Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: Gregory C. Dodd x
  • Refine by Access: All Content x
Clear All Modify Search
Kenneth Sassen and Gregory C. Dodd

Abstract

A mixed-phase hydrometer growth model has been applied to determining the nucleation mode and rate responsible for the glaciation of a highly supercooled liquid cloud studied jointly by ground-based polarization lidar and aircraft in situ probes. The cloud droplets were detected at the base of an orographically induced cirrus cloud at temperatures between −34.3° and −37.3°C. The vertical distribution above cloud base of two independent data quantities, the aircraft-measured water and ice particle concentrations and the lidar linear depolarization ratio, have been compared to model predictions for both the homogeneous and heterogeneous drop-freezing. modes. It is concluded that, although activated ice nuclei may have contributed to the glaciation of the cloud, homogeneous nucleation was the dominant mode. Accordingly, a homogeneous nucleation rate ∼106 times greater than that predicted by classical theory, but ∼103 times less than laboratory measurements would suggest is found to be appropriate at the measured cloud temperatures.

Full access
Kenneth Sassen and Gregory C. Dodd

Abstract

A one-dimensional cloud microphysical model is applied to exploring the basic conditions under which ice crystal nucleation, from the homogeneous freezing of ammonium sulfate haze particles, can occur in cirrus clouds at temperatures ≲ −35°C. Cirrus generating regions maintained by uniform updrafts of 0.1–0.25 m s−1, and an idealized ice crystal precipitation mechanism dependent on vertical wind shear are treated in the model. The findings indicate that ice crystals are generated in a pulse-like fashion as a result of water vapor competition effects from ice crystals nucleated within an updraft, followed by precipitation. Water saturation is not required for ice crystal nucleation at ≲ −35°C, and the relative humidities required at decreasing temperatures gradually decrease. The temperature dependency of the relative humidities associated with ice production does not depend significantly on model inputs, suggesting that cirrus cloud processes follow an adjusted pseudoadiabat, which produces ice mass contents that become increasingly smaller than those possible from a pseudoadiabatic process involving nucleation at water saturation. Finally, to determine whether polarization lidar observations can identify haze particles in cirrus generating regions, as has been suggested by recent studies, Mie scattering simulations were performed for the properties of the model-generated haze particles.

Full access
Kenneth Sassen, Michael K. Griffin, and Gregory C. Dodd

Abstract

The optical and microphysical properties of subvisual cirrus clouds are derived from ground-based polarization lidar, shortwave radiation flux, and solar corona measurements of two ∼0.75 km deep cirrus located near the tropopause. The first cloud produced no visual manifestations under excellent viewing conditions, and the second appeared to be a persistent aircraft contrail that was generally visible except in the zenith direction. Average lidar linear depolarization ratios and volume backscatter coefficients for the two clouds were 0.19 and 0.35, and 0.6 × 10−3 and 1.4 × 10−3 (km sr)−1, respectively. It is estimated that the zenith-subvisual cirrus contained ice crystals of 25 μm effective diameter at a mean concentration of 25 L−1 and ice mass content of 0.2 mg m−3. The threshold cloud optical thickness for visual-versus-invisible cirrus derived from both broadband shortwave flux and 0.694 μm lidar data, is found to be τc ≈ 0.03. Such τ values are comparable to those of 5–10 km deep stratospheric aerosol clouds of volcanic origin and polar stratospheric clouds, which are episodic in nature. Hence, we conclude that if these clouds are a fairly common feature of the upper troposphere, as recent SAGE satellite measurements would suggest, then the impact of natural and contrail subvisual cirrus on the planet's radiation balance may be relatively significant.

Full access
Jan Paegle, Julia N. Paegle, and Gregory C. Dodd

Abstract

The origins of atmospheric states that are non-elliptic for the height constrained balance equation are examined from observational perspectives. Such states are commonly present in the deep tropics in Objectively analyzed data sets. In order to analyze the source of this phenomenon, we compute terms of the divergence equation (from which the balance equation derives) for disturbed periods of the GATE experiment. Meaningful residuals cannot be obtained because they are obscured by observational uncertainty of the geopotential gradients that are calculated from the hydrostatic equation using temperature observations. The geopotential fields recomputed from the divergence equation using observed wind data appear to be much better determined, but they still produce fields that are non-elliptic for the height constrained nondivergent balance equation.

For the convective GATE cases, it appears that the essential balance of the divergence equation is between divergent accelerations, deforming accelerations, the divergent pressure form field and friction, while centripetal accelerations (including the Coriolis is effect) are secondary. Thus, the underlying assumption of solenoidal flow in the balance equation is fundamentally wrong in regions of tropical convection. This appears to be the physical source of the poorly posed balance equation in many non-elliptic cases.

Full access
Thomas T. Warner, Rong-Shyang Sheu, James F. Bowers, R. Ian Sykes, Gregory C. Dodd, and Douglas S. Henn

Abstract

Ensemble simulations made using a coupled atmospheric dynamic model and a probabilistic Lagrangian puff dispersion model were employed in a forensic analysis of the transport and dispersion of a toxic gas that may have been released near Al Muthanna, Iraq, during the Gulf War. The ensemble study had two objectives, the first of which was to determine the sensitivity of the calculated dosage fields to the choices that must be made about the configuration of the atmospheric dynamic model. In this test, various choices were used for model physics representations and for the large-scale analyses that were used to construct the model initial and boundary conditions. The second study objective was to examine the dispersion model's ability to use ensemble inputs to predict dosage probability distributions. Here, the dispersion model was used with the ensemble mean fields from the individual atmospheric dynamic model runs, including the variability in the individual wind fields, to generate dosage probabilities. These are compared with the explicit dosage probabilities derived from the individual runs of the coupled modeling system. The results demonstrate that the specific choices made about the dynamic-model configuration and the large-scale analyses can have a large impact on the simulated dosages. For example, the area near the source that is exposed to a selected dosage threshold varies by up to a factor of 4 among members of the ensemble. The agreement between the explicit and ensemble dosage probabilities is relatively good for both low and high dosage levels. Although only one ensemble was considered in this study, the encouraging results suggest that a probabilistic dispersion model may be of value in quantifying the effects of uncertainties in a dynamic-model ensemble on dispersion model predictions of atmospheric transport and dispersion.

Full access