Current Research Projects

Sub-grid scale terrain variations (with horizontal scale from 10m to 100km) can exert significant drag on the atmospheric boundary-layer flow over them and can consequently have a siginificant influence on large-scale atmospheric flows. To incorporate this sub-grid scale effect in regional and global weather and climate models, a parameterization of small-scale topographic impacts is needed. This can, in principle, be achieved by adjustments to drag coefficient or by evaluating an effective roughness length.

We use the mixed spectral finite difference (MSFD, Ayotte et al. 1994), the non-linear extension of MSFD (NLMSFD, Xu et al 1994) and finite difference models as tools to investigate the impacts of small-scale topography on drag and effective roughness. We find in the neutrally stratified situation that drag varies quadratically with the slope of the underlying surface (Xu and Taylor 1995). Work associated with stably stratified atmosphere are underway and progressing.

References
Ayotte, K.W., Xu, D. and Taylor, P.A., 1994, 'The impacts of turbulence closure on predictions of the mixed spectral finite difference model for flow over topography', Boundary-Layer Meteorology, 68, 1-33.
Xu, D., Ayotte, K.W. and Taylor, P.A., 1994, 'Development of a non-linear mixed spectral finite difference model for turbulent boundary-layer flow over topography', Boundary-Layer Meteorology, 70, 341-367.
Xu, D. and Taylor, P.A., 1995, 'Boundary-Layer parameterization of drag over samll scale topography', Quarterly Journal of Royal Meteorological Society, 121, 433-443.

The project has two principle objectives. The first is to model planetary boundary-layer structure under a range of conditions appropriate to Northern Canada and to test these models against field observations. Conditions considered will include strong winds with blowing snow and springtime snowmelt situations and would initially involve 1-D (z), time dependent models. Particular attention will be paid to sublimation under blowing snow situation and the determination of evaporation/ablation from wet or dry snow covered surfaces. A second objective is to couple models of boundary-layer flow over complex terrain with models of blowing snow to parameterize moisture and other fluxes over heterogeneous terrain with emphases on the snow redistribution and the effects of heterogeneity on evaporation.

Meteorology plays an important role in ozone formation and transportation. As a result, the substantial variations in meteorological conditions (on all time scales) can exert such large impacts on ozone concentrations that they may mask any long term trends in ozone that could reasonably be traced to changes in precursor emissions such as NOx ans VOC.
We have developed a regression model with variables including a linear trend, seasonal variations (i.e. an annual cycle), meteorological parameters (temperature, sunshine hours and relative humidity) and a categorical variavle. By comparing the trends obtained with and without considering meteorological impacts, we are able to estimate the meteorological effects. Moreover, with this regression model we can identify the ground level ozone associated with changes in precursor emissions (Xu et al. 1996).