latent heat fluxes are directed toward the river during daytime and from the river at night, and they are controlled by differences between water and air temperatures. The magnitude of the vertical fluxes between the river and the atmosphere increases during daytime with increasing temperature gradient up to a threshold, beyond which the increasing stability starts to dampen the fluxes. Water and air temperatures show very little variations across the width of the river, indicating that the measurements taken at one margin may be representative of the mean river exchange. Local scalar budgets show that daytime warming and moistening rat...

Atmospheric boundary layer adjustment to the synoptic cycle at the Brazil‐Malvinas Confluence, South Atlantic Ocean

[1] This study analyzes and discusses atmospheric boundary layer vertical profiles of potential temperature, specific humidity, and wind speed at each of the sides of the Brazil-Malvinas Confluence in the southwestern Atlantic Ocean. Such confluence is characterized by the meeting of water masses with very different characteristics: the southern waters of the Malvinas current can be several degrees colder and appreciably less salty than the northern Brazil current waters. At the same time, a synoptic cycle can be identified at the region, marked by the successive passages of frontal systems and extratropical cyclones. The different phases of the synoptic cycle lead to different thermal advections at the confluence, causing respective different patterns of atmospheric boundary layer adjustment to the surface heterogeneity induced by the confluence. In the present study, this adjustment along the synoptic cycle is analyzed using data from five experiments performed across the confluence from 2003 to 2008. In each of the campaigns a number of soundings were launched from a ship at both sides of the confluence. A climatological analysis with respect to the closest frontal passage is presented, and it suggests that the observations collected at each of the years analyzed are referent to a different day of the synoptic cycle. The average profiles at each side of the confluence are in agreement with previous modeling studies of warm and cold thermal advection patterns over an oceanic front. Furthermore, our study shows that peculiar transitional characteristics are also observed between the conditions of well-established warm and cold advection. At many phases of the synoptic cycle a strongly stratified boundary layer occurs at one or both sides of the confluence. Some of the observed characteristics, such as a large moisture accumulation near the surface, suggest that existing sensible and latent heat fluxes parameterizations fail under very strong stratifications, and the consequences of this deficiency are analyzed.

Simulation of a Serial Upstream-Propagating Mesoscale Convective System Event over Southeastern South America Using Composite Initial Conditions

Abstract Serial upstream-propagating mesoscale convective system (MCS) events over southeastern South America are important contributors to the local hydrologic cycle as they can provide roughly half of the total monthly summer precipitation. However, the mechanisms of upstream propagation for these events have not been explored. To remedy this situation, a numerical simulation of the composite environmental conditions from 10 observed serial MCS events is conducted. Results indicate that the 3-day simulation from the composite yields a reasonable evolution of the large-scale environment and produces a large region of organized convection in the warm sector over an extended period as seen in observations. Upstream propagation of the convective region is produced and is tied initially to the development and evolution of untrapped internal gravity waves. However, as convective downdrafts develop and begin to merge and form a surface cold pool in the simulation, the cold pool and its interaction with the env...

WRF-chem Simulation of a Saharan dust Outbreak over the Mediterranean Regions

A fully coupled meteorology-chemistry-aerosol model (WRF-Chem) is applied to simulate the Saharan dust outbreak over the Mediterranean regions. Two dust emission schemes, namely, those of Jones et al., (2010), and Shao (2001) are evaluated using the the GOCART aerosol model. To investigate the performance of each dust emission scheme, a case study was carried out for a Mediterranean dust event that took place between 21 and 23 May 2014. Considering the time average Aerosol Optical Depth, simulation results reproduced satisfactorily the outbreak and transport pattern of dust plumes. However, the estimated dust emission amounts in each scheme differ greatly due to the presence of several tuning parameters, that must be adjusted considering satellite and ground based experimental data.

Analyzing the Validity of Similarity Theories in Complex Topographies

The dispersion of trace gases in the atmosphere depends on the state of the atmospheric boundary layer (ABL) and one of the most important parameters characterizing it is the intensity of turbulence within the ABL. Hence, the reliability of the atmospheric dispersion models depends on the way turbulent parameters are calculated and related to the structure of the ABL. Similarity theories are the usual tool used to study the ABL structure, and it is able to describe not only the distributions of turbulent statistical variables, but also profiles of the mean variables and spectra. Different kinds of similarity have different similarity scales, similarity relations and application ranges. There is a long history of field experiments contributing to the continuing development of similarity theories (Businger et al., 1971; Niewstadt, 1984; Sorbjan 1986; Mahrt et al. 1998) and the results of such studies lead to a well known description of the structure of turbulence over flat, homogeneous surfaces and various atmospheric conditions. However only in recent years attention has been devoted to the structure of the atmospheric turbulence over complex terrain. Kaimal and Finnigan (1994) addressed the problem of heterogeneity surface on several scales. On the smallest scale the effects are confined to the surface layer and it is related to local advection in micrometeorology, i.e. how far downwind of a change we must go to find a flow in equilibrium with the local surface. Over very complex terrain, however, this equilibrium may never be attained. Generally studies over non-homogeneous topography try to estimate surface fluxes from profile measurements once the one-dimensional conditions, upon which Monin-Obukhov similarity is predicted, are lost. On the other hand information on turbulence response to the surface changes is much less complete than that on the mean fields (Kaimal and Finnigan, 1994). Based on theory, Jackson and Hunt (1975) have suggested the existence of a two-layer to describe

Evaluation of Nocturnal Temperature Forecasts Provided by the Weather Research and Forecast Model for Different Stability Regimes and Terrain Characteristics

The quality of nocturnal temperature forecasts made by the Weather Research and Forecast (WRF) numerical model is evaluated. The model was run for all July 2012 nights, and temperature fields compared to hourly observations made at 26 weather stations in southern Brazil. Four different planetary boundary-layer (PBL) schemes are considered: Bougeault–Lacarrere (BouLac), Quasi-Normal Scale Elimination (QNSE), Yonsei University (YSU) and Mellor–Yamada–Janjic (MYJ). Additional simulations to assess the role of higher horizontal and vertical resolutions were performed using the MYJ scheme. All schemes, except BouLac, underestimated the 2-m temperature, and in all cases the temperature bias is dependent on wind speed. At high wind speeds, all schemes exhibit a cold bias, which is greater for those that yield lower nocturnal surface-layer turbulent intensity. The elevation difference between each station and the model nearest grid point

PADRÕES SINÓTICOS ASSOCIADOS A UM EVENTO DE ADVECÇÃO DE NEBULOSIDADE RASA OCORRIDO EM 11 DE DEZEMBRO DE 2005 SOBRE O RIO GRANDE DO SUL

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