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

The Adem thermodynamic climate model (ATCM) has been adapted to carry out three-month extended and seasonal numerical weather predictions. The model uses the Northern Hemisphere NMC polar stereographic grid with 1977 points and a grid distance of 408.5 km. The performance of the model has been evaluated for the period February 1981 to November 1983, which includes the “El Nino” 1982-83, and more recently in a. real time basis for the “El Niño” period from June 1997 to August 1998 for Mexico. The results show good skill in predicting seasonally temperature and precipitation anomalies during this "El Nino" period, which also confirms the results for the “El Nino” 1982-83. During the whole period June 1997 to August 1998, the model predicted above normal air-temperature and below normal precipitation, in agreement with the observations, which corresponded to a severe persistent drought in Mexico that existed during that period. The results show that the ocean temperatures play an important role in the predictions, and it may be suggested that the above normal ocean temperatures in the Pacific Ocean regions contiguous to Mexico associated with “El Nino” produced the above normal surface air-temperature and the below normal precipitation possibly favoring the drought situation in Mexico.

Variation of aerosol concentration has been studied in the view of some meteorological parameters like wind speed, temperature, relative humidity and rainfall during South-East (SE) monsoon (June-September, 1996) and winter (November 1996-February, 1997) at Roorkee. The measurements were done with the help of laser beam scatterometer for a wide range of size of aerosols (0.05-3.0 /µm). The present study brings out the fact that the number density of aerosols is very much affected by the meteorological parameters. The aerosol concentration was minimum in the August, September and November, 1996 and remained around maximum in June-July, 1996 and January-February, 1997. The variation of aerosols is in phase with relative humidity during winter season while it is out of phase with temperature. Its variation with relative humidity has been explained in terms of condensation taking place in the atmosphere. During monsoon period the rain plays very important role in characterizing the aerosol density. The aerosol concentration and size were found to be decreased during September, 1996 although the relative humidity was very high. The credit goes to scavenging of aerosol particles.

Measurements of ambient carbonyls were carried out at one sampling site at the University of Mexico campus, Mexico City in dry season (November and December 1993, March 1994, and February and March 1996), and rainy season (June and September 1995, May, June, July, and September 1996). In general, formaldehyde had higher average hourly concentrations than those of the other sampled carbonyls, however, in May 1996, acetaldehyde concentrations were above 65 ppbv; much higher than the obtained for all the period of study. In June 1996, these levels decreased to 20 ppbv, even higher than the observed values in all the period of study, except February 1996 with 22.5 ppbv. It was not found a satisfactory explanation for the acetaldehyde levels observed in February, May and at the beginning of June 1996. Average hourly concentrations of acetone from 4 to 28 ppbv for all the period study were slightly higher or equal to formaldehyde and acetaldehyde concentrations at some days. Propionaldehyde (1 to 9 ppbv) and butyraldehyde (1 to 4 ppbv) levels were small but significant. In general, a decrease in carbonyl concentrations during the evening was observed. Comparison of carbonyl concentrations between March 1994 and 1995, June 1995 and June 1996 and September 1995 and 1996, indicated a significant difference (p ≤ 0.05) for acetaldehyde, acetone and propionaldehyde. On the other hand, formaldehyde concentration (14.9 ppbv) was higher in rainy season and acetaldehyde, acetone and butyraldehyde were higher in dry season (14.1, 12.4, 2.5 y 2.3 ppbv, respectively).

Spectral and broad-band solar radiation measurements were made non-continuously between 1990 and 1994 at Abisko (68º 21' N, 18º 49' E), which is about 200 km north of the Arctic Circle in northern Sweden. The increase in the atmospheric turbidity due to the huge amounts of volcanic aerosols which were injected into the stratosphere by the volcanic eruptions of Mount Pinatubo in June 1991 consequently led to a decrease in the direct irradiance which is considerable especially in the visible region, varying between 18% and 30% at solar elevations of 40º and 15º respectively. Although less than the direct irradiance due to the concomitant increase in the diffuse radiation, the change in the total radiation was also significant (about 1.4 and 8 at solar elevation angles of 40º and 15º respectively).

A model of the zonally averaged structure of the atmosphere is designed. It employs exchange coefficients to describe the meridional transports of quasi-conservative quantities. In contrast to earlier models the exchange coefficients have a specified meridional variation replacing the constant coefficients used in previous studies. The basic model is the two-level, quasi-geostrophic model. The time integration of the model equations is carried out in the spectral domain using the spherical equations. The cut-off for the Legendre polynomials in the model is n=10, but can be selected freely. Employing symmetry around the equator the heating of the atmosphere is prescribed based on observational studies. The integrations permit a determination of the meridional variation of the resulting temperature, the geopotentials, the zonal winds at the upper and the lower levels, the transports of sensible heat for the model atmosphere, the vertical velocity and the transports of momentum at the upper and the lower levels. The energetic of the model may also be determined. The results are in reasonable agreement with the observed zonal structure of the atmosphere.

Based on the analysis of variations in the five day mean OLR and low-level wind fields (1,000 and 850hPa), intra- seasonal changes in the convective activity in Mexico are documented. The results show that as the convection over northwestern Mexico tend to increases from the middle of June, the opposite tendency is visible in the southern part of the country, the Caribbean, Central America, and the eastern tropical Pacific, where a period of less convection, locally termed "canicula", occurs in the middle of the rainy season. Changes in the intensity of low-level wind over Central America and the eastern Pacific are consistent with the onset and evolution of the rainy season over the studied region.

Observations show that the atmospheric kinetic energy on average obeys a relation saying that its dependence on the longitudinal wave number is n-3 in an interval with wave numbers larger than the wave number with maximal conversion of eddy available potential energy to eddy kinetic energy and the wave numbers dominated by frictional dissipation. Numerical experiments with a barotropic model have indicated that the power law for the kinetic energy is n-1 for the small wave numbers. Such relations are a result of the atmospheric forcing due to the diabatic heating, the cascade processes of kinetic energy due to nonlinear interactions and the frictional dissipation. The paper presents a study of a homogeneous fluid with a free surface forced by adding and subtracting fluid. For a given forcing the steady state may be determined although the model is nonlinear. For a special forcing the kinetic energy spectrum shows the -1 and the -3 relations. An important question is whether the potential energy also obeys a -3 power law. Other cases of the forcing are investigated in order to determine the sensitivity of the results to the forcing.

This paper examines the relation between easterly waves that propagate from aver the caribbean ssea toward the east pacific ocean and deep convective bursts that frequently occur over the gulf of tehuantepec and often intensify into tropical cyclones. This study shows that there are disturbances present over the caribbean at the time the convection is developing over the gulf and that the disturbances have a horizontal and vertical structure and propagation features characteristic of easterly waves.

At Pune (18º32 N, 73º51'E, 559 m AMSL), a tropical urban station in India, aerosol characteristics during the winter season exhibit unique features such as low-level inversions and dust haze formation during morning hours. Moreover, this station experiences abundance of sub-micron sized aerosols due to passage of continental air mass during winter. With a view to examine these winter aerosol properties, in detail, the PC-based high spectral resolution radiometer has been operated in the 0.4-0.7 µm wavelength region at the Indian Institute of Tropical Meteorology, Pune, India. The details of the spectroradiometer together with the data acquisition and analysis procedures have been briefly described. In this paper, the results obtained from the observations carried out on 75 cloud-free days during the successive winters of 1993-94, 1994-95 and 1995-96 have been presented and discussed in the light of local meteorological and environmental conditions. Of the three winters considered for the study, aerosol optical depths during the 1995-96 winter indicate greater values and forenoon aerosol size spectra exhibit bimodal distribution suggesting the dominant combined influence of transport of continental aerosols and trapping of surface-generated aerosols due to close-to-ground inversion during the season over the experimental station.

A barotropic model with momentum transport, a simple baroclinic model with transports of heat only and a baroclinic model with both momentum and heat transports are used to illustrate the instabilities and the nonlinear longer term behavior of barotropic, baroclinic and mixed barotropic-baroclinic processes in models without forcing and frictional dissipations. While the instabilities of such models are well known through analytical studies of the linear perturbation equations, it is obvious that these studies give no information on the long term behavior which can be studied only by nonlinear equations. Numerical integrations of low order nonlinear equations will be used to illustrate the long term behavior of the two models. The main aspects of the observed atmospheric energy processes may be reproduced by the most general low-order model used in this study. This model contains meridional transports of sensible heat and momentum by the eddies which are included in the model. To reproduce the atmospheric energy diagram based on observational studies it is necessary to include heating and frictional dissipations. The latter two processes will be excluded in the present study in order to obtain the long term behavior in non-forced models. The integrations of the various models will indicate that a long-term almost periodic behavior is observed with rather large time scales. The almost periodic variations are obtained from initial states with moderate values of the zonal parameters.