The absorption and albedo of low, thin, marine stratus clouds are investigated using data collected by two aircraft during the Radiation, Aerosol and Cloud Experiment, conducted off the coast of Nova Scotia, Canada. Measured values of absorption and albedo are compared to calculations using a plane parallel radiative transfer model and drop size distributions determined from cloud profiles conducted close in time to the radiation measurements. The calculated values of albedo and absorption were consistent with the measurements. Uncertainties in absorption measurements due to instrument error were high. The model is used to determine cloud susceptibility, the sensitivity of cloud albedo to changes in droplet concentration. The susceptibilities determined from these calculations agree well with previous studies. The measured changes in albedo are consistent with those predicted by the model.

Daily 1200 UTC sea-level pressure, 850-hPa height and temperature, and 500-hPa height fields are used in a compositing analysis to document the synoptic evolution of wintertime precipitation events over Montreal. Since there has been a bias in past studies towards analyzing extreme events, four precipitation intensity categories are defined. A summary of the findings are as follows: (i) Heavier precipitation events are associated with a ridge-trough couplet as opposed to a trough for lighter events. The magnitude of the downstream ridge is influenced by the strength of warm advection at the 850-hPa level. (ii) Differences in the intensity of precipitation can be related to preferred climatological storm tracks and possible moisture inflow from the Atlantic Ocean, the Gulf of Mexico, and the Great Lakes. (iii) A negative PNA pattern is associated with winter events lightest in magnitude.

A comprehensive DMS oxidation reaction mechanism was added to a chemical box model (Model of Chemistry Considering Aerosols - MOCCA) to investigate how atmospheric sulfate levels are affected by pollution in the marine boundary layer. The interaction of a remote air mass with a polluted coastal air mass containing elevated NOx, alkane, and alkene levels was simulated for a mid-latitude region of the Northern Hemisphere. Model runs from different settings were compared with respect to species concentrations, branching ratios, and product yields. Results showed moderate decreases in gas phase DMS, DMSO, DMSO2, and MSA concentrations, and increases of approximately 7% and 100% for SO2 and H2SO4 , respectively. A sensitivity analysis was completed to determine how variations in temperature, DMS emission, NO emission, aerosol characterization, accommodation coefficients, PAN reactions, and a background SO2 emission influence the results.

The relationship between precipitation and atmospheric vertical motion is investigated over the globe. By combining and averaging precipitation rates within small ranges of vertical motion, the mean precipitation rate is found to vary smoothly with vertical motion. The relationship is modelled with a simple function that assumes zero precipitation for subsidence and linearly increasing precipitation for ascending motion. Function parameters are computed from NCEP reanalysis data individually for every grid point and every calendar month. Variations in the slope account for geographical and seasonal variations in moisture and other precipitation factors. / At each grid point, the scheme diagnoses precipitation rate from a single concurrent value of vertical motion. It is shown to have moderate skill over climatology in predicting mean monthly precipitation. Midlatitudes, oceans and the winter season are favoured. The scheme is designed to extract precipitation fields from a dry GCM. Application to climate modeling is discussed.

We adapt the theory of Empirical Normal Modes (ENMs) to diagnose gravity waves generated by a relatively high resolution numerical model solving the primitive equations. The ENM approach is based on the Principal Component Analysis (which consists of finding the most efficient basis explaining the variance of a time series), except that it takes advantage of wave-activity conservation laws. In the present work, the small-amplitude version of the pseudoenergy is used to extract from data quasi-monochromatic three-dimensional empirical modes that describe atmospheric wave activity. The spatial distributions of these quasi-monochromatic modes are identical to the normal modes of the linearized primitive equations when the underlying dynamics can be described with a stochastic linear and forced model, thus establishing a bridge between statistics and dynamics. We use this diagnostic method to study inertia-gravity wave generation, propagation, transience, and breaking over the Rockies, the North Pacific, and Central America in the troposphere-stratosphere-mesosphere GFDL SKYHI general circulation model at a resolution of 18° of latitude by 1.2° of longitude. Besides the action of mountains in exciting orographic waves, inertia-gravity wave activity has been found to be generated at the jet stream level as a possible consequence of a sustained nonlinear and ageostrophic flow. In the Tropical region of the model, the "obstacle effect" has been found to be the major source of inertia-gravity waves. A significant proportion of these inertia-gravity waves was able to reach the model mesosphere without much dissipation and absorption.

The goal of this study is to improve knowledge about how cloud inhomogeneities affect the reflection of solar radiation. In particular, it addresses two main questions: what the processes through which inhomogeneities influence solar reflection are, and how this influence can be taken into account in albedo retrievals based on future satellite measurements. / The first question is important, since present methods give numerical results only about the overall radiative effect of cloud inhomogeneities, but cannot determine the degree to which various mechanisms are responsible for this overall effect. This study establishes a theoretical framework which defines and evaluates the various processes through which cloud inhomogeneities influence solar radiation. This framework is then used to examine quantitatively the inhomogeneity effects that occur in irregular cloud fields. Among other insights, it is shown and explained that identical variations in cloud optical thickness can cause much stronger inhomogeneity effects if they are due to variations in geometrical cloud thickness, and not in volume extinction coefficient (as assumed in previous studies of irregular cloud fields). The differences in albedo can exceed 0.05, and the relative differences in reflectance toward the zenith can be greater than 25% for overhead sun, and 50% for oblique sun. Also, a possible explanation is given for a phenomenon observed in previous studies: that cloud reflectivity toward the zenith increases with decreasing solar elevation. / This study also develops an albedo retrieval algorithm that considers radiative inhomogeneity effects. The algorithm takes advantage of the unique multi-view capability of the Multi-angle Imaging SpectroRadiometer (MISR) placed on the Earth Observing System-AM satellite (to be launched in 1998). This instrument will offer new possibilities for albedo retrievals since, unlike present instruments, it will measure the radiation reflected in not only one, but nine directions. The potential accuracy of the algorithm is analyzed for a dataset obtained by using a Monte Carlo model to simulate radiative transfer through a large number of irregular cloud fields. The results indicate that using multi-view measurements can improve the accuracy of satellite-based albedo retrievals by a factor of three or more.

A study of Kalman filtering in atmospheric data assimilation is presented. Our research aims at an understanding of the physical and statistical mechanisms as well as the principles underlying its application to atmospheric data assimilation. Both the continuous and the discrete formulations of the filter were considered. Using nonlinear advection diffusion dynamics, a number of aspects in data assimilation were addressed, often by exploring the parameter space or by performing Monte Carlo simulations. The filtering properties, the spatial regularity and indirect inference about the model error covariance were examined with a discrete linear Kalman filter. The dynamics of the mean, variance, and correlation of forecast errors for Burgers' equation were studied. The validity of the tangent linear approximation for Burgers' equation was examined. An ensemble of realizations of the extended Kalman filter has permitted a statistical investigation of its performance, errors and limit of validity.

This study examines the large-scale fields of eight stronger and eight weaker cyclones prior to rapid cyclogenesis to help explain their subsequent deepening rate. The cyclones all commenced their rapid development within the same small geographical region over the Kuroshio and are compared using composites, ensembles and individual case studies. / Colder air and stronger anticyclogenesis over East Asia track the stronger cases along the Kuroshio, while the weaker cases tend to track across Japan. Anticylogenesis is also present to the east of the stronger cases which hence have warm advection ahead of them. The weaker cases generally form as transitory features in the northerly flow to the west of large cyclones and so have cold advection ahead of them. This affects a deep layer, producing a ridge-trough couplet at 500 mb for the stronger cases and a more zonal flow above the weaker cases.

An X-band transmitter ($ lambda$ = 3,2 cm) should perform better than an S-band transmitter ($ lambda$ = 10,4 cm) in the remote measurement of snowfall by radar. Comparisons between groundtruth measurements and their radar measured counterparts are done for two similar snowstorms (November 20th and 21st 1986, January 30th and 31st 1987), scanned respectively with an S-band and an X-band transmitter, mounted on the 10 m wide antenna of the Weather Radar Observatory of McGill University; these observations confirmed the hypothesis. A mixed rain-snow precipitation event was also observed on February 21st 1989, using alternatively the X-band and the S-band transmitter on the same antenna during the course of the event; this allowed an almost simultaneous comparison of the transmitters. Modifications to the regular sequence of observation of the weather radar during winter are proposed to take advantage of the potential of the X-band transmitter in the measurement of snowfall.

A two-dimensional anelastic model is used to study slantwise convection. The initial atmosphere is specified to be statically, inertially and symmetrically stable for dry convection. By varying the distribution of environmental relative humidity, a moist basic state potentially stable for upright convection but unstable for slantwise convection was obtained. The conditional symmetric instability (CSI) is released by specifying an initial streamfunction impulse. / The results show a typical slantwise convective circulation. A cloudband extends along the updraft of the CSI circulation. An energy budget analysis shows that the main source of perturbation kinetic energy comes from the vertical shear of the basic state flow, although the buoyancy term is important during the initial onset stage. It is also found that the CSI circulation induces both upright convective unstable layers and horizontal inertial unstable layers. / Although the decay stage has already been reached at the end of simulation, a neutral state with zero moist potential vorticity was not achieved. Our results also show that a high vertical resolution is necessary for the simulation of moist symmetric instability.