Synoptic processes of winter precipitation in the Upper Indus Basin

Precipitation in the Upper Indus Basin is triggered by orographic interaction and the forced uplift of a cross-barrier moisture flow. Winter precipitation events are particularly active in this region and are driven by an approaching upper-troposphere western disturbance. Here statistical tools are used to decompose the winter precipitation time series into a wind and a moisture contribution. The relationship between each contribution and the western disturbances are investigated. We find that the wind contribution is related not only to the intensity of the upper-troposphere disturbances but also to their thermal structure through baroclinic processes. Particularly, a short-lived baroclinic interaction between the western disturbance and the lower-altitude cross-barrier flow occurs due to the shape of the relief. This interaction explains both the high activity of western disturbances in the area and their quick decay as they move further east. We also revealed the existence of a moisture pathway from the Red Sea to the Persian Gulf and the north of the Arabian Sea. A western disturbance strengthens this flow and steers it towards the Upper Indus Plain, particularly if it originates from a more southern latitude. In cases where the disturbance originates from the north-west, its impact on the moisture flow is limited, since the advected continental dry air drastically limits the precipitation output. The study offers a conceptual framework to study the synoptic activity of western disturbances as well as key parameters that explain their precipitation output. This can be used to investigate meso-scale processes or intra-seasonal to inter-annual synoptic activity.

A three-dimensional numerical analysis of moisture flow in wood and of the wood’s hygro-mechanical and visco-elastic behaviour

A three-dimensional numerical model was employed in simulating nonlinear transient moisture flow in wood and the wood’s hygro-mechanical and visco-elastic behaviour under such conditions. The model was developed using the finite element software Abaqus FEA®, while taking account of the fibre orientation of the wood. The purpose of the study was to assess the ability of the model to simulate the response of wood beams to bending and to the climate of northern Europe. Four-point bending tests of small and clear wood specimens exposed to a constant temperature and to systematic changes in relative humidity were conducted to calibrate the numerical model. A validation of the model was then performed on the basis of a four-point bending test of solid timber beams subjected to natural climatic conditions but sheltered from the direct effects of rain, wind and sunlight. The three-dimensional character of the model enabled a full analysis of the effects of changes in moisture content and in fibre orientation on stress developments in the wood. The results obtained showed a clear distinction between the effects of moisture on the stress developments caused by mechanical loads and the stress developments caused solely by changes in climate. The changes in moisture that occurred were found to have the strongest effect on the stress state that developed in areas in which the tangential direction of the material was aligned with the exchange surface of the beams. Such areas were found to be exposed to high-tension stress during drying and to stress reversal brought about by the uneven drying and shrinkage differences that developed between the outer surface and the inner sections of the beams.

CALCULATION ACCURACY OF EVAPORATION AT DIFFERENT AVERAGING PERIODS BY OBSERVATION DATA IN THE SOUTHERNBALTIC

The evaporation or moisture flow in the boundary layer is one of important components in the ocean-atmosphere interaction system. The most commonly used formulas for calculating evaporation include standard hydrometeorological parameters — atmospheric pressure, wind speed, drop of humidity, and height, at which measurements are made, and the exchange rate of moisture, which also depends on these parameters. The correct method is to calculate moisture flow based on current or hourly parameter values and then average the flow values for the required period: day, month or year. In the absence of current data of observation parameters, flow values should be calculated based on averaged parameter values. In this case, the calculated values of evaporation strongly depend on the method and the averaging period. Using the example of rather long observation series (2002—2016) at stations in the Southern Baltic, a quantitative assessment of the effect of averaging on the calculated values of the moisture flow both for direct calculation from the current observation data and for averaged data is made. The probable explanations are given for a significant variation in the calculated values of evaporation in the Baltic Sea by different authors. The coefficients for the correction of evaporation values obtained from the averaged values of hydrometeorological parameters are calculated. It is shown that averaging the parameters per day is acceptable with almost no loss of accuracy (error no more than 2-4 %). Averaging the values of hydrometeorological parameters over the period of a month leads to an underestimation of the calculated evaporation values by 20-30 %. When averaging over a period of about a year, the error increases to 30-35 %. The conclusions and correction coefficients are valid for calculations of evaporation in the Baltic Sea and can be used in other areas of middle and high latitudes, where the evaporation values do not differ by orders of magnitude (tropics, equator, zones of significant variability of environmental parameters).