Showing papers on "Convective mixing published in 1997"

Transport of 222radon to the remote troposphere using the Model of Atmospheric Transport and Chemistry and assimilated winds from ECMWF and the National Center for Environmental Prediction/NCAR

Abstract: The Model of Atmospheric Transport and Chemistry (MATCH) is used to simulate the transport of 222Rn using both European Centre for Medium-Range Weather Forecasts (ECMWF) winds and National Center for Environmental Prediction/National Center for Atmospheric Research (hereafter referred to as NCEP) reanalysis winds. These winds have the advantage of being based on observed winds but have the disadvantage that the subgrid-scale transport processes are not routinely archived. MATCH derives subgrid-scale mixing rates for the boundary layer using a nonlocal scheme and for moist convective mixing using one of two parameterizations (Tiedtke [1989] or Pan and Wu [1997]). This paper describes the ability of the model to recreate mixing rates of 222Rn using the forecast center winds. Radon 222 is a species with a continental crust source and a simple sink involving radioactive decay with an e-folding timescale of 5.5 days. This atmospheric constituent is therefore a good tracer for testing the vertical transport in the chemical transport model, as well as the horizontal transport from continental regions to remote oceanic regions. The various simulations of 222Rn are compared with observations as well as with each other, allowing an estimate of the uncertainty in transport due to uncertainties in the winds and subgrid-scale processes. The calculated vertical profiles over the western United States are somewhat similar to observed, and the upper tropospheric concentrations compare reasonably well in their spatial distribution with data collected during Tropospheric Ozone II (TROPOZ II), although the model values tend to be higher than observed values, especially in the upper troposphere. The model successfully simulates specific observed pollution events at Cape Grim. It has more difficulty at sites farther from continental source regions, although the model captures the seasonal structure of the pollution events at these sites (Macquarie Island, Amsterdam Island, Kerguelen Island, and Crozet Island). Inclusion of a moist convective mixing scheme in MATCH increases 222Rn concentrations in the upper troposphere by 50% compared to not having moist convective mixing, while surface concentrations do not appear to be very sensitive to moist convection. In addition, differences between the upper tropospheric concentrations of radon predicted using the ECMWF and NCEP winds can be 30% for large areas of the globe, due to either differences in the forecast center winds themselves or the moist convective mixing schemes used in conjunction with them. This has implications for model simulations of radiatively and chemically important trace species in the atmosphere.

Convection in ice-covered lakes: effects on algal suspension

Abstract: Convection occurs in ice-covered lakes if solar radiation warms near-surface water from the freezing point towards the temperature of maximal density. One effect of convective mixing may be to suspend non-motile phytoplankton in the upper water column, providing cells with enough light for growth during ice-covered periods. Observations of the diatom Aulacoseira baicalensis under the ice cover of Lake Baikal, Siberia, support the hypothesis that convective mixing causes net suspen- sion of cells. This paper presents a theoretical examination of the conditions under which convective flow fields can suspend algae in the photic zone of the upper water column. It is shown that the efficiency of algal suspension depends on the ratio of the still-water algal sinking rate, Wp, to con- vective updraft speed, Wu. The suspension efficiency is also shown to be affected by asymmetries in the flow field and night-time cessation of convection, but only if Wp and Wu are comparable in value. It is concluded that convection in Lake Baikal should be vigorous enough to increase the mixed-layer residence time of A.baicalensis from a few days to over a month, at least during years with thin snow

Chlorofluorocarbon uptake in a World Ocean model: 2. Sensitivity to surface thermohaline forcing and subsurface mixing parameterizations

Abstract: Part 1 of this study [ England et al., 1994] examined the sensitivity of simulated oceanic Chlorofluorocarbon (CFC) to changes in the way the air-sea gas exchange rate is parameterized in a World Ocean model. In part 2 we consider more closely the role of surface thermohaline forcing and subsurface mixing parameterizations in redistributing CFC-11 and CFC-12 in the ocean. In particular, a series of five different model ocean experiments are forced with the same air-sea CFC flux parameterization. The five cases include (1) a control run with a standard seasonal cycle in surface forcing and traditional Cartesian mixing, (2) a run in which the production rate and salinity of Antarctic Bottom Water (AABW) is increased, (3) a run in which the production, outflow rates, and density of North Atlantic Deep Water (NADW) is increased, (4) a run with enhanced isopycnal mixing of passive tracers, and finally (5) a run in which the effects of eddies on the mean ocean flow are parameterized following Gent and McWilliams [1990]. The simulated CFC uptake in the Southern Ocean far exceeds observations in the first four experiments. The excessive uptake is linked to the poor model simulations of Southern Ocean deep water masses, where, for example, model Circumpolar Deep Water is typically 0.2 to 0.4 kg m3 too buoyant. The insufficient density of the deep water allows for extensive penetration of convective adjustment to great depth during winter, in contrast to observations, and this results in excessive downward mixing of the CFC-enriched surface waters. Compared with the control experiment, the Southern Ocean CFC uptake is reduced in the cases with increased AABW salinity and NADW density, as a result of slightly higher deep water density and reduced wintertime convection in those experiments. Nevertheless, CFC uptake in the Southern Ocean still substantially exceeds observed ocean CFC content in the adjusted surface forcing cases. The most extreme uptake occurs in case 4, where, in addition to deep convective mixing of CFC, there is also mixing into the ocean interior along isopycnal surfaces having an unrealistic orientation. The Southern Ocean CFC uptake in case 5, using the mixing scheme of Gent and McWilliams [1990], is dramatically reduced over that in the other runs. Only in this run do deep densities approach the observed values, and wintertime convection is largely suppressed south of the Antarctic Circumpolar Current. Deep penetration of CFC-rich water occurs only in the western Weddell and Ross Seas. This run yields CFC sections in the Southern Ocean which compare most favourably with observations, although substantial differences still exist between observed and simulated CFC. The simulation of NADW production is problematic in all runs, with the CFC signature indicating primary source regions in the Labrador Sea and immediately to the southeast of Greenland, while the Norwegian-Greenland Sea overflow water (which is dominant in reality) plays only a minor role. Lower NADW is insufficiently dense in all runs. Only in the run with surface forcing designed to enhance NADW production does the CFC signal penetrate down the western Atlantic boundary in a realistic manner. However, this case exhibits an unrealistic net ocean surface heat loss adjacent to Greenland and so cannot be advocated as a technique to improve model NADW production. Conventional depth sections and volumetric maps of CFC concentration indicate that on the decadal timescales resolved by CFC uptake the dominant determining factor in overall model ventilation is the choice of subsurface mixing scheme. The surface thermohaline forcing only determines more subtle aspects of the subsurface CFC content. This means that the choice of subgridscale mixing scheme plays a key role in determining ocean model ventilation over decadal to centennial timescales. This has important implications for climate model studies.

An evaluation of deep convective mixing in the Goddard Chemical Transport Model using International Satellite Cloud Climatology Project cloud parameters

Abstract: The simulation of deep convective mixing in the Goddard Chemical Transport Model (GCTM) is evaluated by comparing 1990-1992 distributions of upper tropospheric convective mass flux and cloud top pressure from the Goddard Earth Observing System data assimilation system (GEOS-1 DAS) with deep convective cloud fields from the International Satellite Cloud Climatology Project (ISCCP). Deep convective mixing in the GCTM is calculated using convective information from the GEOS-1 DAS. Therefore errors introduced when deep convection is parameterized in the GEOS-1 DAS affect the distribution of trace gases in the GCTM. The location of deep convective mixing in the tropics is fairly well simulated, although its north-south extent is overestimated by >5°. The frequency of deep convective mixing also appears to be overestimated in the tropics, resulting in GCTM-calculated upper tropospheric concentrations of carbon monoxide in the tropics that are larger and less variable than those observed. The spatial extent of deep convective mixing in the subtropics is overestimated at several locations including the Caribbean throughout the year and the South Pacific Convergence Zone during June-August. The extent of deep convection is underestimated over midlatitude marine storm tracks. DAS-calculated cloud top pressures differ from ISCCP cloud top pressures by less than one-half a GCTM layer (35 hPa) at most longitudes in the tropics; however, cloud top pressures are overestimated by more than 35 hPa (i.e., the vertical extent of deep convection is underestimated) over wintertime midlatitude storm tracks and the Indian Ocean and underestimated by more than 35 hPa at locations that include the Gulf of Mexico during December-February and central South America during June-August.

Analysis of Mixing Process for Wet Particle System in a Double Blade Batch Kneader Mixer.

Abstract: Kneading with a double-blade batch kneader mixer for wet particle systems combines both the convective and dispersive mixing processes. In this paper, we show that the mixing phenomena in the kneader can be analyzed by a mathematical model. Namely, for analysis of this mixing process, we derived an analytical equation considering both convective and dispersive mixing with the following three dimensionless numbers. The first dimensionless number represents the convective mixing of wet particle systems, which is the dimensionless interchange amount between two kneading vessels. The second dimensionless number represents dispersive mixing, which is advanced by compression and the rolling effect between the wall and the rotating blades. The last dimensionless number indicates the contribution of dispersive mixing to the total mixing. To determine each dimensionless number value, we also derived an empirical equation using only the rheological properties of the wet particle systems, namely, plastic viscosity, μ 0 , and yield stress, τ 0 . We ascertained that the analytical equation proposed in this study can estimate the mixing phenomena with high precision by verifying the estimated values with experimental results for various kinds of wet particle systems in a batch kneader with sigma blades.

Mixing, Patchiness and Sub-Mesoscale Dynamics in the Coastal Zones.

Abstract: : During the contract period the work was performed on the following topics: Description and modeling of the vertical thermohaline and turbulent structure formed by wind induced and convective mixing on shallow sea shelves. Detailed analysis of microstructure and turbulent measurements at the Black Sea shelf in order to study the origin and decay of stratified turbulent patches. Various scaling arguments that are commonly used for stratified turbulent patches as well as certain predictions on marine fossil turbulence were investigated in this work. Evaluation of the effects of boundary mixing in wakes behind small islands on heat flux enhancement in the upper layer of equatorial Pacific. A series of computer programs were developed to process the field experimental data of the first two cases. A numerical model of vertical turbulent mixing was applied to coastal waters affected by active atmospheric forcing.

Studies on Convection in Polar Oceans

Abstract: : A laboratory experimental program was carried out to investigate fundamental physical processes related to deep-ocean and under-ice convection occurring in high latitude oceans. With regard to deep convection, the aspects of interest were the preconditioning of a stratified region prior to the onset of convection, breakdown of stratification leading to turbulent convection, growth of convective layer against stable stratification, scales of convection, lateral processes leading to horizontal buoyancy exchanges and the final collapse of deep-convective regions. Studies on convection under an ice cap included the formation and melting of ice due to surface cooling of a two-layer stratified fluid. This problem is rich in a variety of physical processes such as double-diffusive transports of heat and salt and turbulent mixing across the pycnocline that separates the two layers. Important new mechanisms related to above-described convective processes were delineated and simple parameterizations were proposed to represent convective events in numerical models.