Characteristics of Precipitating Convective Systems Accounting for the Summer Rainfall of Tropical and Subtropical South America

2013 ◽  
Vol 14 (1) ◽  
pp. 25-46 ◽  
Author(s):  
Ulrike Romatschke ◽  
Robert A. Houze
Keyword(s):  
South America ◽  
Amazon Basin ◽  
Tropical Rainfall Measuring Mission ◽  
Summer Rainfall ◽  
Radar Data ◽  
South Atlantic Convergence Zone ◽  
Horizontal Scale ◽  
Low Level ◽  
Convective Systems ◽  
Radar Echoes

Abstract Ten years of Tropical Rainfall Measuring Mission precipitation radar data are used to study the physical properties of the precipitating cloud systems that account for the summer rainfall of tropical and subtropical South America. Radar echoes in the continental subtropics tend to be of an intensely convective nature, especially at the eastern foothills of the Andes where diurnally forced deep convective cells of small horizontal scale form when moist low-level flow is driven toward the foothills in connection with a midlatitude disturbance. As the disturbance moves east over the La Plata basin, nocturnal convective systems of larger horizontal scale with wide stratiform regions occur in a zone of general convergence. Precipitation in the continental tropics is generally produced by convective systems with greater stratiform composition. At the northeastern foothills of the central Andes, radar echoes of nocturnal convective systems of medium to large horizontal scale occur where moist low-level flow is lifted over the foothills. Growth of systems to large size is inhibited by daytime divergence at the foothills. Over the Amazon basin, daytime systems are also smaller than nocturnal systems. Radar echoes of precipitation over the Brazilian Highlands are generally smaller in horizontal scale, more convective, and mostly occur during the afternoon over elevated terrain. In the oceanic South Atlantic convergence zone, radar echoes grow to extremely large sizes. They are highly stratiform in nature and occur during all times of the day except late evening when convergence is weakened as a response to continental heating.

Extreme Summer Convection in South America

2010 ◽  
Vol 23 (14) ◽  
pp. 3761-3791 ◽  
Author(s):  
Ulrike Romatschke ◽  
Robert A. Houze
Keyword(s):  
South America ◽  
Amazon Basin ◽  
Three Dimensional ◽  
Tropical Rainfall Measuring Mission ◽  
Central Andes ◽  
South Atlantic Convergence Zone ◽  
Reanalysis Data ◽  
The North ◽  
The Andes ◽  
Environmental Prediction

Abstract Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data are used to indicate mechanisms responsible for extreme summer convection over South America. The three-dimensional reflectivity field is analyzed to define three types of extreme echo, deep convective cores, wide convective cores, and broad stratiform regions. The location and timing of these echoes are sensitive to midlatitude synoptic disturbances crossing the Andes. At the leading edges of these disturbances the nocturnal South American low-level jet (SALLJ) transports moisture along the eastern edge of the Andes from the tropical to the subtropical part of the continent. Where the SALLJ rises over lower but steep mountains on the east side of the southern central Andes, deep and wide convective cores are triggered in the evening. When the SALLJ withdraws to the north as the disturbance passes, nocturnal triggering occurs in the northeastern foothills of the central Andes. Extreme convection over the Amazon basin takes the form of broad stratiform regions that evolve from systems with wide convective cores moving into the center of the region from both the southwest and northeast. The systems from the northeast form at the northeast coast and are likely squall lines. Along the coast of the Brazilian Highlands, diurnal/topographic forcing leads to daytime maxima of deep convective cores followed a few hours later by wide convective cores. Wide convective cores and broad stratiform regions form in the South Atlantic convergence zone (SACZ) with a diurnal cycle related to continental heating.

scholarly journals A numerical study of cell merger over Cuba – Part I: implementation of the ARPS/MM5 models

2006 ◽  
Vol 24 (11) ◽  
pp. 2781-2792 ◽  
Author(s):  
D. Pozo ◽  
I. Borrajero ◽  
J. C. Marín ◽  
G. B. Raga
Keyword(s):  
Mesoscale Model ◽  
Numerical Study ◽  
Initial Boundary ◽  
Radar Data ◽  
Horizontal Resolution ◽  
Convective Systems ◽  
Maximum Reflectivity ◽  
Radar Echoes ◽  
Good Agreement ◽  
Made In

Abstract. On 21 July 2001 a number of severe storms developed over the region of Camaguey, Cuba, which were observed by radar. A numerical simulation was performed in order to realistically reproduce the development of the storms observed that day. The mesoscale model MM5 was used to determine the initial, boundary and update conditions for the storm-scale simulation with the model ARPS. Changes to the source code of ARPS were made in order to assimilate the output from the MM5 as input data and a new land-use file with a 1-km horizontal resolution for the Cuban territory was created. A case representing the merger between cells at different stages of development was correctly reproduced by the simulation and is in good agreement with radar observations. The state of development of each cell, the time when the merger occurred, starting from the formation of clouds, the propagation motion of the cells and the increase in precipitation, due to the growth of the area after the merger, were correctly reproduced. Simulated clouds matched the main characteristics of the observed radar echoes, though in some cases, reflectivity tops and horizontal areas were overestimated. Maximum reflectivity values and the heights where these maximum values were located were in good agreement with radar data, particularly when the model reflectivity was calculated without including the snow. The MM5/ARPS configuration introduced in this study, improved sensibly the ability to simulate convective systems, thereby enhancing the local forecasting of convection in the region.

Morphology, Intensity, and Rainfall Production of MJO Convection: Observations from DYNAMO Shipborne Radar and TRMM

2015 ◽  
Vol 72 (2) ◽  
pp. 623-640 ◽  
Author(s):  
Weixin Xu ◽  
Steven A. Rutledge
Keyword(s):  
Mesoscale Convective System ◽  
Tropical Rainfall Measuring Mission ◽  
Radar Data ◽  
Convective System ◽  
Precipitation Radar ◽  
Convective Systems ◽  
Trmm Pr ◽  
Mesoscale Convective ◽  
Trmm Precipitation

Abstract This study uses Dynamics of the Madden–Julian Oscillation (DYNAMO) shipborne [Research Vessel (R/V) Roger Revelle] radar and Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) datasets to investigate MJO-associated convective systems in specific organizational modes [mesoscale convective system (MCS) versus sub-MCS and linear versus nonlinear]. The Revelle radar sampled many “climatological” aspects of MJO convection as indicated by comparison with the long-term TRMM PR statistics, including areal-mean rainfall (6–7 mm day−1), convective intensity, rainfall contributions from different morphologies, and their variations with MJO phase. Nonlinear sub-MCSs were present 70% of the time but contributed just around 20% of the total rainfall. In contrast, linear and nonlinear MCSs were present 10% of the time but contributed 20% and 50%, respectively. These distributions vary with MJO phase, with the largest sub-MCS rainfall fraction in suppressed phases (phases 5–7) and maximum MCS precipitation in active phases (phases 2 and 3). Similarly, convective–stratiform rainfall fractions also varied significantly with MJO phase, with the highest convective fractions (70%–80%) in suppressed phases and the largest stratiform fraction (40%–50%) in active phases. However, there are also discrepancies between the Revelle radar and TRMM PR. Revelle radar data indicated a mean convective rain fraction of 70% compared to 55% for TRMM PR. This difference is mainly due to the reduced resolution of the TRMM PR compared to the ship radar. There are also notable differences in the rainfall contributions as a function of convective intensity between the Revelle radar and TRMM PR. In addition, TRMM PR composites indicate linear MCS rainfall increases after MJO onset and produce similar rainfall contributions to nonlinear MCSs; however, the Revelle radar statistics show the clear dominance of nonlinear MCS rainfall.

scholarly journals Statistical properties of aerosol-cloud-precipitation interactions in South America

2010 ◽  
Vol 10 (5) ◽  
pp. 2287-2305 ◽  
Author(s):  
T. A. Jones ◽  
S. A. Christopher
Keyword(s):  
South America ◽  
Optical Thickness ◽  
Amazon Basin ◽  
Cloud Droplet ◽  
Heavy Rain ◽  
Total Variance ◽  
Atmospheric Conditions ◽  
Low Level ◽  
Aerosol Cloud ◽  
Significant Difference

Abstract. Given the complex interaction between aerosol, cloud, and atmospheric properties, it is difficult to extract their individual effects to observed rainfall amount. This research uses principle component analysis (PCA) that combines Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and cloud products, NCEP Reanalysis atmospheric products, and rainrate estimates from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) to assess if aerosols affect warm rain processes. Data collected during September 2006 over the Amazon basin in South America during the biomass-burning season are used. The goal of this research is to combine these observations into a smaller number of variables through PCA with each new variable having a unique physical interpretation. In particular, we are concerned with PC variables whose weightings include aerosol optical thickness (AOT), as these may be an indicator of aerosol indirect effects. If they are indeed occurring, then PC values that include AOT should change as a function of rainrate. To emphasize the advantage of PCA, changes in aerosol, cloud, and atmospheric observations are compared to rainrate. Comparing no-rain, rain, and heavy rain only (>5 mm h−1) samples, we find that cloud thicknesses, humidity, and upward motion are all greater during rain and heavy rain conditions. However, no statistically significant difference in AOT exists between each sample, indicating that atmospheric conditions are more important to rainfall than aerosol concentrations as expected. If aerosols are affecting warm process clouds, it would be expected that stratiform precipitation would decrease as a function increasing aerosol concentration through either Twomey and/or semi-direct effects. PCA extracts the latter signal in a variable labeled PC2, which explains 15% of the total variance and is second in importance the variable (PC1) containing the broad atmospheric conditions. PC2 contains weightings showing that AOT is inversely proportional to low-level humidity and cloud optical thickness. Increasing AOT is also positively correlated with increasing low-level instability due to aerosol absorption. The nature of these weightings is strongly suggestive that PC2 is an indicator of the semi-direct effect with larger values associated with lower rainfall rates. PC weightings consistent with the Twomey effect (an anti-correlation between AOT and cloud droplet effective radius) are only present in higher order PC variables that explain less than 1% of the total variance, and do not vary significantly as a function of rainrate. If the Twomey effect is occurring, it is highly non-linear and/or being overshadowed by other processes. Using the raw variables alone, these determinations could not be made; thus, we are able to show the advantage of using advanced statistical techniques such as PCA for analysis of aerosols impacts on precipitation in South America.

Orogenic Convection in Subtropical South America as Seen by the TRMM Satellite

2011 ◽  
Vol 139 (8) ◽  
pp. 2399-2420 ◽  
Author(s):  
Kristen L. Rasmussen ◽  
Robert A. Houze
Keyword(s):  
South America ◽  
Tropical Rainfall Measuring Mission ◽  
The United States ◽  
South American ◽  
Convective Storms ◽  
Mountain Ranges ◽  
Low Level ◽  
Global Understanding ◽  
Low Level Jet ◽  
Downslope Flow

AbstractExtreme orogenic convective storms in southeastern South America are divided into three categories: storms with deep convective cores, storms with wide convective cores, and storms containing broad stratiform regions. Data from the Tropical Rainfall Measuring Mission satellite’s Precipitation Radar show that storms with wide convective cores are the most frequent, tending to originate near the Sierra de Cordoba range. Downslope flow at upper levels caps a nocturnally enhanced low-level jet, thus preventing convection from breaking out until the jet hits a steep slope of terrain, such as the Sierra de Cordoba Mountains or Andean foothills, so that the moist low-level air is lifted enough to release the instability and overcome the cap. This capping and triggering is similar to the way intense convection is released near the northwestern Himalayas. However, the intense storms with wide convective cores over southeastern South America are unlike their Himalayan counterparts in that they exhibit leading-line/trailing-stratiform organization and are influenced by baroclinic troughs more similar to storms east of the Rocky Mountains in the United States. Comparison of South American storms containing wide convective cores with storms in other parts of the world contributes to a global understanding of how major mountain ranges influence precipitating cloud systems.

scholarly journals Contribution of Extreme Convective Storms to Rainfall in South America

2015 ◽  
Vol 17 (1) ◽  
pp. 353-367 ◽  
Author(s):  
K. L. Rasmussen ◽  
M. M. Chaplin ◽  
M. D. Zuluaga ◽  
R. A. Houze
Keyword(s):  
South America ◽  
Amazon Basin ◽  
Austral Summer ◽  
Tropical Rainfall Measuring Mission ◽  
Life Cycles ◽  
High Resolution Data ◽  
Tropical Regions ◽  
Convective Storms ◽  
La Plata ◽  
La Plata Basin

Abstract The contribution of extreme convective storms to rainfall in South America is investigated using 15 years of high-resolution data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). Precipitation from three specific types of storms with extreme horizontal and vertical dimensions have been calculated and compared to the climatological rain. The tropical and subtropical regions of South America differ markedly in the influence of storms with extreme dimensions. The tropical regions, especially the Amazon basin, have aspects similar to oceanic convection. Convection in the subtropical regions, centered on La Plata basin, exhibits patterns consistent with storm life cycles initiating in the foothills of the Andes and growing into larger mesoscale convective systems that propagate to the east. In La Plata basin, convective storms with a large horizontal dimension contribute ~44% of the rain and the accumulated influence of all three types of storms with extreme characteristics produce ~95% of the total precipitation in the austral summer.

scholarly journals Improving Simulations of Convective Systems from TRMM LBA: Easterly and Westerly Regimes

2007 ◽  
Vol 64 (4) ◽  
pp. 1141-1164 ◽  
Author(s):  
S. Lang ◽  
W-K. Tao ◽  
J. Simpson ◽  
R. Cifelli ◽  
S. Rutledge ◽  
...  
Keyword(s):  
Large Scale ◽  
Collection Efficiency ◽  
Deep Convection ◽  
Tropical Rainfall Measuring Mission ◽  
Squall Line ◽  
Gradual Transition ◽  
Microphysics Scheme ◽  
Easterly Wind ◽  
Low Level ◽  
Convective Systems

Abstract The 3D Goddard Cumulus Ensemble model is used to simulate two convective events observed during the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM LBA) experiment in Brazil. These two events epitomized the type of convective systems that formed in two distinctly different environments observed during TRMM LBA. The 26 January 1999 squall line formed within a sheared low-level easterly wind flow. On 23 February 1999, convection developed in weak low-level westerly flow, resulting in weakly organized, less intense convection. Initial simulations captured the basic organization and intensity of each event. However, improvements to the model resolution and microphysics produced better simulations as compared to observations. More realistic diurnal convective growth was achieved by lowering the horizontal grid spacing from 1000 to 250 m. This produced a gradual transition from shallow to deep convection that occurred over a span of hours as opposed to an abrupt appearance of deep convection. Eliminating the dry growth of graupel in the bulk microphysics scheme effectively removed the unrealistic presence of high-density ice in the simulated anvil. However, comparisons with radar reflectivity data using contoured-frequency-with-altitude diagrams (CFADs) revealed that the resulting snow contents were too large. The excessive snow was reduced primarily by lowering the collection efficiency of cloud water by snow and resulted in further agreement with the radar observations. The transfer of cloud-sized particles to precipitation-sized ice appears to be too efficient in the original scheme. Overall, these changes to the microphysics lead to more realistic precipitation ice contents in the model. However, artifacts due to the inability of the one-moment scheme to allow for size sorting, such as excessive low-level rain evaporation, were also found but could not be resolved without moving to a two-moment or bin scheme. As a result, model rainfall histograms underestimated the occurrence of high rain rates compared to radar-based histograms. Nevertheless, the improved precipitation-sized ice signature in the model simulations should lead to better latent heating retrievals as a result of both better convective–stratiform separation within the model as well as more physically realistic hydrometeor structures for radiance calculations.

Regional and Diurnal Variability of the Vertical Structure of Precipitation Systems in Africa Based on Spaceborne Radar Data

2005 ◽  
Vol 18 (7) ◽  
pp. 893-916 ◽  
Author(s):  
Bart Geerts ◽  
Teferi Dejene
Keyword(s):  
Vertical Structure ◽  
Austral Summer ◽  
Tropical Rainfall Measuring Mission ◽  
Radar Data ◽  
Equatorial Region ◽  
Diurnal Variability ◽  
Low Level ◽  
Freezing Level ◽  
Rainfall Frequency ◽  
The Difference

Abstract The Tropical Rainfall Measuring Mission (TRMM) 2A25 radar reflectivity profiles and derived surface rain rates are used to describe the vertical structure of precipitation systems in Africa. Five years of data are used in both the boreal and austral summer rainy seasons. A number of climate regions are isolated and compared. To place the composite reflectivity profiles in context, they are contrasted against TRMM 2A25 observations over the Amazon. In all of tropical Africa, precipitation systems tend to be deeper and more intense than in the Amazon, and shallow warm-rain events are less common. In all African regions, but especially in the Sahel and northern Savanna, storms are characterized by high echo tops, high hydrometeor loading aloft, little indication of a radar brightband maximum at the freezing level, and evidence for low-level evaporation. Storms in Africa are generally most common, and deepest, in the late afternoon, and weaker shallow systems are relatively more common around noon. The diurnal modulation is regionally variable. The amplitude of the diurnal cycle of the mean echo top height decreases from the arid margins of the zenithal rain region toward the equatorial region, and is smallest in the Amazon. A secondary predawn (0000–0600 LT) maximum occurs in the Congo, in terms of rainfall frequency, rainfall intensity, and echo tops. The storm intensity indicators generally peak a few hours later in the Sahel and northern Savanna than in other regions in Africa. The difference between all African regions and the Amazon, and the relatively smaller differences between regions in Africa, can be understood in terms of the climatological humidity, CAPE, and low-level shear values.

scholarly journals Statistical properties of aerosol-cloud-precipitation interactions in South America

2009 ◽  
Vol 9 (5) ◽  
pp. 21463-21507
Author(s):  
T. A. Jones ◽  
S. A. Christopher
Keyword(s):  
South America ◽  
Optical Thickness ◽  
Amazon Basin ◽  
Cloud Droplet ◽  
Heavy Rain ◽  
Total Variance ◽  
Atmospheric Conditions ◽  
Low Level ◽  
Aerosol Cloud ◽  
Significant Difference

Abstract. Given the complex interaction between aerosol, cloud, atmospheric properties, it is difficult to extract their individual effects to observed rainfall amount. This research uses principle component analysis (PCA) that combines Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and cloud products, NCEP Reanalysis atmospheric products, and rainrate estimates from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) to assess the specific combinations of these inputs that most affect warm rain processes. Data collected during September 2006 over the South America, which includes the Amazon basin, are used as aerosols, clouds, and precipitation are all present in this region at this time. The goal of this research is to combine these observations into a smaller number of variables through PCA with each having a unique physical interpretation. In particular, we are concerned with PC variables whose weightings include aerosol optical thickness (AOT), as these may be an indicator of aerosol indirect effects. If they are indeed occurring, then PC values that include AOT should change as a function of rainrate. To emphasize the advantage of PCA, changes in aerosol, cloud, and atmospheric observations are compared to rainrate. Comparing no-rain, rain, and heavy rain (>5 mm h−1) samples, cloud thicknesses, humidity, and upward motion are all larger for the rain and heavy rain samples. However, no statistically significant difference in AOT exists, indicating that atmospheric conditions are more important to rainfall than aerosol concentrations as expected. If aerosols are affecting warm process clouds, it would be expected that stratiform precipitation would decrease as a function increasing aerosol concentration through either Twomey and/or semi-direct effects. PCA extracts the latter signal in a variable labeled PC2, which explains 15% of the total variance and is second in importance the variable (PC1) containing the broad atmospheric conditions. PC2 contains weightings showing that AOT is inversely proportional to low-level humidity and cloud optical thickness. Increasing AOT is also positively correlated with increasing low-level instability due to aerosol absorption. The nature of these weightings is strongly suggestive that PC2 is an indicator of the semi-direct effect with larger values associated with lower rainfall rates. PC weightings consistent with the Twomey effect (an anti-correlation between AOT and cloud droplet effective radius) are only present in PC13, which explains less than 1% of the total variance. Also, it does not vary significantly with rainrate. Thus, if the Twomey effect is occurring, it is highly non-linear and/or being overshadowed by other processes. Using the raw variables alone, these determinations could not be made; thus, we are able to show the advantage of using advanced statistical techniques such as PCA for analysis of aerosols impacts on precipitation in South America.

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