Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection

Schiro, Kathleen A.; Neelin, J. David

doi:10.5194/acp-18-1997-2018

Cited by 32 publications

(48 citation statements)

References 73 publications

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“…In total, 45 MCS events were identified for this deployment, with 24 “Wet” season (December through April), 10 “Dry” season (June through September), and 11 “Transitional” season (March, October, and November) examples (Table S1 in the supporting information). Amazon events also satisfy MCS criteria outlined in the complementary work by Schiro and Neelin (). These criteria include that MCS events have a surface equivalent potential temperature ( θ e ) drop greater than 5 K within 30 min of line passage and a maximum precipitation rate exceeding 10 mm/hr within the convective line.…”

Section: Methodsmentioning

confidence: 99%

“…These criteria include that MCS events have a surface equivalent potential temperature ( θ e ) drop greater than 5 K within 30 min of line passage and a maximum precipitation rate exceeding 10 mm/hr within the convective line. The representativeness of thermodynamic profiles has been modified from the requirements used by Schiro and Neelin () to selecting events having radiosonde launches in advance of the convective line/cold pool that are launched less than 4 hr prior to line passage over the RWP.…”

Section: Methodsmentioning

confidence: 99%

“…Surface precipitation breakdowns (e.g., convective‐stratiform precipitation) and cold pool properties for the two regions are discussed. These efforts include additional analyses following Schiro and Neelin () on the effect of the MCSs on the surface thermodynamics, the origin height of MCS downdrafts, and discussions on regional and seasonal variability therein.…”

Section: Introductionmentioning

confidence: 99%

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The Characteristics of Tropical and Midlatitude Mesoscale Convective Systems as Revealed by Radar Wind Profilers

et al. 2019

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This study contrasts characteristics of mature squall‐line mesoscale convective systems (MCSs) observed by extended ground‐based radar wind profiler (RWP) deployments from the U.S. Department of Energy Atmospheric Radiation Measurement program. This analysis compares the dynamical structure, precipitation, and cold pool properties associated with MCS events over RWP sites in Oklahoma, USA, (midlatitude) to those observed during a 2‐year RWP deployment to Manaus, Brazil, during GoAmazon2014/5 campaign (tropical). The MCSs indicate similar convective line rainfall rates and total rainfall accumulations. However, midlatitude events suggest a larger fractional stratiform contribution to total precipitation. For both regions, convective line cold pools are associated with sharp decreases (approximately 10 K) in the surface equivalent potential temperature (θe) near the time of line passage. Surface θe properties for both regions suggest a modest relationship between rainfall rate and the probability of observing measurable surface rainfall. The probability of observing convective updrafts in both tropical and midlatitude MCS events is found to be similar as a function of low‐level radar reflectivity. However, midlatitude MCSs are associated with more intense convective updrafts, with upward air motions (mean, maximum) peaking at higher altitude. The most pronounced contrast is the propensity for deeper and more intense downdrafts in midlatitude MCSs. An analysis based on observed downdraft properties is performed using simple mixing assumptions. For these events, the vertical gradient of θe in the lower troposphere is relatively consistent between the Amazon and Oklahoma, suggesting similar mixing rates for downdrafts originating below 3 km (0.1 km−1). However, if downdrafts originate nearer to the level of minimum θe at SGP, mixing may be occurring at rates comparable to 0.3 km−1.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Characteristics of Tropical and Midlatitude Mesoscale Convective Systems as Revealed by Radar Wind Profilers

et al. 2019

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“…The downward transport of particles from the lower free troposphere to the BL can occur either through convective downdrafts , typically observed in Amazonia (Machado et al, 2004;Nunes et al, 2016), or through continuous entrainment. Previous studies reported heights of 1500 to 2500 km as the probable level of the origin of convective downdraft events in Amazonia (Betts et al, 2002;Schiro and Neelin, 2018). In this way, convective downdrafts could explain the transport of particles from the lower free troposphere into the BL; however the downward transport of particles from outflow regions of deep convective clouds is still uncertain, and requires more investigation.…”

Section: Introductionmentioning

confidence: 96%

Multi-year statistical and modeling analysis of submicrometer aerosol number size distributions at a rain forest site in Amazonia

et al. 2018

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Abstract. The Amazon Basin is a unique region to study atmospheric aerosols, given their relevance for the regional hydrological cycle and the large uncertainty of their sources. Multi-year datasets are crucial when contrasting periods of natural conditions and periods influenced by anthropogenic emissions. In the wet season, biogenic sources and processes prevail, and the Amazonian atmospheric composition resembles preindustrial conditions. In the dry season, the basin is influenced by widespread biomass burning emissions. This work reports multi-year observations of high time resolution submicrometer (10-600 nm) particle number size distributions at a rain forest site in Amazonia (TT34 tower, 60 km NW from Manaus city), between 2008 and 2010 and 2012 and 2014. The median particle number concentration was 403 cm −3 in the wet season and 1254 cm −3 in the dry season. The Aitken mode (∼ 30-100 nm in diameter) was prominent during the wet season, while the accumulation mode (∼ 100-600 nm in diameter) dominated the particle size spectra during the dry season. Cluster analysis identified groups of aerosol number size distributions influenced by convective downdrafts, nucleation events and fresh biomass burning emissions. New particle formation and subsequent growth was rarely observed during the 749 days of observations, similar to previous observations in the Amazon Basin. A stationary 1-D column model (ADCHEMAerosol Dynamics, gas and particle phase CHEMistry and radiative transfer model) was used to assess the importance of the processes behind the observed diurnal particle size distribution trends. Three major particle source types are required in the model to reproduce the observations: (i) a surface source of particles in the evening, possibly related to primary biological emissions; (ii) entrainment of accumulation mode aerosols in the morning; and (iii) convective downdrafts transporting Aitken mode particles into the boundary layer mostly during the afternoon. The latter process has the largest influence on the modeled particle number size distributions. However, convective downdrafts are often associated with rain and, thus, act as both a source of Aitken mode particles and a sink of accumulation mode particles, causing a net reduction in the median total particle number concentrations in the surface layer. Our study shows that the combination of the three mentioned particle sources is essential to sustain particle number concentrations in Amazonia.

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“…The program collected diverse data during 2014-2015 on the distributions of pollutants, surface fluxes, and surface and upper-air meteorological conditions (detailed information can be found in Martin et al, 2016). The data availability motivated several studies Cecchini et al, 2016;Collow et al, 2016;Kuo et al, 2017;Marengo et al, 2017;Cecchini et al, 2017;Schiro and Neelin, 2018;Wang et al, 2018;Chakraborty et al, 2018; among many others). In particular, Schiro and Neelin (2018) found that the downdrafts associated with MCSs and isolated F I G U R E 1 (a) Amazon basin (green area) and GoAmazon2014/5 region (black box) locations; and (b) GoAmazon2014/5 sites locations (white circles; see also convection had similar magnitudes and probabilities with height.…”

Section: Introductionmentioning

confidence: 99%

Mesoscale convective systems over the Amazon basin: The GoAmazon2014/5 program

Rehbein

Ambrizzi

Mechoso

et al. 2019

Intl Journal of Climatology

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The Green Ocean Amazon (GoAmazon2014/5) scientific program focused on the influence of aerosols and surface fluxes on tropical cloud formation. This major research effort gathered high‐quality environmental data over the central Amazon basin during 2014 and 2015. The present work is a contribution to the GoAmazon2014/5 investigations with an emphasis on the behaviour of the most important mechanism of precipitation over the tropics: mesoscale convective systems (MCSs). To provide a background, MCSs' tracks obtained from infrared satellite images over the entire Amazon basin in 2014–2015 are compared with climatological values. The number of MCSs and precipitation in the basin is about 50% lower than compared to the climatology for 2000–2013. We argue that the below average occurrence of MCSs during the GoAmazon2014/5 program can be explained, at least in part, by the effects of positive anomalies in sea surface temperature over the equatorial Pacific Ocean, negative moisture transport toward the Amazon basin, and by the anticyclonic phase of the mode of interannual and intraseasonal variability over South America. Special attention is given to the 99 MCSs that occurred over the GoAmazon2014/5 site. The impact of MCSs on the meteorological variables over the GoAmazon2014/5 sites is examined, and the contribution of MCSs to rainfall over that region is estimated to be about 70% of the total. Finally, the synoptic and thermodynamic conditions related to the MCSs' genesis and dissipation are discussed. It is suggested that in days with reduced MCS genesis over the GoAmazon2014/5 region, the ventilation over the continent by easterly winds from the relative cold South Atlantic Ocean favours convection at locations near the ocean as compared to those inland.

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Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection

Cited by 32 publications

References 73 publications

The Characteristics of Tropical and Midlatitude Mesoscale Convective Systems as Revealed by Radar Wind Profilers

The Characteristics of Tropical and Midlatitude Mesoscale Convective Systems as Revealed by Radar Wind Profilers

Multi-year statistical and modeling analysis of submicrometer aerosol number size distributions at a rain forest site in Amazonia

Mesoscale convective systems over the Amazon basin: The GoAmazon2014/5 program

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