Roscoe R. Braham

What is the Role of Ice in Summer Rain-Showers?

Abstract Recent observations indicate that ice pellets and snow pellets are present in most convective clouds in the Central United States by the time these clouds reach top temperatures of −10C. The attendant circumstances raise the question of whether the ice plays an active role in rain development in these clouds or whether its presence is purely incidental. The ice pellets are usually preceded by the development of liquid precipitation particles large enough to produce rain by coalescence with cloud droplets. The pellet concentrations are not related to ground-level ice nuclei concentrations. Apparently the pellets form as a result of freezing of the drops, contrary to most laboratory studies of droplet freezing. Observations can be brought into harmony by invoking the droplet splintering measurements of Mason and Maybank. The presence of numerous small ice particles in these clouds at temperatures warmer than −10C casts doubt upon the value of seeding with ice nuclei for rain inducement.

The Role of the Winter Land Breeze in the Formation of Great Lake Snow Storms

Abstract Radar and aircraft data are presented, in the form of case studies, for three shoreline-parallel snow bands that occurred over Lake Michigan. In all three cases a winter land breeze from one or both shores is shown to have an important role in organizing the low-level convergence and convective motions. These cases are compared with earlier studies of lake-effect snow bands on Lakes Erie and Ontario.

Observational Study of a Convective Internal Boundary Layer over Lake Michigan

Abstract Using aircraft data collected during the University of Chicago Lake-Effect Snow Storm project, the results of a case study of the convective thermal internal boundary layer (TIBL) over Lake Michigan are presented. An intense cold air outbreak on 20 January 1984 featured a rapid growth of the convective TIBL thickness and the concurrent development of cloud and snow. The average slope of the TIBL top over a fetch of 123.7 km was 1.0%. Microphysical characteristics of cloud and snow along with the TIBL development are also presented. Results of the TIBL integrated budgets of heat and total water (including cloud and snow water) are given in detail. Over the surface of Lake Michigan the average downward snow flux (snow precipitation rate) was 0.79 mm (water) per day. The average sensible and latent heat fluxes at the water surface were 323 and 248 W m−2, respectively. About 13 percent of the total warming of this cloud-topped TIBL was due to radiation.

Mean profiles of moisture fluxes in snow-filled boundary layers

Profiles of moisture fluxes have been examined for convective boundary layers containing clouds and snow, using data derived from aircraft measurements taken on four dates during the 1983/1984 University of Chicago lake-effect snow project. Flux profiles were derived from vertical stacks of aircraft cross-wind flight legs taken at various heights over Lake Michigan near the downwind shore. It was found that, if ice processes are taken into account, profiles of potential temperature and water content were very similar to those presented in past studies of convective boundary layers strongly heated from below. Profiles of total water content and equivalent potential temperature adjusted for ice were nearly invariant with height, except very near the top of the boundary layer, suggesting that internal boundary-layer mixing processes were rapid relative to the rates at which heat and vapour were transported into the boundary layer through entrainment and surface fluxes. Ice was found to play a significant, measurable role in boundary-layer moisture fluxes. It was estimated that 40 to 57% of the upward vapour flux was returned to the surface in the form of snow, converting about 45 to 64% of the surface latent heat flux into sensible heat in the snow-producing process. Assuming advective fluxes are relatively small (thought to be appropriate after the first few tens of km over the lake as suggested by past studies), the boundary layer was found to warm at a rate faster than could be explained by surface heat fluxes and latent heat releases alone, the remainder of the heating presumably coming from radiational processes and entrainment. Discussions of moisture phase change processes throughout the boundary layer and estimates of errors of these flux measurements are presented.

On Calculating the Buoyancy of Cores in a Convective Boundary Layer

Abstract Aircraft measurements of vertical air motions are used in a process of conditional sampling to select updraft and downdraft cores during a period of strong lake-effect convection. Corresponding measurements of temperature and moisture are used to calculate the buoyancies of the cores and to evaluate the dependence of the calculated buoyancy on the horizontal extent of core environment used in the calculations. Results suggest that calculated buoyancies are relatively insensitive to the definition of core environment.

Precipitation Particle Measurements in Cumulus Congestus

Abstract Precipitation particle measurements from the upper levels of cumulus congestus clouds are analyzed with regard to general cloud characteristics, liquid water content, and precipitation water content as related to the theoretical radar reflectivity. Conclusions are: (1) the majority of the cumulus congestus clouds examined, whose tops exceed 14,000 ft, contained precipitation particles (250-microns diameter) in the upper levels sometime during their life cycle, (2) particle concentrations in excess of 1000 per m3 were found in about 20 per cent of the clouds examined, (3) the relationship Z=1.6×10−2M1.46 for radar reflectivity is applicable for cumulus congestus in the early stages of precipitation development.

Phloroglucinol Seeding of Undercooled Clouds

Abstract A series of twelve releases of phloroglucinol were made into stratus clouds at temperatures of −7C to −17C. Showers produced by dry ice seeding were used to identify particular spots in the layer clouds from which the exact locations of the phloroglucinol releases could be obtained by simple navigation. Visual observations of the cloud behavior and Formvar replicas of cloud and precipitation particles provided a means for judging the effects of the phloroglucinol. It is concluded that phloroglucinol will induce the formation of ice in undercooled clouds. However, in these experiments, it was not nearly as effective as the dry ice in causing shower formation.

A Laboratory Study of Atmospheric Ice Particles

Abstract As a means of studying ice nucleation, snow and ice pellets collected from the tops of clouds were melted and refrozen in order to determine their freezing temperatures. In all cases where a definite cloud top temperature could be assigned, the melted ice pellets froze at a temperature colder than that of the cloud top, indicating that these pellets did not originate through the heterogeneous freezing of cloud drops. Essentially no difference was indicated in the freezing temperatures of ice pellets collected on seeded and non-seeded days. A firm statement on this point could not be made as the number of observations is limited, and it is not certain that the seeding agent had been ingested into the cloud being studied.