Showing papers in "Theoretical and Applied Climatology in 1989"

Physical, mineralogical and chemical properties of harmattan dust at Ile-Ife, Nigeria

Abstract: Meteorological conditions associated with the transport and deposition of harmattan dust particles in West Africa are discussed as well as particle-size distribution analyses of Saharan aeolian dust monitored at Ile-Ife (729 N, 434 E) nearly 2,000 km South-West of its predominant source region, the Chad Basin Mineralogical analysis of the dust indicated that it is predominantly composed of quartz (> 70%) followed by microcline, kaolinite and traces of mica and halloysite Chemical analyses indicated a predominance of SiO2 (> 60%) followed by Al2O3 (174%), Na2O (101%), Fe203 (56%), TiO2 (333%), K2O (188%), MgO (056%), and CaO (037%) The following trace elements were identified: Zn (931 ppm), Mn (834 ppm), Ni (113 ppm), Cr (103 ppm), Cu (106 ppm), Co (77 ppm) V (65 ppm) and Li (25 ppm) The effect of the poor visibility caused by the harmattan dust on aviation operation and safety as well as the health hazards posed on the respiratory system by the air-borne toxic elements should be of concern to relevant decision makers and should be further investigated

Enhancement of surface-atmosphere fluxes by desert-fringe vegetation through reduction of surface albedo and of soil heat flux

Abstract: Under the arid conditions prevailing at the end of the dry season in the western Negev/northern Sinai region, vegetation causes a sharp increase relative to bare soil in the daytime sensible heat flux from the surface to the atmosphere. Two mechanisms are involved: the increase in the surface absorptivity and a decrease in the surface heat flux. By increasing the sensible heat flux to the atmosphere through the albedo and the soil heat flux reductions, the desert-fringe vegetation increases the daytime convection and the growth of the planetary boundary layer. Removal of vegetation by overgrazing, by reducing the sensible heat flux, tends to reduce daytime convective precipitation, producing higher probabilities of drought conditions. This assessment of overgrazing is based on observations in the Sinai/Negev, where the soil albedo is high and where overgrazing produces an essential bare soil. Even if the assessment for the Sinai/Negev does not quantitatively apply throughout Africa, the current practice in many African countries of maintaining a large population of grazing animals, can contribute through the mesoscale mechanisms described to reduce daytime convective precipitation, perpetuating higher probabilities of drought. Time-of-day analysis of precipitation in Africa appears worthwhile, to better assess the role of the surface conditions in contributing to drought.

The Tropical easterly Jet and Sudan rainfall: A review

Abstract: Recent work on Sahelian rainfall anomalies has concentrated on modelling of both ocean heat anomaly forcing of regional low-mid troposphere circulation and land surface forcing of precipitation processes. This paper, however, assesses the role of an upper troposphere synoptic feature of importance in modulating surface rainfall over Sudan in the eastern Sahel: the Tropical easterly Jet (TEJ). The TEJ provides an example of an inter-regional circulation feature linking the Sahelian and southeast Asian monsoons and ultimately, perhaps, forced by ENSO-related anomalies. Using the literature, intra-and inter-year variations in TEJ intensity, latitude and extent over northeast Africa are discussed and related to the lower tropospheric ITD and precipitation anomalies. The paper outlines ideas to be pursued in relating TEJ behaviour to surface rainfall, upper air windflow, and mid-troposphere water vapour (WV) as derived from METEOSAT data.

Late Quaternary volcanic modulation of Milankovitch climate forcing

Abstract: A preliminary global volcanicity index time series was developed from the total available record of radiocarbon dated eruptions. This was combined with a previously developed simple model of Pleistocene glaciation based on the integral of the seasonal contrast of hemispheric irradiance. The combined integral gave a modeled glacial volume time-series which matched both the general variation of glacial volume in the last forty thousand years and the short-term events as well. Particularly well-modeled were the Allerod-Younger Dryas-Two Creeks complex of Late Glacial events and the Cochrane stillstand.

Statistical specification of local surface weather elements from large-scale information

Abstract: A procedure for stochastic simulation of the spatial structure of daily mesoscale temperature and precipitation values is presented. Specific values are generated on the basis of, and consistent with, observational data representing averages over areas corresponding to Atmospheric General Circulation Model grid elements. The procedure is based on a rotated principal component representation of the correlation structure of the observed local data, and procedures synthetic data which statistically resemble the corresponding observations. Use of a stochastic, rather than a deterministic, specification procedure may not be required if generation of only temperature data are of interest.

Delimitation of the summer-dry period in Greece according to the frequency of weather-types

Abstract: The mean absolute and relative frequencies of daily synoptic weather-types over the Greek area, have been calculated, for a period of 36 years (1950–1985), and for each decade of the seven-month period April to October. The real summer-dry period is delimited first empirically according to the relative frequencies of the weather-types for each period of 10 days; then objectively by use of the Principal Component Analysis. Results obtained by the two methods appear to converge. The summer-dry period lasts for six months: it begins in the third ten-day period of April and reaches to the second tenday period of October. From the second ten-day period of June to the third of August constitutes the high-summer-dry period; the periods before and after that stretch of timelasting for five ten day periods each-constitute the first and the second low-summer-dry periods.

Principal Component Analysis of Western Mediterranean Air Temperature Variations 1866-1985

Abstract: A study of Western Mediterranean temperature variations (annual and seasonal data) is presented covering a period of 120 years (1866–1985) and using the principal component analysis (PCA). The meteorological stations are splitted into two groups: Group A comprises northern stations, group B southern stations. The variations in group A-data indicate a decreasing temperature tendency from the year 1866 to the second decade of the 20th century; a temperature rise follows until the middle of the decade 1940–1950 and then temperatures, although remaining higher than the long-term mean value, show a marked decrease. The variations in group Bdata indicate a similar picture but with a time lag of 20 to 25 years. The temperature drop during the last 35 years (group A) is due to pronounced decrease in summer temperatures whereas the winter temperatures for the same period show a marked tendency to increase.

Principal component analysis of Belgian rainfall

Abstract: A principal component analysis is applied to the monthly normals of the daily intensity of rainfall at the stations of the Belgian rainfall network. Four principal components are found to be significant, explaining 99.8% of the total variance. Their climatic significance is to be found respectively in: (1) the general atmospheric circulation, (2) the air mass instability in summer, (3) the neighbourhood of the sea, (4) the mean pressure gradient. At the computational level, it is also shown how to operate with a large correlation matrix and a small number of observations.

A bioclimatic classification of the Greek area

Abstract: In this study the results of a detailed classification of the Greek area are given, based on the Gaussen's xerothermic index. In this way various climatic zones are designated, which could be of help to the programming of various cultivations in the different areas. So we found out that Greece, according to the xerothermic index, has a climate going from submediterranean to xerothermomediterranean, which makes not only forestry but also seasonal irrigated agriculture appropriate.

Surface downward flux computed by using geophysical parameters derived from HIRS 2/MSU soundings

Abstract: The global field of outgoing longwave radiation (OLR) and surface downward flux (SDF) are computed for January and July 1979 (FGGE year). The computation was done by using geophysical parameters retrieved from space in conjunction which a radiation code. The geophysical parameters (such as temperature and humidity profiles, surface/land temperature, and cloud information) are retrieved from analysis of HIRS 2/MSU using the Goddard Laboratory for Atmospheres (GLA) physical retrieval system. The radiation code is a modified version of the Wu-Kaplan radiation code used in the GLA 4th order GCM. Other fluxes (such as clear OLR and clear SDF, and Earth emission) flux divergences (upward flux divergence and net upward flux divergence), flux differences and longwave radiative forcing (clear OLR — cloudy OLR, cloudy SDF — clear SDF, and atmospheric cloud forcing) which have physical meaning but can not be measured directly from space have also been computed and presented.

Canopy structure and light climate of different alpine plant communities: Analysis by means of a model

Abstract: Using the classical model of Monsi and Saeki (1953) as a starting point a series of models has been developed for the simulation of the penetration of direct beam solar radiation into the canopy. In each step of the model an additional factor affecting the light climate of the stand is explicitly taken into consideration. The hierarchical nature of the models makes possible a separate quantification of the individual parameters (vertical distribution of plant area and inclination, solar position, horizontal distribution patterns of foliage in different layers of the canopy). The models are successfully applied to data from field observations on three structurally differing plant stands: a revegetated grassland (graded and resown) on a ski slope, an alpine pasture and aVaccinium stand within this pasture. It is shown that the vertical distribution of the plant area and its inclination account for 80–94% of the observed variations in measurements of the light climate in the three stands investigated. A marked improvement in the results, especially in the case of the erectophilic grassy stand on the ski slope, can be achieved by taking into account the angle of incidence of light (explained variance = 99%). Whereas the assumption of random leaf distribution appears to be justified for the two grass stands (alpine pasture and revegetated grassland), computer simulation shows a strongly clumped leaf distribution in theVaccinium stand, especially in its upper third.

Magnitude and sources of variation in albedo within an alpine tundra

Abstract: Temporal and spatial variations of albedo in a mid-latitude alpine tundra are assessed in order to develop a classification of surface cover mapping units which is useful for surface climate simulations. The largest temporal changes in albedo result from alterations in moisture conditions at the tundra surface associated with snowpack ripening and precipitation. Surface albedo varies under high atmospheric transmission conditions (clear skies) from 0.168 to 0.205; under low transmission conditions (cloudy) there was little variation in the surface albedo with the solar zenith angle and the value of the albedo was approximately equal to that under clear skies when 45° >z > 30°.

A note on the frequency of occurrence of severe winters as evidenced in monastery and historical records from Greece during the period 1200–1900 A.D.

Abstract: Monastery and historical records for Greece during the period 1200–1900 A.D. have been used to obtain the number of “severe” winters which occurred during this time period. A winter was considered as being “severe” if long duration of snow cover and/or freezing of lakes in Greece were archived. Evidence is presented that the coldest periods in Greece during the period 1200–1900 A.D. occurred in the first half of the 15th century, in the second half of the 17th and in the 19th century. Geographical coherence is indicated with other areas in Europe, e.g., with The Netherlands, where historical records exist over the same period. The number of severe winters in 50-year intervals does show a positive correlation with the volcanic dust veil index.

The solar cycle and Indian rainfall

Abstract: The objective of this study is to examine critically the relationship between solar cycles and Indian monsoon rainfall, for the period 1871–1984, and to search for significant periodicities, by utilizing the maximum entropy spectral technique (MEST). The results of this study using MEST show clearly a significant 11-year cycle in solar activity and rainfall. Also present is a significant 7.33-year cycle in rainfall. The double (Hale) sunspot cycle is not discernible here either in sunspot number or in rainfall. The cross-spectral analysis between the sunspot number and rainfall confirms the existence of a reasonable correlation over an 11-year cycle with a relative phase lag of 0.16 year (sun lead).

On the estimation of surface radiation using satellite data

Abstract: Model calculations are used to investigate the uncertainties in the surface radiative flux empirically derived from satellite radiation measurements and theoretically calculated from radiation models using satellite-inferred cloud parameters. The empirical approach depends upon how well the satellite-measured radiances (represented here by the top-of-the-atmosphere flux) correlate with the net flux at the surface. The model calculations show that while the TOA flux and the net surface flux are correlated with respect to changes in optical thickness, they are not correlated with respect to changes in cloud height and negatively correlated with respect to changes in water vapor content. It is also found that the solar zenith angle has a strong effect on these relationships. It is, therefore, important to correct for the effects of atmospheric water vapor content and the solar zenith angle in the empirical estimation of surface radiative flux. The theoretical approach depends upon the net effect of the uncertainty in satellite-inferred cloud parameters. In the solar spectral region, the effects of the uncertainty in satellite retrieval of could cover and optical thickness on the net downward surface flux are systematically in opposite directions, so that the combined effects is typically small (< 7 Wm−2). In the thermal infrared region, an error of 7 Wm−2 could be induced by an uncertainty of 100 mb in the cloud-base height or an uncertainty of 0.1 in the fractional cloud cover. As opposed to what is commonly perceived, the error in the surface flux is likely to be larger in the IR region than in the solar spectral region.

Review of meteorological aspects on hail defense activities in Austria

Abstract: Hail defense in Austria is supported by meteorological offices since 1979. Testing the efficiency of these weather modification activities hailpad-networks have been erected 1981/1982 in two target areas (Lower Austria, size 500 sqkm and Styria, size 700 sqkm), with a mean distance of 2 km. All data, i.e. seeding information, hailpad data, crop damage (estimation by the Austrian Hail Insurance) are collected to establish a relation between hail registration and harm on plants. For the period 1981–1987 first results of the target areas are presented. For further investigation on the seeding effect a ten year period with more intensive radar registration will be necessary.

The seasonal cycle of the global zonally averaged energy balance

Abstract: The annual and seasonal cycle of the global zonally averaged storage and transport of moist static energy (MSE), the surface eddy flux of MSE, and the radiation budget at the top of the atmosphere (TOA) and surface are investigated in this paper. We use the GFDL (Geophysical Fluid Dynamics Laboratory) zonal mean data to compute the storage rate, divergence, and poleward flux of MSE. Surface eddy fluxes are determined based on an empirical formula developed from available climatological data. Radiative fluxes at the TOA and surface are computed from a radiation model. A number of significant features are illustrated by the present analysis in regard to the global annual and seasonal energy balance of the earth-atmosphere system. The storage/release rate of MSE is greater in spring and autumn than in summer and winter. The rate of poleward transport of MSE has a maximum at ~40°N and ~35°S with a value of ~3 × 1015 W. In terms of the radiation budget, the persistent ITCZ minimum and subtropical maxima of the IR flux distribution are reproduced well in the radiation parameterization program. The incoming solar flux maximum shifts from ~20°N in spring to ~40°N in summer due to the poleward increase in the fraction of daytime. Finally, an examination of the global surface energy balance reveals that the surface net fluxes derived from parameterizations for eddy and radiative fluxes agree well with values derived from observations. This agreement appears to imply that on the global mean, the physical parameterizations used in the present study are reliable in modeling the seasonal surface flux components.

On the mean monthly equivalent potential temperature and rainfall in West Africa

Abstract: Rainfall in West Africa is examined in relation to monthly mean equivalent potential temperature (θe)at the earth's surface. The study revealed that monthly mean equivalent potential temperature (θe) and monthly rainfall (R) generally decreased northwards from the equator.

Some features of the periods contributing specified percentages of rainfall to annual total in Kerala, India

Abstract: Based on daily rainfall data over a period of 80 years (1901–1980) taken at 75 stations, this paper reports interannual and long term fluctuations of the following parameters of the periods contributing each of 2, 5, 10, 20–90 and 95% rainfall to annual total over each of North Kerala and South Kerala rainfall subregions of India: (i) the starting date, (ii) the ending date, (iii) the length of the rainfall period or duration, (iv) the total rainfall (which is a fixed percentage of the annual total) of the rainfall period, and (v) the rainfall time-distribution characteristic which has been quantified by computing Oliver's precipitation concentration index (PCI) using daily rainfall data of the concerned rainfall period.

Precipitation effects of hail suppression in Serbia

Abstract: Daily precipitation amounts ofconvective days, for some 20 May–September seasons (covering periods both before and after activation of the hail suppression), were analysed for 3 stations in the protected and 4 in the unprotected area. A tentative conclusions derived from this study is that precipitation at the protected stations in the period of suppression changed from the preceding period in a way which cannot be explained by similar changes at the unprotected stations. This change is attributed to the inadvertent hail suppression effect. The point estimate of the change is an increase of 23% (on convective days). At the 95% confidence level the estimate shows an increase in the interval (8%, 40%). A rough estimate of theseasonal increase for the protected area is about 30 mm.

Annual Variation of Pressure over the Mediterranean Area

Abstract: This study deals with the annual variation of pressure over the Mediterranean area using monthly values from 24 stations, for a period of 22 years. The Azores and the Siberian anticyclones as well as the low pressure system of SW-Asia are responsible for the observed distribution pattern of the pressure over the area and for the seasonal changes at each station. Fourier analysis shows that for Central Mediterranean stations, which record two minima and two maxima of pressure during the year, the amplitude of the second harmonic is smaller than the amplitude of the third.

Annual precipitation values at 20° N/0° E in the Early Holocene

Abstract: 9,000 years ago, the climate of North-West Africa allowed the existence of Lake Ine Kousamene at ~ 20° N/0° E. The calculations of a climatological precipitation-runoff model, which contains also a description of the energy balance of the lake basin, show that in the Early Holocene the size of Lake Ine Kousamene of about 300 km2 matches a precipitation rate of about 500–600 mm/year in that area.

Development of sea surface temperature, surface wind and divergence anomalies during a composite ENSO episode

Abstract: The interannual variations in sea surface temperature (SST) in the equatorial east Pacific, which are dominated by the El Nino phenomenon, are shown for the period 1870–1983. Since 1870 25 significant warm events have occurred. These events are classified as weak, moderate, strong and very strong, according to the normalized SST anomalies in the region 130° W–80° W, 0°–5° S. The spatial and temporal development of a composite El Nino/Southern Oscillation (ENSO) episode, based on 10 very strong or strong events, is presented in terms of SST, surface wind and divergence anomalies for the tropical Pacific (10° N–30° S). During its evolution the following phases are distinguished: “Antecedent Conditions”, “Onset Phase”, “Peak Phase”, “Mature Phase” and “Dissipation Stage”. Some aspects of ocean-atmosphere interaction associated with this evolution and, more specifically, the initiation of the composite event, are described. Seasonally varying feedback processes between SST, surface wind and convergence anomaly patterns in the western Pacific/Indonesian region suggest a possible mechanism for the initiation of typical ENSO events.

Discrimination between hail and rain precipitation types from dual polarization radar, raingage and hailpad data

Abstract: Dual polarization radar data (ZH andZDR) are analysed in 4 convective cells. Quite different relationships betweenZH andZDR are induced by the different hydrometer characteristics in these cells. The radar data, compiled with raingage and hailpad data obtained at the ground, show that in the mixed-phase (hail and rain) type of precipitations the differential reflectivityZDR is about 1.5 dB smaller than in the rain type of precipitations, without reaching zero values as in the hail type. In zones of strong precipitations the differential reflectivityZDR increase as the horizontal reflectivityZH, reaching values > 4 dB which indicate the existence of very big drops.

Further results of the analysis of central England temperature data

Abstract: Central England temperature data for 1659–1986 are analysed using a new procedure for separating the trend from monthly mean temperatures. The method has been developed without any important assumption relating to the trend function. The monthly trends are illustrated. It is suggested that a general warming tendency underlies various cooling and warming periods. Several autoregressive-moving average (ARMA) processes have been fitted to the stationarized time series of which an ARMA (2, 1) process proved to be the most appropriate model. Calculations on the data after adjustment to remove non-linear trends show much lower autocorrelations of yearly and individual monthly mean temperatures than estimated before.

Plant spacing effects on net irradiance extinction in cotton

Abstract: Decreasing the fraction of water that evaporates from the soil (E) is one means of increasing the water available to crops and thereby increasing the transpiration and the water use efficiency of a cropping system. In addition to soil surface wetness, net irradiance at the soil surface (R ns ) is the predominant environmental element affectingE. TheR ns is infrequently measured and is often estimated using an equation of the formR ns /R n = exp (−k · LAI), whereR n is the abovecanopy net irradiance,k is an extinction coefficient, andLAI is the leaf area index. The effect of row spacing and plant population onk was examined for cotton (Gossypium hirsutum L.) grown on a Houston Black clay (fine, montmorillonitic, thermic Udic Pellustert). Treatments consisted of 10 and 15 plants m−2 in 0.68- and 0.34-m north-south oriented rows and in a uniformly-spaced pattern. HourlyR n andR ns were measured with a domed and tubular net radiometer, respectively. TheR n was approximately a constant fraction of global irradiance across treatments and throughout the season. Minimum values of theR ns /R n ratio ranged from 0.14 to 0.24. Values ofk, calculated as the slope of the regression of ln (R ns /R n ) with theLAI, ranged from 0.44 to 0.66. Plant population affectedk more than row spacing. Thek value calculated from pooled data was 0.52.

The 11-yr ozone modulation of extreme surface air temperatures

Abstract: The opposite effect of the total ozone on daily maximum and minimum air temperatures is one of the reasons for the failure of the correlation between solar activity and surface air temperature by means of mean daily values. The solar-related fluctuations in the atmospheric total ozone are found to be responsible for the observed correlations between solar activity and separately daily maximum and minimum air temperatures. The present evidence indicates that ozone serves as a stratospheric shutter modulating both the incoming daytime visible solar energy flux and the Earth's night-time upward infrared radiation transferred downward.

Thunderstorms and hail

Abstract: This study presents an overview of the processes causing and affecting strong thunderstorms. As storms are in general relatively small meteorological phenomena, the standard observational systems are not capable to detect all events. Hence, it is difficult to obtain climatological descriptions of thunderstorms and hail. According to the dynamical and morphological classification thunderstorms are divided into those with considerable wind shear and those without it: shear enables the long-lasting, strong storms; without it, the shortliving ones develop. Further, strong storms can be classified according to spatial organisation of convection cells. Mesometeorological factors also determine the spatial distribution, the frequency of occurence, and the organisation of thunderstorms and hail. Strong dependance on topographical characteristics of the territory is illustrated as well. Regarding energetics, some estimation of typical potential energy and latent heat release is given together with a comparison between the thunderstorm development enhancing and suppressing processes. An overview of the microphysical processes leading to hail growth is also included with special emphasis on the present state of knowledge on the question of the natural ice nuclei concentration in clouds, this being the basic factor for almost all artificial cloud modification hypotheses.

Influence of the El Chichon eruption in Central Europe determined as change in atmospheric transparency at Davos, Switzerland

Abstract: From the beginning of 1982, new instrumentation for the continuous measurement of direct solar irradiance has been installed at Davos, Switzerland From these data the atmospheric transmission, computed by the so-called ratioing technique, is presented The truly continuous recording allows the establishment of a very homogeneous data set by applying selection rules according to well defined criteria With the long-term record of the atmospheric transparency (1909–1970) as reference, the influence of the stratospheric aerosol cloud produced by the El Chichon eruption in Mexico 1982 can be readily identified, and also quantified by comparison with LIDAR measurements made in Southern Germany during the same period of time Other significant decreases of the transparency are found during spring 1984 and winter/spring 1985 As these are not seen in the LIDAR data they must have their origin in the troposphere