Radar cells from the GATE experiment, from Spain and from the Montreal area were followed and measurements of their total rain flux and area were extracted. The values of the flux and area, for each one of the cells, for initial time increments T(,0) were used to evaluate various extrapolation schemes for forecasting purposes. The extrapolation procedure that gave the smallest error in forecasting the changes in flux and area, was found to be the linear one and the optimum increment T(,0) was about 30 min. However, even though these techniques have the advantage of establishing a trend in the behavior of the flux and area with time, a comparison of the forecast errors from the linear extrapolation scheme with those from the assumption of no change in cell area and rain flux shows insignificant improvements. A technique including both cell motion and internal changes in flux and area of the rain cells was developed to evaluate the accuracy of rain accumulation forecasts. Again, it was found that the errors were similar with those generated by the assumption of no change in rain flux and area from the moving cell. / The preceding results were used as a possible input into the design and evaluation of cloud seeding experiments. Based on this, a method is developed which gives the necessary number of experiments (i.e., seeded cases) in order for a seeding factor to be statistically significant at specified confidence levels. As a conclusion, it can be stated that the use of short term rain predictions (which are translated as differences of the rain flux in time) is very promising. In comparison to other techniques described in the literature this method appears to be superior, in the sense that fewer experiments are needed to detect seeding factors at specified confidence levels.

No description available.

In order to address rain attenuation scattering of millimetric waves and microwave sin Botswana, we have employed a comparison technique to determine the Ro.o1 at fourteen diverse locations in Botswana. In addition we have identified two rain climatic zones for Botswana. We note that Matzler employs Mie Scattering technique to determine the specific attenuation due to rain in Central Europe. Both Matzler and Olsen use the exponential distribution of N(D) to calculate y. In this dissertation we use the Mie scattering approach, but assume several distributions, including the log-normal distribution of N(D) as expounded by Ajayi et aI., to determine y for tropical and subtropical regions of Africa. The results show that the extinction coefficients depend more strongly on temperature at lower frequencies than at higher frequencies for lognormal distribution: at selected frequencies, we record high attenuation values at rising SHF bands: at 300 GHz, tropical showers take on values of 12, 12.5, 11.9 and 14 dB/km for Gaborone, Francistown, Kasane and Selebi-Phikwe, respectively. The absorption coefficient is significant but decreases exponentially with rain temperature at lower microwave frequencies. The application of the proposed model (Continental Thunderstorm is shown using practical results from Durban) is corroborated using practical results from Durban. Further, based on attenuation measurements, it is found that the lognormal distribution is suitable for Durban at rain rates greater than or equal to 21 mm/h. At rain rates below this, the loss-Thunderstorm is the better fit. Finally in this dissertation the results show that for rainfall intensity below about 10 mm/h for Marshall-Palmer (MP), Joss-Drizzle (JD), Joss-Thunderstorm (JT) and Law-Parson (LP) distributions, and below about 4 mm/h for Continental-Showers (CS), Tropical Showers (TS), Continental Thunderstorms (CT) and Tropical Thunderstorm (TT) distributions, the specific rain backscattering follows Rayleigh scattering law where the rain drops are small with respect to the wavelength when the frequency is 19.5 GHz. At rain rates above 10 mm/h for exponential distribution, and above 4 mm/h for lognormal distribution, the specific backscattering follows Mie scattering law. When the received echo power from rain becomes significant, it contributes to the rise in the noise floor and the radar receiver can lose its target. In addition, the result shows that Mie backscattering efficiency is highest at a raindrop diameter of 4.7mm. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2008.

No description available.

A novel system for leaching of reconstituted cores of soils taken from horizons of <I>Calluna</I> moorland podzol profiles derived from granites has been used to see if absolute, or at least, relative, weathering rates assessed by simple leaching techniques and by using the PROFILE model agreed. Initial base cations are first removed by leaching with ammonium acetate. Agreement was reasonable, but the results highlighted problems posed by mineralization effects in column-based procedures. A successful attempt was made to develop a procedure to overcome this problem, based upon pre-oxidation with H₂O₂. This procedure was tested on a selection of B and C horizon soils from podzol profiles, and shown to give weathering rates which correlated well with those derived using the PROFILE model. The equilibrated soil cores remaining at the end of the study were then used for testing the effect of acidification of infiltrating water upon weathering rates of base cations. Weathering rate did not increase regularly to combat the acid input. Intact soil core microcosms, fitted with rhizon samplers at three depths and subjected to appropriate simulated precipitation, have been used to elucidate how land use influences the mobility of copper and zinc. Three land uses were considered, improved pasture, semi-improved pasture, and <I>Calluna</I> moorland. Copper and zinc concentrations were also measured in river water samples collected at 59 points from throughout the River Dee network under diverse flow conditions, with a view to elucidating catchment factors influencing the mobilisation of these two elements into river water. The results demonstrated large inputs of copper arising as a consequence of arable land use, and suggested that TOC in drainage water from moorland soils mobilises copper from mineral sediments in the river network. Both zinc and copper concentrations were increased under high discharge conditions.

No description available.

No description available.

No description available.

Dry cloud forests on the western slopes of the Peruvian Andes were mapped from aerial photographs, 306 stands being recorded from 4^o50'S to 12^o47'S. The frequency and area of these stands, as well as most parameters analyzed in the eight sample sites, show a steep decreasing latitudinal trend and are strongly correlated with the latitudinal rainfall gradient. The mean area of the forest stands decreases from 115 ha in northern Peru to 42 ha in central Peru. The number of species recorded decreases along the study area from 52 to 13 and there is a well defined latitudinal sequence of species. Mean density and basal area per hectare of stems ≥10 cm gbh decreases from 2995 individuals and 79.91 m^2 in the north to 500 individuals and 17.27 m^2 in central Peru. The vertical structure is similar throughout the study area, emergent trees reaching on average 22 m and the main canopy 12 m in the north and 13 m and 7 m respectively in central Peru. Regeneration is very active in northern Peru. Juveniles have been found for a high proportion of species, including all common ones, and most species show a logarithmic decline in number of stems with increasing girth. There is a steep decreasing trend towards central Peru, where few species regenerate, mostly shrubs. The pattern found may be the result of the combined effect of grazing and a climatic change towards drier conditions evidenced in the regeneration pattern of most sites.

Efflorescence is the presence of insoluble salts upon the surface of concrete or masonry products. The aims of this project were to understand efflorescence and to evaluate metakaolin as an additive for efflorescence prevention. Computer modelling was used in order to simulate the effect of alkali upon relevant phases. Alkali was shown to depress the solubility of calcium hydroxide via the common ion effect, increase, in general, the solubility of calcium carbonate via the foreign ion effect and dramatically increase the solubility and rate of uptake of carbon dioxide into solution. The raindrop model is an attempt to simulate the processes leading to efflorescence. A raindrop falling onto a roof tile is assumed firstly to leach ions from the tile, secondly, to absorb atmospheric carbon dioxide and thirdly to precipitate calcium carbonate. The raindrop model calculates that the formation of efflorescence is primarily dependant upon leaching rather than drying effects. Sensitivity studies show that even when significant sulphate is leached, calcium carbonate remains the prevalent efflorescent phase. Metakaolin, a type of burnt clay pozzolan, had previously been cited as preventing efflorescence. However, this study showed that the performance of metakaolin is affected by the presence of micaceous and other impurity phases which are activated upon heating to metakaolinization temperatures, and readily release alkalis. This study found that the main products of metakaolin hydration in alkaline solutions are gehlenite hydrate and amorphous C-A-S-H gels. The latter was shown to absorb alkalis, particularly potassium, from solution. Results are presented of wet-dry and renewal of leachant leaching tests on two metakaolin blends and a neat OPC paste control. Wet-dry leaching, judged to be the most realistic leach test, produces leaching trends not noted in other leaching tests.