Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

This paper presents results from numerical simulations of a severe storm producing a microburst, using a convective numerical model. The numerical experiments are done in order to understand the role of the environmental vertical wind profile and/or the ice phase in organizing cloud dynamics and the strength of the microburst. Three particular downdraft types (the mid level, the penetrative and the precipitation related) and their relationship with cloud microphysics and the environmental wind profile are analyzed. To accomplish this objective, a well documented storm developed on 20 July 1986 over northern Alabama (USA), is used as case study. Similar environmental conditions to those that characterize this event have been observed prior to thunderstorms that produce severe low-level winds over northeastern Argentina. Model results show a good representation of the observed main storm features during the different cloud stages for the control experiment. Numerical simulations confirm the role of the vertical wind shear, in spite of its weak magnitude, in the genesis and support of the mid-level downdraft. The inclusion of the ice phase provides additional buoyancy that promotes a stronger updraft and vertical growth and reduces the loading contribution related to supercooled raindrops to the mid-level downdraft. Melting provides an additional cooling that reinforces both the low-level precipitation related downdraft and the divergent outflow velocity differential in the microburst.  

t is known that cities are warmer and drier that the surrounding, vegetated areas. In addition to their aesthetic aspects, plants play important roles in urban ecosystems function by providing various favourable effects. The City or Erzurum has extreme continental climatic features and is very hot in summer time, i.e., the period between June and September. There are few tree species capable of growing in the city; it is thought that in such cities, grass may have superior effects to the trees. The present study was carried out to determine temperature differences between asphalt concrete (AC), soil and grass surfaces on and 2m above them and to show the advantages of the use of grass surfaces in this kind of new developing cities. After the study, it was seen a mean temperature difference of 6.5 ºC between AC and soil, 5.3 ºC between soil and grass, and 11.79 ºC between AC and grass surfaces respectively, while temperature differences 2m above the surfaces were 5.22 ºC between AC and soil, 2.32 ºC between soil and grass, and 7.54 ºC between AC and grass surfaces. It was suggested at the end of the study that in the new developing cities having the same features as Erzurum, open spaces between the structures and areas along the roads must be covered with grass surfaces and recreational and aesthetical spaces must be left.

The high particle concentrations that are observed in Santiago de Chile during winter have prompted the government to pursue several approaches to reduce the contamination. One of these strategies was to change the traffic direction, and privileges in several streets. The main avenue (Alameda), which has 5 lanes each way, was segregated such that in three of them, only public buses can circulate and the other two lanes can be used by other vehicles. The objective is that the buses can circulate more freely, thus reducing emissions. During winter of 2001, we have measured the elemental carbon (EC) concentration along Alameda Avenue and several other streets with a light-absorption coefficient equipment built at the University of Santiago. The morning rush hour peak can be seen for all months indicating that the influence of traffic in this area is high. The EC concentration during rush hour represents an average of 25% of the total concentration. In addition, the average in EC concentration (for all months) due to rush hour traffic is higher in the street with no segregated traffic than the other two stations that have segregated traffic. However, the large variability in the data does not allow concluding with statistical significance that there is a reduction in EC pollution during rush hour. The average values of the elemental carbon concentration in 4 stations have been measured, and two of them show high values, the other two show lower values that depend on the location across the city. These results, allow drawing approximately the limits of an area with high elemental carbon concentration.  

Throughfall and soil chemistry were compared in two sites with differing atmospheric deposition: Desierto de los Leones National Park (high atmospheric deposition) and Zoquiapan National Park (low atmospheric deposition). Throughfall fluxes of NO3-, SO42-, Ca, Mg and K were compared under two canopy cover types: Abies religiosa Schl. (fir) and Pinus hartwegii Lindl. (pine), in comparison with sites without cover canopy, e.g. forests clearings. Throughfall fluxes decreased in the following order: fir > pine > forest clearing. Nitrogen balance under canopy of fir and pine resulted in negative values for net throughfall of NH4+ at Desierto de los Leones and Zoquiapan, while NO3-, only resulted in negative values under canopy cover at the low deposition site. With few exceptions, concentrations of total C, N and S, soluble SO42-, and Ca2+ were higher in soil under fir canopies than under pine or in forest clearings. In polluted sites, the densely foliated fir canopies generally resulted in higher throughfall fluxes and soil accumulation of N, S and Mg compared to pine canopies or open areas. The elevated atmospheric depositions affect the functional process of forest ecosystem, particularly the throughfall and nutrients intern cycle, and these effects depend of the cover and present tree species.

Tropical cyclones during September affect the semi-arid northwestern region of México with a relatively high frequency, bringing much-needed precipitation. This study provided a better understanding of tropical cyclone inter-annual variability, their relationships with other atmospheric-oceanic phenomena, and their inter-decadal occurrences. Daily rain data from 534 meteorological stations were analyzed and used to calculate the percentage of the annual precipitation related to tropical cyclones of the eastern North Pacific Ocean affecting the region from 1949 to 2002. Using interpolation techniques, the stations were grouped in 1º × 1º areas, and the area structure of the tropical cyclone influence was obtained using empirical orthogonal function (EOF) variable analysis to identify five regions. Representative inter-annual variation series from each region were analyzed to identify changes in the influence of tropical cyclones as part of the annual precipitation. A gradient of tropical cyclone influence was found declining from south to north, mainly in the peninsula area. A regime shift in 1976 is coincident with a shift trend in series from areas with larger tropical cyclones influence. The Southern Oscillation Index (SOI) driving is stronger for the northern part of the region, while the southern part has stronger Pacific Decadal Oscillation (PDO) influence.  

The most austral zone of the Southern Cone of South America, which is sometimes under the influence of the Antarctic Ozone Hole (AOH), occasionally receives enhanced levels of ultraviolet B radiation (UV-B: 280-320 nm). Ultraviolet erythemal irradiance measurements began in 1999 by the University of Magallanes in Punta Arenas (Lat. 53.0º S; Lon. 70.9º W), Chile, with the installation of four instruments in different locations in the Magallanes region, which is the southernmost region in Chile and the nearest to the Antarctic Continent. Data from Solar Light instruments, the Brewer spectrophotometer (Serial 180) and the Total Ozone Mapping Spectrometer (TOMS) from 1999 to 2005 for the Magellan’s Region is presented in this paper. These data show a significant number of days in 2004 with low ozone concentrations, specifying that recovery of the ozone column at these latitudes is still uncertain. Data of erythemal measurements from the stations that are part of the Magallanes network were analyzed, with extra attention given to the spring-summer period when the activity of the AOH is more intense. On several occasions important decreases down to 20-53% in the total ozone column were observed. Along with these decreases, increased levels of UV-B radiation were observed. When compared to normal daily concentration values of the total ozone, the days with increased UV-B levels reached values between ~50 and ~200% above normal at the different stations.

The goal of this article is to show simple, analytical solutions of the general dispersion equation for an homogeneous plane source, parallel to the surface of the earth. At a first step, we construct a one-dimensional model, where a perfect vertical mixture is assumed (PVMM). At a second step, vertical diffusion is added to the problem (GM). Both models predict that the concentration increases downwind and, due to deposition, it remains bounded. In order to analyze the validity of the models, the space distribution of particulate matter PM10 in the saturated zone Temuco-Padre Las Casas, Chile (38.77º S, 72.63º W) is analyzed and compared with the prediction of the models.

Variability of Mexican hydroclimate, with special attention to persistent drought, is examined using observations, model simulations forced by historical sea surface temperature (SST), tree ring reconstructions of past climate and model simulations and projections of naturally and anthropogenically forced climate change. During the winter half year, hydroclimate across México is influenced by the state of the tropical Pacific Ocean with the Atlantic playing little role. Mexican winters tend to be wetter during El Niño conditions. In the summer half year northern México is also wetter when El Niño conditions prevail, but southern México is drier. A warm tropical North Atlantic Ocean makes northern México dry and southern México wet. These relationships are reasonably well reproduced in ensembles of atmosphere model simulations forced by historical SST for the period from 1856 to 2002. Large ensembles of 100 day long integrations are used to examine the day to day evolution of the atmospheric circulation and precipitation in response to a sudden imposition of a El Niño SST anomaly in the summer half year. Kelvin waves propagate east and immediately cause increased column-integrated moisture divergence and reduced precipitation over the tropical Americas and Intra-America Seas. Within a few days a low level high pressure anomaly develops over the Gulf of México. A forced nonlinear model is used to demonstrate that this low is forced by the reduced atmospheric heating over the tropical Atlantic-Intra-America Seas area. Tree ring reconstructions that extend back before the period of instrumental precipitation data coverage are used to verify long model simulations forced by historical SST. The early to mid 1950s drought in northern México appears to have been the most severe since the mid nineteenth century and likely arose as a response to both a multiyear La Niña and a warm tropical North Atlantic. A drought in the 1890s was also severe and appears driven by a multiyear La Niña alone. The drought that began in the 1990s does not exceed these droughts in either duration or severity. Tree ring records extending back to the fourteenth century suggest that the late sixteenth century megadrought may have been the longest drought to have ever affected México. While the last decade or so in north and central México has been drier than preceding decades, the associated continental pattern of hydroclimate change does not fit that which models project to occur as a consequence of rising greenhouse gases and global warming. However, models robustly predict that México will dry as a consequence of global warming and that this drying should already be underway. At least for now, in nature, this is likely obscured by strong natural atmosphere-ocean variability.

Multi-channel sun photometer observations of precipitable water content (PWC) in the near-infrared band over a tropical Indian station, Pune, during the period May 1998-May 2006 have been used to investigate temporal and seasonal variations. Precipitable water estimated from surface meteorological parameters (temperature and relative humidity) and that obtained from MODIS satellite for the same station have been compared with those observations made by the sun photometer. There exists a well defined seasonal variation in precipitable water content with maximum during southwest monsoon months (June-September) and minimum during the month of March. Variability in PWC is higher during post-monsoon and winter months (October to February) and smaller during pre-monsoon and monsoon months (March to September). The overall mean PWC measured by sun photometer at Pune is 13.27 mm. Precipitable water estimated from surface meteorological parameters and that obtained from satellite retrieval are higher in magnitude compared to that from the ground-based sun photometer measurements. However, the temporal variations on all time scales agree very well and are in-phase. A well calibrated portable sun photometer operated at the near infrared wavelengths would give very good temporal observations of precipitable water at any location.

Sea breezes blow under anticyclonic weather types, weak surface pressure gradients, intense solar radiation and relatively cloud-free skies. Generally, total cloud cover must be less than 4/8 in order to cause a thermal and pressure difference between land and sea air which allows the development of this local wind circulation. However, many numerical and observational studies have analyzed the ability of sea breezes to generate clouds in the convective internal boundary layer and in the sea breeze convergence zone. Accordingly, the aim of this study is to statistically analyze the impact of sea breezes on cloud types in the convective internal boundary layer and in the sea breeze convergence zone. The study area is located in the southeast of the Iberian Peninsula (province of Alicante, Spain) and the survey corresponds to a 6-yr study period (2000-2005). This climatological study is mainly based on surface cloud observations at the Alicante-Ciudad Jardín station (central coastal plain) and on an extensive cloud observation field campaign at the Villena-Ciudad station (Prebetic mountain ranges) over a 3-yr study period (2003-2005). The results confirm the hypothesis that the effect of sea breezes on cloud genera is to increase the frequency of low (Stratus) and convective (Cumulus) clouds. Sea breezes trigger the formation of thunderstorm clouds (Cumulonimbus) at the sea breeze convergence zone, which also have a secondary impact on high-level (Cirrus, Cirrocumulus, Cirrostratus), medium-level (Altostratus, Altocumulus) and low-level clouds (Stratus, Stratocumulus, Nimbostratus) associated with the Cumulonimbus clouds (e.g., Cumulonimbus anvil).