Ernesto Jauregui

The energy balance of central Mexico City during the dry season

Abstract The first measurements of the energy balance fluxes of a dry, densely built-up, central city site are presented. Direct observation of the net radiation, sensible and latent heat flux densities above roof-top in the old city district of Mexico City allow the heat storage flux density to be found by residual. The most important finding is that during daytime, when evaporation is very small ( 8), the uptake of heat by the buildings and substrate is so large (58%) that convective heating of the atmosphere is reduced to a smaller role than expected (38%). The nocturnal release of heat from storage is equal to or larger than the net radiation and sufficient to maintain an upward convective heat flux throughout most nights. It is important to see if this pattern is repeated at other central city, or dry urban sites, or whether it is only found in districts dominated by massive stone structures. These findings have implications for the height of the urban mixing layer and the magnitude of the urban heat island.

Heat island development in Mexico City

Abstract The present paper describes the climatology of the near surface urban heat island of Mexico City, using hourly data from two recently installed automatic stations at a rural and an urban site. The results show that the nocturnal heat island was more frequent (75% of the time for the period examined) than daytime cases (25%). The maximum nocturnal heat island intensity, 7.8°C was observed in a dry month (February) characterized by calm clear nights. Although less frequent, the daytime heat islands did occur and may have been caused by differences in evaporative cooling from wet surface during the wet season. Midday heat islands had a frequency of 13% and an intensity of 3–5°C during the wet season. The afternoon/evening heat islands had an intensity of 4–5°C. They occurred during both dry and wet season with a frequency of 12% of the time in a year. The average urban/rural thermal contrasts are positive throughout the year varying from 5°C at day break in the middle of the dry season to 1–3°C around noon during the wet months.

Urban effects on convective precipitation in Mexico city

Abstract This paper reports on urban-related convective precipitation anomalies in a tropical city. Wet season (May–October) rainfall for an urban site (Tacubaya) shows a significant trend for the period 1941–1985 suggesting an urban effect that has been increasing as the city grew. On the other hand, rainfall at a suburban (upwind) station apparently unaffected by urbanization, has remained unchanged. Analysis of historical records of hourly precipitation for an urban station shows that the frequency of intense (> 20 mm h−1) rain showers has increased in recent decades. Using a network of automatic rainfall stations, areal distribution of 24 h isoyets show a series of maxima within the urban perimeter which may be associated to the heat island phenomenon. Isochrones of the beginning of rain are used to estimate direction and speed of movement of the rain cloud cells. The daytime heat island seems to be associated with the intensification of rain showers.

The surface energy balance in Mexico City

Abstract Results of a field observation programme to investigate the surface energy balance of Mexico City, D.F., are presented. Direct measurements of net all-wave radiation and the turbulent sensible heat flux were conducted from a 28 m tower located about 4 km from the city centre. Heat storage in the urban fabric was parameterized and the turbulent latent heat flux was found by the residual in the energy balance equation. Most of the data were gathered in the dry season from February to the end of March 1985. The results from this tropical highland city resemble those from temperate cities in respect of the magnitude of the Bowen ratio and the diurnal trend in the amount of the net radiation partitioned into turbulent sensible heat. On the other hand, the role of heat storage is much larger than in the temperate case so that the partitioning of the sensible heat between turbulent and conductive transfer seems to be more nearly equal. Nevertheless evaporation appears to be an important term in both the surface water and energy balances. The nature of the energy balance may help explain some of the previously observed features of the heat island and humidity regimes of Mexico City.

Aspects of heat-island development in Guadalajara, Mexico☆

Abstract The magnitude of the urban effect on temperature in the tropical city of Guadalajara is examined. Parallel to the citys growth, air temperature shows an increasing trend; over a 40-year period (1931–1970) this rate was of the order of 0.03°C yr−1. As would be expected, this rate of temperature increase has been uneven over the period. When population increase per decade was large (90%), as in the 1940s the corresponding rate was significant (0.4°C per decade). The largest warming rate (0.7°C per decade) occurred during the 1960s when population growth was 73%. These results suggest that other factors (on a regional/global scale) may have been at play. Results show that estimates of the intensity of the heat island in a tropical city are likely to be dependent (besides the physical factors implicit in the city size), on the land-use characteristics in the rural/suburban control. Since the suburban/rural substrates undergo a significant annual variation in their physical properties in Guadalajara, heat-island intensity is highest during the dry season and declining in the wet season when contrasts in urban/rural thermal admittance are likely to be minimal. The presence of lower temperatures in the city during the afternoon hours suggests that the city (especially in the dry season) acts as a moisture source. This “cool” island has also been observed in other cities with similar regional climate.

URBAN–RURAL HUMIDITY CONTRASTS IN MEXICO CITY

Data from one pair of urban–suburban (Tacubaya and Airport) andone pair of urban–rural (School of Mines and Plan Texcoco) temperature and humidity measuring stations were used to illustrate specific humidity(q) contrasts in Mexico City. Results show a marked seasonal variation of q from around 7·9 g kg−1 during the dry months to 10 g kg−1 in the wet season (May–October) on both urban and suburban sites. The mean monthly contrasts for this pair of stations, albeit small, show that the city air is somewhat drier during the first half of the year. Comparison of urban and rural q on an hourly basis shows that although urban air is more humid at night the reverse is true during the afternoon. Areal distribution of q shows two centres of maximum humidity over the city at night and a corresponding minimum during the afternoon. On average the urban–rural contrasts in q were found to be somewhat smaller than the estimated uncertainty. The above results are in agreement with mid-latitude experience.

Simulated urban climate response to historical land use modification in the Basin of Mexico

Numerical simulations of thermal field variation due to land use changes, such as the reduction of the lacustrine system and the growth of the urban area, in the Basin of Mexico are presented. It is shown that the historically recorded warming in the basin could be attributed, not only to the growth of the urban area, but more importantly to the drastic reduction in the lacustrine system that existed in the basin.

Urban heat island development in medium and large urban areas in Mexico

King, D.: The Quaternary stratigraphie record at Lake Eyre North and the evolution of existing topographic forms. In: Trans.R.Soc.S.Aust. 79, 1956, 93-103. Loffler, E. a. Sullivan, M. E.: Lake Dieri resurrected: an interpretation using satellite imagery. In: Z. Geo morph. N.F. 23, 1979, 233-242. Twidale, C. R.: Age and origin of longitudinal dunes in the Simpson and other sand rigde deserts. In: Die Erde 112, 1981, 231-247. Wasson, R. J.: The Cainozoic history of the Strzelecki and Simpson dune fields (Australia), and the origin of the de sert dunes. In: Z. Geomorph. N.F., Suppl. Bd. 45, 1983 (a), 85-116. : Dune types, sand colour, sediment provenance and hy drology of the Strzelecki-Simpson dunefield, Australia. In: Brookfield, M. E. a. Ahlbrandt, T. S. (Eds.): Eo lian Sediments and Process, Amsterdam 1983 (b), 165-195.