Modelling solar radiation in steeply sloping terrain.

A methodology for predicting global shortwave radiation into any slope based on the ratio of the incoming shortwave on to the horizontal and the exoatmospheric beam is described. The model is based on individual calculations of direct and diffuse components, enabling its application under a wide range of sky conditions. An algorithm accounting for the effect of topographic shading is also included and this enables accurate estimations of spatially distributed global shortwave radiation in steep sided terrain. The model in then applied to the summer and winter solstices at Monachyle catchment at Balquhidder, Scotland. Although incident radiation is strongly affected by slope angle and aspect, the influence of shading is seasonal and more dependent upon overall catchment orientation. In winter, areas of the catchment receive only diffuse radiation, emphasizing the importance of modelling radiation components individually

Global structure of the Madden-Julian Oscillations during two recent contrasting summer monsoon seasons over India

The global nature of the Madden-Julian Oscillations (MJOs) have been investigated by applying a frequency filter to daily data for the summer monsoon months (June to September) during two contrasting years-1987, a deficient monsoon year and 1988, an excess monsoon year. Several meteorological parameters at five levels in the troposphere have been examined. Regions with large amplitude of these oscillations are isolated for each year. The results indicate that the global spatial distribution of these oscillations is more in a deficient year than in an excess year, in particular over the Indian subcontinent and the EI Niño Southern Oscillation (ENSO) regions. The principal modes of variability during these two years have been investigated through Empirical Orthogonal Functions (EOFs). The first two eigenmodes of 850 hPa zonal wind explain nearly 50 of the variance. The dipole type of structure between the Indian and the Pacific region is more apparent in 1987 than in 1988. Time-longitude cross sections of the filtered zonal wind over the equatorial regions clearly show that eastward propagation is detected in 1987, but is virtually absent in 1988. It is also seen that the 30-60 day filtered winds are stronger during the monsoon of 1987 than in 1988

Palaeomagnetic field intensity variation recorded in a Brunhes epoch deep-sea sediment core

Deep-sea sediments have been shown to possess a natural remanent magnetisation (NRM) that often can be attributed to the statistical alignment of detrital magnetic grains in the Earth's magnetic field at or shortly after the time of their deposition. In favourable circumstances this remanence can be interpreted as a record of palaeomagnetic field behaviour. In the study reported here we have attempted to describe relative variations in palaeomagnetic field intensity on a time scale of 10^4-10^5 yr, during the past 700,000 yr, using the palaeomagnetic record of deep-sea sediment piston core RC10-167 (33°2′N, 150°23′E), which has an exceptionally thick section of sediment deposited during the Brunhes normal polarity epoch (Fig. 1). After subtracting the stratigraphic contribution of several distinct volcanic ash layers interspersed with the otherwise uniform pelagic sediment, we calculate an average deposition rate of 2.1cm kyr^-1 between the adjusted level (1,470cm) of the Brunhes-Matuyama boundary (t=700,000 yr) and the top of the core, assumed t=0 yr. A 2-cm thick sample, representing about 1,000 yr of deposition, was taken at an average interval of 3.3 cm (representing about 1,600 yr). This sampling placed a theoretical limit of 3,000-4,000 yr on the period of a resolvable sinusoidal variation

Impact of vegetation changes on the dynamics of the atmosphere at the Last Glacial Maximum

Much work is under way to identify and quantify the feedbacks between vegetation and climate. Palaeoclimate modelling may provide a mean to address this problem by comparing simulations with proxy data. We have performed a series of four simulations of the Last Glacial Maximum (LGM, 21,000 years ago) using the climate model HadSM3, to test the sensitivity of climate to various changes in vegetation: a global change (according to a previously discussed simulation of the LGM with HadSM3 coupled to the dynamical vegeta- tion model TRIFFID); a change only north of 35°N; a change only south of 35°N; and a variation in stomatal opening induced by the reduction in atmospheric CO2 concentration. We focus mainly on the response of temperature, precipitation, and atmosphere dynamics. The response of continental temperature and precipita- tion mainly results from regional interactions with veg- etation. In Eurasia, particularly Siberia and Tibet, the response of the biosphere substantially enhances the glacial cooling through a positive feedback loop between vegetation, temperature, and snow-cover. In central Africa, the decrease in tree fraction reduces the amount of precipitation. Stomatal opening is not seen to play a quantifiable role. The atmosphere dynamics, and more specifically the Asian summer monsoon system, are significantly altered by remote changes in vegetation: the cooling in Siberia and Tibet act in concert to shift the summer subtropical front southwards, weaken the easterly tropical jet and the momentum transport asso- ciated with it. By virtue of momentum conservation, these changes in the mid-troposphere circulation are associated with a slowing of the Asian summer monsoon surface flow. he pattern of moisture convergence is slightly altered, with moist convection weakening in the western tropical Pacific and strengthening north of Australia