No description available.

No description available.

No description available.

The dynamics of the atmospheric mesoscale and oceanic submesoscale are characterized by strong stratification and weak rotation, and the resulting energy spectra, though surprisingly universal, are still poorly understood. The aim of this work is to study the nonlinear dynamics and interactions of vortical motion (with potential vorticity) and internal inertia-gravity waves in the idealized context of homogeneous stratified turbulence, and to examine the extent to which they can account for the observations. / We consider separately turbulence generated by vortical motion and internal waves using a combination of theory and numerical simulations. When vortical motion dominates the flow, the statistical mechanical equilibrium of the Boussinesq equations points to the absence of an inverse cascade of vortical energy. Instead, energy leaks into waves and cascades downscale. Our simulations show that the kz spectrum of vortical energy is flat out to kz ~ N/U (where N is the Brunt-Vaisala frequency and U is the root mean square velocity), which is consistent with the asymptotic limiting equations but not with the observations. Steeper spectra are obtained when waves are forced, but they are nevertheless shallower than the observations. At sufficiently strong stratifications, the wave energy spectra are found to be sensitive to the resolution of wave breaking and the presence of vortical motion. Bumpy spectra are obtained when no breaking occurs, but interactions with vortical modes cause the bumps to disappear. Overall, these results indicate that the observed atmosphere and ocean energy spectra are not universal properties of stratified turbulence, and theories for the spectra must take other factors into account. / In both the vortical and wave-dominated cases, U/N emerges as a key vertical length scale. Overturning is generated only when U/N is larger than the dissipation scale. Furthermore, in the vortical case, it is the scale at which different layers are coupled together. When rotation is introduced, the coupling scale evolves from U/N to the quasi-geostrophic scale (f/N)L (where f is the Coriolis parameter and L is the horizontal scale). This transition occurs at a relatively large Rossby number of O(1).

The current practice of snowfall forecasting in Canada is to determine the snow water equivalent (SWE) expected to precipitate using a numerical weather predication model and then multiplying this amount by a snow/SWE ratio to determine the forecast snow depth. The 10:1 "rule of thumb" is still widely used operationally as this ratio, even though it is well-known to introduce error in the forecasts because the density of snow is highly variable. In 2003 Ivan DubE developed a decision tree type algorithm to find the snow/SWE ratio which has subsequently been automated in 2004 by the Meteorological Service of Canada (MSC). The objectives of this study are to explore the behaviour of snow/SWE ratio by developing a Canada-wide climatology of this quantity and examining performance of the MSC algorithm over the winter 2004-2005 using several verification techniques. We found that the mean annual snow/SWE ratio across Canada is 13:1 with large variations temporally and spatially and that the MSC algorithm performed with equal or better skill than the 10:1 algorithm in 84% of the events.

No description available.

We compare the results of two regional atmospheric models, Canadian Regional Climate Model (CRCM) and Mesoscale Compressible Community Model (MC2) at a mesoscale resolution of 51 km, using 5 cases with different synoptic characteristics over the Mackenzie River Basin. CRCM has physics appropriate for coarse resolution typical of climate models, while MC2 is a mesoscale model with fine resolution physics. / The results of both models are largely similar, indicating CRCM physics is able to reproduce mesoscale features. There are however differences. CRCM has stronger and expanded warm and cold bias at low levels compared to MC2. We identified surface initialization and coastal topographic precipitation as two processes responsible for these differences. The underprediction of precipitation over the western slopes of the Rockies is related to the cold bias found at the lee of the mountain barrier in CRCM. Furthermore, our results suggest that the use of different cloud schemes is another factor causing differences between the models.

Statistical downscaling (SD) procedures have been frequently used for assessing the potential impacts of climate change and variability on hydrological regime. These procedures are based on the empirical relationships between large-scale atmospheric variables (predictors) and surface environment parameters (e.g., precipitation and temperature). The present research work is hence concerned with the development of new predictor climate variables that could be used for improving the accuracy of downscaling of daily precipitation process at a local site. The new predictors should be able to provide a more accurate simulation of the local variable since they could describe more accurately the physical characteristics of the precipitation process. In particular, a better reproduction of summer rainfall event is expected through an improved inclusion of main thermodynamic forcings from humidity and stability parameters. / The first part of this study focuses on the re-computation of the geostrophic circulation predictor variables developed by Wilby and Wigley (2000), reconstructed from mean sea level pressure or geopotential heights. The same circulation variables are re-computed from prognostic winds of the National Centre for Environmental Prediction (NCEP) re-analysis data set (Kalnay et al., 1996). Assessment of the performance of the re-computed predictors is carried out using the Statistical DownScaling Model (SDSM), developed by Wilby et al. (2002), and based on a number of climate indices characterizing the frequency, intensity and extremes of daily precipitation process. Two different predictor sets are considered, the first consisting of circulation-only variables, the second including a raw specific humidity predictor. For each predictor set, results obtained from the two computation techniques are compared. Daily precipitation data available at Montreal-Dorval Airport station for the 1961-1990 period were used in this assessment. Results indicated that the re-computation of geostrophic variables for both sets could yield significant improvements in the reproduction of local precipitation characteristics for the validation 1976-1990 period. The most striking improvement can be achieved for winter, as expected from the greater influence of large-scale circulation forcings on precipitation in this season. In the second part, new advection variables are developed based on a generalized omega equation. It is found that the Laplacian of temperature advection and the differential vorticity advection appear as direct forcings of the vertical velocity, strongly correlated with the precipitation process. Precipitable water and atmospheric instability indices are also included in the predictor range, mainly to reach a better simulation of convective precipitation. Next, a new statistical downscaling scheme is developed, combining a Principal Component Analysis (PCA) of the new predictors and the SDSM model. Analysis of the different computed principal components confirms the major role of the two identified advection terms and the humidity/instability predictors. Assessment of the new PCA+SDSM scheme shows significant improvements of the simulation of precipitation intensity, although results are less conclusive regarding the precipitation occurrence. / Finally, the influence of the calibration period length on the new downscaling scheme performance was carried out by comparing the simulation results obtained from two calibration runs of 15 and 30 years of length: for the 1961-1975 period and for the 1961-1990 one. It was found that doubling the calibration period length could lead to significant improvements in the reproduction of the local precipitation characteristics.

In addition to a proper radar calibration, quantitative estimation of precipitation from VHF radars requires the extraction of the precipitation signal out of the Doppler spectra. It also requires the proper conversion of this precipitation signal into a reflectivity factor. / This research develops a multi-faceted approach for the calibration of VHF vertically-pointing radars, by combining a first calibration method that compares the recorded VHF signal to power coming from a noise generator and a second calibration method that compares recorded VHF signal to cosmic radiation. This approach allows the retrieval of antenna and receiver parameters (such as noise levels, efficiency, and gain), and four other equations for the corresponding errors. In addition, we develop an equation for calibrating Doppler spectra. / The analysis is focused on rain observations with VHF radar. We verify the hypothesis that |K|2 = 0.93 for most of the rain observations at VHF band. A signal-processing algorithm for extracting the rain signal out of the VHF power spectra is then presented. This work also derives a general version of the radar equation valid for vertically pointing radars, as well as a particular version of this equation valid for the McGill VHF radar. The study then makes numerical simulations of several profiles of precipitation signal at VHF band, by combining high-resolution profiles of precipitation signal (from a calibrated X-band radar) and the VHF antenna pattern in our general version of the radar equation. The analyses indicate that VHF reflectivity at gates above the melting layer is artificially enhanced by the precipitation signal collected from the side lobes. / This work also studies the effect of precipitation in the scattering properties of clear air. We analyze several cases of stratiform and convective rain, occurring in a continental mid-latitude environment (Montreal, Lat.45.41°N, Long.73.94°W). For these cases, Doppler spectra taken by a VHF vertically-pointing radar were used to retrieve simultaneous co-located values of precipitation intensity (rainrates) and degrees of refractive index fluctuation (structure-function parameter for refractivity turbulence, Cn2). We validated these retrievals using co-located, calibrated measurements of precipitation signal at X-band. The comparison between equivalent reflectivity factors at X and VHF bands agrees within 1 dB. The study includes rainrates between 0.3 and 78 mm/h, and Cn2 values between 10-16 and 10-12 m-2/3 , retrieved from the VHF spectra at 2.5 km height. The study finds that the occurrence of rain is associated with distinctive changes in the structure of air refractive index fluctuations, and that these changes are of a turbulent nature for the most intense rainrates.

Using the reflectivity and Doppler data of the Carvel radar, located near Edmonton, Alberta, for the summer of 2000 to 2004, we produced a mesocyclone climatology for a region within 120 km from the radar using the mesocyclone detection algorithm of the McGill Radar data Analysis, Processing and Interactive Display (RAPID) software system. The Upper level Vertically Integrated Liquid water content (UVIL) algorithm of the same software package has been used to detect strong convection. Two datasets were built. The first one consists of dates when mesocyclones occurred while the other includes dates characterized by strong convection but without mesocyclonic activity. A synoptic-scale analysis is conducted to identify the main differences between the atmospheric circulations of the two datasets. / The upper-tropospheric flow associated with the mesocyclonic events shows a highly amplified meridional circulation. A strong trough-ridge couplet is evident 48 hours prior to the event. This atmospheric feature is responsible for the development of a vorticity maximum that is advected into Alberta. Lee cyclogenesis in noticeable on the sea-level pressure field associated with mesocyclone activity along with a significant low-level warm temperature input in the region of study. The corresponding atmospheric patterns associated with non-mesocyclonic events do not present such large-scale precursors. The different atmospheric fields act to favour large-scale forcing for ascent when mesocyclones are going to occur.