Maria A. F. Silva Dias

Radar observations of the kinematic, microphysical, and precipitation characteristics of two MCSs in TRMM LBA

[1] Dual-Doppler and polarimetric radar observations are used to analyze two mesoscale convective systems (MCSs) that occurred during the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere field campaign. The MCSs formed in different meteorological regimes, based on profiles of atmospheric wind and thermodynamic data. The first MCS event (26 January 1999) was a squall line that formed in low-level easterly flow and had an intense leading line of convection. In contrast, the 25 February 1999 MCS formed in low-level westerly flow and was best characterized by stratiform precipitation with embedded convective elements. The radar analyses suggest that the MCSs were distinct in terms of overall vertical structure characteristics. In particular, polarimetric radar cross sections indicated the presence of an active mixed phase zone in the easterly MCS that was largely absent in the westerly case. The easterly MCS had considerably more precipitation ice in the middle to upper troposphere compared to the westerly MCS. Composite analyses showed that the easterly MCS had higher peak reflectivities and a smaller reflectivity gradient above the 08C level in convective regions of the storm compared to the westerly MCS event. Moreover, mean profiles of both vertical air motion and vertical mass transport in the convective portion of the easterly MCS were larger (over a factor of 2 at some heights below the 08 C level) than those in the westerly event. These observations suggest that the easterly and westerly wind regimes in the southwest Amazon region produce convection with different vertical structure characteristics, similar to regimes elsewhere in the global tropics (e.g., maritime continent). INDEX TERMS: 3314 Meteorology and Atmospheric Dynamics: Convective processes; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 3374 Meteorology and Atmospheric Dynamics: Tropical meteorology KEYWORDS: Microphysics, tropical, precipitation, polarimetric radar, mesoscale convective system

Environmental conditions associated with Amazonian squall lines : a case study

Abstract The environmental conditions associated with squall lines (SL) that were observed during the period of 13 April–13 May 1987 (GTE/ABLE-2B) originating at the northern coast of South America and propagating over the Amazon Basin are documented. The SL observed on 5–7 May are examined in more detail. The SL days had in common a stronger and deeper low-level jet when compared with the days without SL. Two possible explanations are found for the intensification of the low-level jet: propagating easterly waves in the tropical Atlantic, which eventually reach Manaus, and localized heat sources in the western Amazon. Both were observed on 5–6 May. It is suggested that numerical simulations should be performed to unravel the relative importance of each large-scale mechanism.

The Chuva Project: How Does Convection Vary across Brazil?

CHUVA, meaning “rain” in Portuguese, is the acronym for the Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (GPM). The CHUVA project has conducted five field campaigns; the sixth and last campaign will be held in Manaus in 2014. The primary scientific objective of CHUVA is to contribute to the understanding of cloud processes, which represent one of the least understood components of the weather and climate system. The five CHUVA campaigns were designed to investigate specific tropical weather regimes. The first two experiments, in Alcantara and Fortaleza in northeastern Brazil, focused on warm clouds. The third campaign, which was conducted in Belem, was dedicated to tropical squall lines that often form along the sea-breeze front. The fourth campaign was in the Vale do Paraiba of southeastern Brazil, which is a region with intense lightning activity. In addition to contributing to the understanding of clo...

Measurements of CO2 exchange over a woodland savanna (Cerrado Sensu stricto) in southeast Brasil

A tecnica de correlacao dos vortices turbulentos (eddy correlation) foi utilizada para se estimar a produtividade liquida do ecossistema (PLE) em uma area de Cerrado Sensu stricto, no sitio experimental da Gleba Pe de Gigante, no sudeste do Brasil. O conjunto de dados coletados incluiu tambem medidas de variaveis climatologicas e de respiracao do solo com câmaras estaticas, no periodo de 10 de Outubro de 1999 a 30 de Marco de 2002. A respiracao do solo media anual foi de 4.8 molCO2m-2s-1, com diferencas sazonais que variaram entre 2 a 8 molCO2 m-2s-1 durante a estacao seca (Abril a Agosto) e na estacao chuvosa, respectivamente, por um padrao de sensivel correlacao com a temperatura (Q10=4.9) e umidade do solo. Com base nos fluxos atmosfericos de CO2, a PLE mostrou uma variabilidade no ciclo diurno grandemente controlada pela radiacao solar, umidade e temperatura do ar. Na escala sazonal, a umidade do solo foi uma variavel de alta correlacao com a PLE, que aparentemente induziu a queda de folhedo, reducao da atividade fotossintetica e da respiracao do solo. O sinal da PLE foi negativo (sumidouro) na estacao chuvosa e no inicio da estacao seca, com taxas de -25 kgCha-1dia-1, e maximos de ate 40 kgCha-1dia-1. Na estacao seca o sinal foi positivo (emissao), o que foi revertido logo no inicio das chuvas. No fim da estacao seca, em dias de grande estresse hidrico, ainda observou-se a resposta da fotossintese na escala do ecossistema, mesmo tendo sido positiva a PLE. Paralelamente ao decorrer da estacao seca, a PLE progressivamente aumentou de 5 ate 50 kgCha-1dia-1. A soma annual da PLE mostrou-se aproximadamente balanceada, tendo sido no entanto, sob um vies de maior precisao, um pequeno mas significativo sumidouro de 0.1 0.3 tCha-1ano-1. Consideramos a hipotese de um pequeno sumidouro como possivelmente realista, dadas as restringentes correcoes impostas no calculo dos fluxos turbulentos, e algumas hipoteses favoraveis de sucessao de estagios do Cerrado, fertilizacao de CO2 atmosferico e variabilidade climatica.

Impact of biomass burning aerosol on the monsoon circulation transition over Amazonia

Received 8 January 2009; revised 25 March 2009; accepted 7 April 2009; published 30 May 2009. [1] Ensemble simulations of a regional climate model (RegCM3) forced by aerosol radiative forcing suggest that biomass burning aerosols can work against the seasonal monsoon circulation transition, thus re-enforce the dry season rainfall pattern for Southern Amazonia. Strongly absorbing smoke aerosols warm and stabilize the lower troposphere within the smoke center in southern Amazonia (where aerosol optical depth >0.3). These changes increase the surface pressure in the smoke center, weaken the southward surface pressure gradient between northern and southern Amazonia, and consequently induce an anomalous moisture divergence in the smoke center and an anomalous convergence in northwestern Amazonia (5S-5N, 60W70W). The increased atmospheric thermodynamic stability, surface pressure, and divergent flow in Southern Amazonia may inhibit synoptic cyclonic activities propagated from extratropical South America, and re-enforce winter-like synoptic cyclonic activities and rainfall in southeastern Brazil, Paraguay and northeastern Argentina. Citation: Zhang, Y., R. Fu, H. Yu, Y. Qian, R. Dickinson, M. A. F. Silva Dias, P. L. da Silva Dias, and K. Fernandes (2009), Impact of biomass burning aerosol on the monsoon circulation transition over Amazonia, Geophys. Res. Lett., 36, L10814, doi:10.1029/ 2009GL037180.

Correlation between smoke and tropospheric ozone concentration in Cuiabá during Smoke, Clouds, and Radiation‐Brazil (SCAR‐B)

Ozone soundings launched from Cuiaba between August 16 and September 10, 1995, during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment show an enrichment of tropospheric ozone when compared with average wet season values and also present a great variability in concentrations depending on the dominant circulation pattern. Smoke tracers, such as aerosol optical thickness, measured from a Sun photometer installed at the Instituto Nacional de Pesquisas Espaciais-Cuiaba site, and black carbon ground measurements, also show an enhancement of smoke during the same period. Although there is a connection between the enrichment of the tropospheric ozone around Cuiaba during the dry season and smoke from biomass burning, the correlation between ozone and smoke indicates different behavior in different periods. Trajectory analyses suggest that the strong ozone peak measured in the period between August 26 and 29, 1995, may be associated not only with direct biomass-burning emissions but also with urban/industrial emissions from big cities on the coast of Brazil and recirculation of old smoke. This view is confirmed by measurements made from the Cloud Lidar System instrument aboard the ER-2 aircraft.

THE ROLE OF LOCAL CIRCULATIONS IN SUMMERTIME CONVECTIVE DEVELOPMENT AND NOCTURNAL FOG IN SÃO PAULO, BRAZIL

A non-hydrostatic, fully compressible, regional-scale numerical model of the atmosphere that includes parameterization of cloud microphysics is run in a two-dimensional mode. It uses two different observed vertical profiles of temperature, relative humidity and wind to simulate the local circulation evolution and impact on convection and nocturnal fog occurrence in Sao Paulo, Brazil. Theatmospheric behaviour generated by the model is similar to the observations of the two summer days with and without sea breeze progression over the plateau. The convective development occurs in the afternoon of the two experiments but with different features and intensities. In the experiment with sea-breeze progression, there is fog formation over the plateau during the following night, in accordance with the observations. A strong northwesterly flow acting in the lower troposphere, with intensity varying between 7 and 10 m s-1,appears as an inhibiting agent of the sea-breeze progression over the plateau and of convective development in the afternoon and of the nocturnal fog occurrence. In this case, observed gusts during the night are well simulated and appear to be related to the topography.A non-hydrostatic, fully compressible, regional-scale numerical model of the atmosphere that includes parameterization of cloud microphysics is run in a two-dimensional mode. It uses two different observed vertical profiles of temperature, relative humidity and wind to simulate the local circulation evolution and impact on convection and nocturnal fog occurrence in São Paulo, Brazil. Theatmospheric behaviour generated by the model is similar to the observations of the two summer days with and without sea breeze progression over the plateau. The convective development occurs in the afternoon of the two experiments but with different features and intensities. In the experiment with sea-breeze progression, there is fog formation over the plateau during the following night, in accordance with the observations. A strong northwesterly flow acting in the lower troposphere, with intensity varying between 7 and 10 m s-1,appears as an inhibiting agent of the sea-breeze progression over the plateau and of convective development in the afternoon and of the nocturnal fog occurrence. In this case, observed gusts during the night are well simulated and appear to be related to the topography.

Amazon Rainforest Exchange of Carbon and Subcanopy Air Flow: Manaus LBA Site—A Complex Terrain Condition

On the moderately complex terrain covered by dense tropical Amazon Rainforest (Reserva Biologica do Cuieiras—ZF2—02°36′17.1′′ S, 60°12′24.4′′ W), subcanopy horizontal and vertical gradients of the air temperature, CO2 concentration and wind field were measured for the dry and wet periods in 2006. We tested the hypothesis that horizontal drainage flow over this study area is significant and can affect the interpretation of the high carbon uptake rates reported by previous works at this site. A similar experimental design as the one by Tóta et al. (2008) was used with a network of wind, air temperature, and CO2 sensors above and below the forest canopy. A persistent and systematic subcanopy nighttime upslope (positive buoyancy) and daytime downslope (negative buoyancy) flow pattern on a moderately inclined slope (12%) was observed. The microcirculations observed above the canopy (38 m) over the sloping area during nighttime presents a downward motion indicating vertical convergence and correspondent horizontal divergence toward the valley area. During the daytime an inverse pattern was observed. The micro-circulations above the canopy were driven mainly by buoyancy balancing the pressure gradient forces. In the subcanopy space the microcirculations were also driven by the same physical mechanisms but probably with the stress forcing contribution. The results also indicated that the horizontal and vertical scalar gradients (e.g., CO2) were modulated by these micro-circulations above and below the canopy, suggesting that estimates of advection using previous experimental approaches are not appropriate due to the tridimensional nature of the vertical and horizontal transport locally. This work also indicates that carbon budget from tower-based measurement is not enough to close the system, and one needs to include horizontal and vertical advection transport of CO2 into those estimates.

Can the Deforestation Breeze Change the Rainfall in Amazonia? A Case Study for the BR-163 Highway Region

Abstract The authors simulated the effects of Amazonian mesoscale deforestation in the boundary layer and in rainfall with the Brazilian Regional Atmospheric Modeling System (BRAMS) model. They found that both the area and shape (with respect to wind incidence) of deforestation and the soil moisture status contributed to the state of the atmosphere during the time scale of several weeks, with distinguishable patterns of temperature, humidity, and rainfall. Deforestation resulted in the development of a three-dimensional thermal cell, the so-called deforestation breeze, slightly shifted downwind to large-scale circulation. The boundary layer was warmer and drier above 1000-m height and was slightly wetter up to 2000-m height. Soil wetness affected the circulation energetics proportionally to the soil dryness (for soil wetness below ∼0.6). The shape of the deforestation controlled the impact on rainfall. The horizontal strips lined up with the prevailing wind showed a dominant increase in rainfall, signific...

Application of a linear spectral model to the study of Amazonian squall lines during GTE/ABLE 2B

A linear nonhydrostatic spectral model is run with the basic state, or large scale, vertical profiles of temperature and wind observed prior to convective development along the northern coast of South America during the GTE/ABLE 2B. The model produces unstable modes with mesoscale wavelength and propagation speed comparable to observed Amazonian squall lines. Several tests with different vertical profiles of low-level winds lead to the conclusion that a shallow and/or weak low-level jet either does not produce a scale selection or, if it does, the selected mode is stationary, indicating the absence of a propagating disturbance. A 700-mbar jet of 13 m/s, with a 600-mbar wind speed greater or equal to 10 m/s, is enough to produce unstable modes with propagating features resembling those of observed Amazonian squall lines. However, a deep layer of moderate winds (about 10 m/s) may produce similar results even in the absence of a low-level wind maximum. The implications in terms of short-term weather forecasting are discussed.