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

Using an instrumented aircraft we made a constant altitude flight over lake Michigan near the Chicago shoreline and about 50 km downwind of it. The flight was made on June 18, 1992 between about 1:30 and 3:30 pm. The wind was southwesterly averaged about 12 m s-1. Employing the eddy correlation method we calculated the fluxes of CO2, O3, water vapor and aerosols in the diameter range of 0.1 to 3.0 µm. The fluxes near the shoreline were found to be significantly higher than those in the middle of the lake. The fluxes near the shoreline for O3 and aerosols were directed toward the surface and corresponding to transfer (deposition) velocities of 0.15 cm s-1 and 0.86 cm s-1, respectively. For CO2, and water vapor, the fluxes were directed upward and corresponding to transfer velocities of 0.04 cm s-1 and 0.54 cm s-1, respectively. At mid-lake the fluxes of O3 and water vapor were directed upward and corresponding to transfer velocity of 0.045 cm s-1 and 0.003 cm s-1. For CO2 and aerosols the fluxes were directed downward and corresponding to transfer (deposition) velocities of 0.006 cm s-1 and 0.226 cm s-1.

Mexico City is located in a valley in the tropical zone with high intensity of total solar radiation; it has poor wind circulation and a great deal of industrial and transportation activity, as a consequence of this it has a serious problem with air pollution. Ozone is representative of total atmospheric oxidants and of air pollution In this work a multiplicative model is proposed and adjusted to 3 years of daily observations at the Pedregal de San Angel Station located at the Southwest part of Mexico's Valley. The importance of some common meteorological parameters in the explanation of daily variance is evaluated. 50% of the total variance is explained with total solar radiation and the previous day ozone concentration and 61% using all variables. This model could be useful in the prediction of ozone concentrations with help of a model to predict solar radiation and could be used in the establishment of criteria for environmental alerts.

Several cases of the synoptic situation associated with intense frosts, which has affected the plantations of coffee and citrus in eastern and nourtheastern Mexico, are analyzed, these events also damaged citrus and vegetables in Texas, Lousiana and Florida. the origin and evolution of the anticyclonic circulations associated with the polar air mass, are traced. these originate during a blocking situation of the westerlies in the Northeastern Pacific with a confluence zone close to Alaska, and an intense cyclogenesis in the northeast of United States. the major cold air outbreak over the east, northeast and Central Mexican Plateau is observed when an anticyclone associated with the polar air covers southeastern United States, Gulf of Mexico and the easthern Mexico.

Aiming at the statistical and sypnoptical characterization of summer precipitation events in the adean portion of their impact on the local river discharge, 94 summer rainfall events within -he period 1970-1992 were defined. For that purpose daily rainfall records in 4 stations above 2000 m elevation and daily average runoff data (1980-1990) in 2 pre-andean stations of the Aconcagua and Maipo watersheds, were utilized. The characteristics weather patterns during those events were determined through pressure-time composite of temperature, relative humidity and wind anomalies at Quintero and Mendoza (radiosonde stations) and thrugh objective analyses from the European Center for Medium Range Weather Forecasting. Considering the magnitude of the zonal wind omponent at 500hPa over Quintero (top of mountain range), the rainfall events were classified into two categories: those with a strong westerly flow (PE). In the PW cases the moisture advection is associated with a deep extratropical trough off central Chile. The decay of the subsidence inversion, the strengthening of the westerly flow and the upper-troposphere divergence favor the upsloope moisture advection and the subsequent onset of convection. In the PE cases central Chile stays to the SW of a short-wave trough with its axis strongly tilted in a NW-SE direction, with weak geopotential gradients and a frequent cut-off low to the N-NE of study area. The cold air associated with the upper air trough and warming of the lower troposphere due to a Foehn effect reduce the static stabulity resulting in potentially unstable conditions. The moisture advection in this case is fron the east, centered at about 700 hPa over Mendoza. The precipitation is convective, being typically distributed into 5 to 6 episodes in the DJF season. Characteristic episode dirations range from 1 to 7 days with daily totals veru seldom exceeding 10 mm. No significant differences were found in the rainfall ammounts between PW and PE episodes.

Coastal meteorological data of five autumn-winter periods (1982/83 to 1986/87), are used to describe the meteorology (and its variability) affecting the Northern Gulf of California, focusing on the strong wind events which have been proposed to cause most of the autumn-winter heat loss. The 1982/83 and 1986/87 autumn-winter periods were found to have anomalous relative humidity (an ENSO event was in progress in both periods): the first one had the highest (~63%) and the second the lowest (~43%) humidity. Also, the highest mean wind speeds were recorded in 1982/83. The differences between these two periods could be due to the pathway followed by the air masses: in 1982/83, the wind direction suggests a more oceanic influence from the Pacific than in 1986/87. The heat fluxes were calculated with a 30 m-deep well-mixed model representing the shallow region at the head of the gulf, where water-mass formation takes place in autumn-winter. The model is driven by the meteorological data and a lateral heat flux obtained by heat balance. Most of the net heat loss (67%) takes place during periods of strong winds (~7 ms-1, mostly NW) and low relative humidity (~56%), lasting from 1 to 15 days. Latent heat flux is the main surface heat loss, accounting for about 67% of the total. The two most important variables for the heat fluxes, wind speed and humidity, have a bimonthly variability, which in turn modulate the latent heat flux: wind speed has maxima in November and January, while humidity has maxima in December and February. In the average the highest net heat loss occurs in November, although the strongest heat-loss events occur in October and March. The interannual meteorological variability has a noticeable effect upon the heat flux: the lowest heat loss (-2 Wm-2) occurred in the autumn-winter of 1982/83, while the highest occurred in that of 1986/87 (-30 Wm-2).

An extensive experiment (the Aspendale ISCCP Regional Experiment) to study the radiative and microphysical properties of mixed-phase clouds yielded aircraft profiles from 3 fully iced cirrus decks, 2 liquid water decks of stratocumulus and 2 mixed-phase decks of altostratus. The short-wave solar radiation results are presented here. The reflection spectra from 1.1 to 2.5µm suffered badly from instrumental noise and the detail of infrared absorption could not be examined. However, the broadest average of cloud reflectivity as a function of wavelength was consistent with the theoretical work of Wiscombe et at. (1984). Analysis of total spectrum albedo A as measured by Eppley pyranometers yielded fairly consistent relations between A and total water or ice path W for solar zenith angles of the order of 60º: namely, A = 0.07+0.037(logW)2 for the stratocumulus, and A = 0.0079(logW)2 for the cirrus. The albedo of the mixed-phase altostratus sat nicely between the two extremes as an apparently simple function of the fraction of liquid water to the total. The unexpected consistency of the cirrus relation is discussed and attributed to the relatively constant mean particle radius (≈ 200µm) of the clouds examined. A parameterization to take account of mean effective particle radius is proposed.  

Total horizontal atmospheric light extinction coefficients as well as particle mass concentrations have been measured in downtown areas of Santiago de Chile, a heavily polluted city. Measurement campaigns were carried out in 1994 and in 1995. Extinction measurements were made by a telephotometric technique in four wavelength bands; oscillating mass balance type instruments were used to measure PM2.5 and PM 10 mass concentrations. The latter type instrument had not been available heretofore. The extensive continuous PM2.5 measurements are the first for this city. Strong and highly significant statistical correlations were found between extinction coefficients and mass concentrations, especially with the fine respirable or PM2.5 mass concentrations. Angström exponents and. in one case. mass extinction coefficients have been estimated.

The climate response of the Northern Hemisphere to the anthropogenic greenhouse gas forcing is studied with a zonally-averaged climate model. First, a simple method is proposed to improve the radiative forcing in climate models with a low vertical resolution for the atmospheric dynamics. Then, the sensitivity of the model response to tropopause height parameterization is examined. It is shown that small changes in tropopause height induced by climate warming introduce an important feedback which strongly affects the 2 x CO2 sensitivity at equilibrium. Finally, transient climate evolution is simulated for the time-dependent radiative forcings corresponding to the scenarios BaU and D of the IPCC 1990. The contributions of various greenhouse gases are taken into account with an effective-CO2 method whose validity is shortly discussed. Several transient simulations have been undertaken with different tropopause height parameterizations. The results show further that the potential tropopause-height feedback can significantly influence the model response.

Examples of measurements and calculations are used for evaluation of the clouds influence on the radiative heat exchange in the whole atmosphere, into the clouds, under and above cloud layers. It is shown that the atmosphere on the whole is heated by radiation approximately as much as the clouds themselves.

Surface air temperature changes during the last century is one of the most disputable topics among researchers concerned with the problem. Nevertheless, the basic question - whether the temperature growth has been connected with human activities or it has been the result of natural climatic variability - has not been solved yet. Over-simplification of processes driving the climate system leads to misundertanding the nature of climatic change. Results of analysis of statistical properties of two independent time series are considered. Changes in statistical structure of air surface temperature with the growth of the scale of spatial averaging from local to the global one are marked. Similarity between global air temperature changes and a random walk is discussed.