1996

Radiative Transfer in Relation to Cloud Studies

Andreas Macke, Columbia University and NASA GISS

4:30pm, Thursday, 26 September 1996

Keywords Cirrus clouds, inhomogenous clouds, solar radiative transfer, ice crystals

Abstract A number of national and international programs such as the International Satellite Cloud Climatology Project ISCCP, the European Clouds and Radiation Experiment EUCREX, or the First ISCCP Regional Experiment FIRE address the role of clouds in atmospheric radiative transfer. Current problems in radiative transfer modeling, remote sensing, and data-analysis for various cloud types as studied within the framework of these projects are discussed. In particular problems associated with nonspherical ice particles and with spatial cloud structures are investigated in more detail.

Recent results in theoretical and observational studies of light scattering by atmospheric ice crystals are summerized. It is shown how they affect direct and indirect remote sensing of cirrus clouds. It appears that a simple model for randomized polycrystals simulates very well the solar radiative properties of real ice clouds containing complex shaped particles.

We discuss different methods of realizing three-dimensional inhomogenous cloud structures in radiation models. The corresponding radiative properties based on Monte Carlo radiative transfer calculations are compared to the widely used idealization of homogenous plane-parallel clouds.

A Direct Inverse Method in FEM: Its Application in Observation Array Design and Data Assimilation

Zhigang Xu, Bedford Inst. of Oceanography

4:30pm, Thursday, 3 October 1996

Keywords Direct inverse method; linear shallow water equations; FEM; SVD; projection of the data onto dynamics; smoothing from null space

Abstract A direct inverse method is developed for observation array design and data assimilation. Based on the finite element method (FEM), the method first converts the so called ``stiffness matrix'' into a weight matrix, so that any interior solution can be explicitly expressed as a weighted average of the boundary values. The weight matrix approach bridges a gap between the numerical modeling and the statistics. The data assimilation can be cast as a regression problem, and the statistical properties, such as goodness-of-fit and confidence regions, can be assigned to the estimated solutions. Visualization of the weight matrix provides a Green-function map to frequency-dependent linearized shallow water equations, which casts much insight into the dynamics in a realistic geometric setting.

The weight matrix is singular, which implies there are infinite sets of boundary conditions fitting the ``DATA'' equally well. The singular value decomposition (SVD) technique is employed to yield a particular solution, which is both minimum misfit and least potential energy. Besides the particular solution, the SVD also produces several factor matrices of the weight matrix. The factor matrices are useful for different purposes: the range space matrix can be used to project the data onto the dynamics, which allows for a prior estimate whether the model and the data are compatible; the null space matrix can be used for making a smoothing device to smooth the SVD solution. The added smoothness will not affect the achieved goodness-of-fit between the model and the data, but may improve the solutions at locations where the observation points have a poor control, especially near the boundaries.

The method is mathematically less complicated, physically informative, and yet not less powerful. It may assimilate the elevation, velocity, and transport data, simultaneously or separately, in a 3D sense.

Influence of the wind on the motion of surface drifters: Application of a data assimilative model to the outer Scotian Shelf

Darryl Williams, Dalhousie

4:30pm, Thursday, 10 October 1996

Keywords drifters, data assimilation, Stokes drift, adjoint method, Ekman spiral, Ekman drift, wind drift layer, rotary spectra

Abstract The dynamics of the upper meter or so of the ocean are complicated by the interaction of current shear, wind shear and the action of waves. An effective model for the motion of surface drifters should take into account: (i) Stokes drift, a small current caused by surface gravity waves; (ii) the leeway effect, due to wind acting on exposed areas of the drifter; (iii) motion in the wind drift layer, which consists of a 5cm thick surface sublayer above a logarithmic sublayer of about one meter thickness.

Drifter trajectories were observed on the outer Scotian Shelf in May 1993. Three types of surface drifters were deployed: one was drogued at 20m, the other two were free floating and had different drafts and mast areas.

To quantify the various components of the drifter velocities, the water column was partitioned into separate layers leading to the following expression for drifter velocity:

Here, is the background flow and is the velocity in the Ekman layer. The remaining subscripts refer to the surface corrections for Stokes drift, the leeway effect, and the logarithmic sublayer (the effect of the surface sublayer was assumed negligible). The calculation of , and is straightforward and based on physical principles. The background flow (below 20m) was modelled using an adjoint method of data assimilation. was modelled statistically, using complex regression. The background flow correction gave the greatest reduction in the variance of the drifter velocities. The variance of the error was 53% of the observed variance. Subtracting the surface correction factors further reduced the residual variance to 45%. Complex regression of the corrected drifter velocity series on the wind stress reduced the residual variance even further, to 37%. Of the surface correction factors, the leeway effect and the logarithmic layer effect resulted in velocities that agreed most closely with Ekman theory.

Black carbon: atmospheric measurement and radiative effect

Linghon Kou (Dalhousie)

4:30pm, Thursday, 17 October 1996

Keywordsblack carbon; field measurement; radiative effect

Abstract Black carbon is an important constituent of the atmospheric aerosol because of its high specific absorption of solar radiation. To evaluate the role of black carbon on the earth-atmosphere radiative balance, information concerning the black carbon distribution throughout the atmosphere is needed. Atmospheric black carbon concentrations have been measured in air, cloud, rain and snow samples collected over Nova Scotia during Radiation, Aerosol, Cloud Experiment (RACE) of 1995. The thermal-optical method was employed in the laboratory to determine the black carbon amount for each individual sample.

The measurement results and the possible radiative effect of black carbon aerosols will be presented. 

Questions on Using U10 Instead of U* to Drive Oceanic Models

Liangming Wang (BIO)

4:30pm, Thursday, 24 October 1996 

Light scattering from conglomerates

Gordon Videen (candidate for new Atm. Sci. position)

4:30pm, Thursday, 31 October 1996

NOTE: special location, Psychology wing of LSC, room 5260

Abstract Although light scattering from atmospheric particles like ice crystals and aerosol particles is often modeled using homogeneous Mie spheres, this approximation is often very poor. Several techniques have recently been developed and employed to improve the estimation of the scattering characteristics of irregularly shaped particles. Multipole expansions are especially applicable to the atmospheric sciences because many of the conglomerate systems of interest are at least partially composed of spherical subsystems. Ongoing research includes examination of gross scattering characteristics used in radiative transfer calculations, comparisons with effective medium approximations, and inversions of scattering signals. 

The Radiative Forcing of Sulfate Aerosols

Jeff Wong (Dalhousie)

4:30pm, Thursday, 7 November 1996

Abstract With the use of a simple thermodynamic model, the evolution of a size distribution of sulfate aerosol was determined as a function of relative humidity. Once the size distribution and composition of the aerosol were known, the optical properties such as the extinction coefficient, single scattering albedo, and asymmetry parameter were then calculated. Simple parameterizations of these relative humidity dependent optical parameters were developed which were used in a radiation model to obtain estimates of the radiative forcing of sulfate aerosol. In this talk, the results of these calculations are presented.  

Parameterizing Sub-Grid Radiative Properties of Oceanic Boundary Layer Clouds for General Circulation Models: Practical Considerations

Howard Barker (Cloud Physics Research Division, AES-Downsview)

4:30pm, Thursday, 14 November 1996

Abstract All climate and NWP models treat all clouds as though they were plane- parallel and homogeneous. This is obviously incorrect and can impose systematic biases on either radiative or atmospheric hydrologic budgets. As long as these models have horizontal resolutions greater than 10's of kms, there will be a need to parameterize sub-grid cloud structures and create corresponding techniques for computing radiative fluxes.

This seminar presents evidence from satellite data and cloud resolving models showing that frequency distributions of liquid water path or optical depth for oceanic boundary layer clouds often follow a gamma distribution. This is convenient and leads to tractable parameterizations for computing both shortwave and longwave radiative fluxes based on the independent pixel approximation. There are, however, some important practical issues that must be reconciled when attempting to account for sub-grid cloud variability in 1D models. These include the problem of cloud overlap, which seems simple but evidently has received only limited consideration, as well as problems associated with having to work within the framework of vector computers. 

Forecasting coastal circulation using an approximate Kalman filter based on dynamical modes

Michael Dowd, Dalhousie

4:30pm, Thursday, 21 November 1996

Keywords Kalman filter; shelf circulation; data assimilation; ocean prediction

Abstract An approximate Kalman filter for nowcasting and forecasting of coastal ocean circulation is presented. Reduction in the effective dimension of the ocean model, and consequently the Kalman filter, is achieved by reformulating the original model in terms of its dynamical modes. A subset of the modes preferentially excited by the model forcing is chosen as the basis for a reduced ocean model. Solving the Kalman filter equations in this reduced dimension modal space retains the important components of the dynamics necessary for model forecasts and error propagation as well as allowing for a computationally efficient means to implement this data assimilation scheme.

The approximate Kalman filter was applied to a prototype model of the Scotian Shelf off Canada's east coast. This limited-area model is based on the linearized, depth-averaged shallow water equations. The dominant modes were identified for both wind and boundary forcing leading to an approximately 90% reduction in the dimension of the system. Synthetic data based on both fixed (coastal sea level and current meter) and moving (ship ADCP) observation arrays were used to test the performance of the filter. 

Response of Lower Stratospheric NOx to Perturbations from Subsonic Aircraft Emissions

Jacquie Witte, Dalhousie

4:30pm, Thursday, 28 November 1996

Abstract NOx concentrations have been observed to be higher within the dense flight corridors of North Atlantic, as compared to background concentrations. Within flight corridors, multidimensional models predict that NOx aircraft emissions are suffuciently large to enhance NOx levels in the lower stratosphere. It is therefore important to consider the effects of these emissions from aircraft at various cruise altitudes, and its chemical transformations.

Past studies designed to test photochemical theory have had varying degrees of success in reconciling NOx measurements with model predictions. One objective is to determine whether or not the origin of these deviations can be more clearly identified. Here, we attempt to estimate the contribution of subsonic aircraft emissions of NOx to observed concentrations. 

TBA

Author: TBA

4:30pm, Thursday, 5 Dec 1996

Abstract 

Floods and icebergs: postglacial events off Nova Scotia around 14 ka.

Ken Skene (Dal/BIO) and David Piper (BIO)

4:30pm, Thursday, 12 December 1996

Keywords ?

Abstract ? 

Energy transfers and resonance in gravity-wave instability: Two sides of a (wobbly) coin

Len Sonmor (Dalhousie) and Gary Klaassen (York)

4:30pm, Thursday, ? ? 1997

Abstract It is generally accepted that atmospheric gravity waves (buoyancy waves) generated at low levels provide the large mesospheric drag required to balance the global non-radiative-equilibrium temperature distribution that has been recognized for many years. Wave instability is thought to play a role in this process, and in establishing the nearly universal spectrum observed in both atmosphere and ocean. Despite its importance, there remains much to be learned about gravity-wave instability. Even the basic mechanisms are not well understood. Some treatments have focussed on the energy transfers, while "freezing" the wave motion, to find vertical and slantwise static and dynamic instabilities. Others have focussed on wave periodicity to find resonant and parametric instabilities. The cumulative picture is a large assortment of independent and competing instabilities; which one dominates for a given wave is not well specified.

In an attempt to improve the understanding, we use a Floquet model that preserves the wave periodicity, tilt, and time dependence, in conjunction with an energy budget analysis to characterize the energy-transfer mechanism, and a mode-tracing technique that follows the continuous evolution of a single instability solution as the wave amplitude and frequency vary. Our study has revealed relationships among various instabilities often thought to be independent. The dominant instabilities of breaking (overturning) gravity waves are directly related to the resonant and slantwise instabilities of small-amplitude waves. At the same time, wave tilt, time dependence, and spatial periodicity modify profoundly the form of the instability, compared to its vertical or frozen counterpart. These characteristics help to understand features observed in noctilucent clouds, and found in numerical simulations. We propose a new classification of gravity-wave instability, by mapping out the dominant mode over the relevant ranges of wave amplitude and frequency.