Marcia B. Baker

Absorption coefficient of atmospheric aerosol: a method for measurement.

Recent models predict that the effect of atmospheric aerosol particles on global temperature depends in part on b(abs,)or on n(2), where b(abs) is the aerosol absorption coefficient, and n(2) is the imaginary part of the aerosol refractive index for radiation in visible wavelengths. Satisfactory techniques for measuring b(abs) have yet to be developed. A method for measurement of b(abs) is described and discussed. The apparatus uses a piece of opal glass to integrate light scattered by particles collected on Nuclepore filters. It is quite mobile, inexpensive, and can be calibrated. Analysis of the errors inherent in the measurement shows it is accurate to well within an order of magnitude for particles of radius larger than 0.1 microm.

Small-scale cloud processes and climate

Clouds constitute the largest single source of uncertainty in climate prediction. A better understanding of small-scale cloud processes could shed light on the role of clouds in the climate system.

Turbulent Mixing, Spectral Evolution and Dynamics in a Warm Cumulus Cloud

Abstract The analysis of Paluch suggests that some cumuli contain cloudy air from only two sources: cloud base and cloud top. A framework is presented for the investigation of droplet spectral evolution in clouds composed of air from only these two sources. The key is the investigation of the dependence of droplet concentration N on the fraction of cloud base air F in a sample of cloudy air. This N-vs-F analysis is coupled with an investigation of droplet spectral parameters to infer the types and scales of entrainment and mixing events. The technique is used in a case study of a small, nonprecipitating continental cumulus cloud which was sampled during the 1981 CCOPE project in eastern Montana. The mixing between cloudy and entrained air in this cloud often appears to occur without total removal of droplets, although there is evidence that total evaporation occurs in some regions with low liquid water content. The observed droplet spectra are compared with those calculated from an adiabatic parcel model....

Entrainment and Detrainment in Cumulus Clouds

Abstract Vertical redistribution of air and its properties inside convective clouds can be studied by standard thermodynamic analyses (Paluch and saturation point diagrams) if the clouds are nonprecipitating and ice free. It is shown from such analysis that the linear patterns often seen in these diagrams can result from continued “lateral” entrainment. The observations are consistent with the hypothesis that buoyancy sorting is an important factor in determining cloud composition. This yields a simple, graphical means for estimating the thermodynamic properties of in-cloud air at any pressure. Field observations of detrained fluxes near cumulus clouds reveal the existence of rather thin detrainment layers which can occur anywhere over the cloud layer. These observations are shown to be consistent with the conclusion of Bretherton and Smolarkiewicz that detrainment is governed by vertical gradients of cloud buoyancy. This allows prediction of detrainment regions from cloud base temperature and pressure an...

Characteristics of cumulus band clouds off the coast of Hawaii

Abstract We have analyzed aircraft observations from seventeen cumulus cells within cloud bands observed off the east coast of Hawaii during the Joint Hawaii Warm Rain Project (JHWRP) of 1985. Low level convergence generated by the encounter of the trade winds and the island determines the location of the initial convection. However, the upward momentum below cloud base seems to be less important in the subsequent evolution of the bands than buoyancy associated with latent heat release. Entrainment into the clouds occurs at all levels, and almost all cloudy parcels below the trade inversion are moving upwards. While evaporative cooling does not seem to enhance entrainment below the inversion, it does play a role in the descent of cloud top air to the bottom of the inversion. Despite the existence of undiluted cores, the average thermodynamic characteristics of the clouds below the inversion appear well described by a very simple, constant lateral entrainment rate parcel model. We have used in-cloud and ne...

The Effects of Turbulent Mixing in Clouds

Abstract Turbulent mixing of cloudy and cloud-free air may play an important role in determining the overall dynamic and microphysical behavior of warm clouds. We present a model of turbulent mixing based on laboratory and theoretical studies of chemically reacting shear layers, extended to include the effects of buoyancy instabilities and droplet sedimentation. It is found to be consistent with recent observations of microphysical variability in natural clouds.

Precipitation and radiation modeling in a general circulation model: Introduction of cloud microphysical processes

Cloud microphysical processes are introduced in the precipitation parameterization of a general circulation model (GCM). Three microphysical processes are included in this representation of warm cloud precipitation: autoconversion of droplets, collection of droplets by falling raindrops, and evaporation of raindrops falling in clear sky. The mean droplet radius, r, is calculated from the cloud water mixing ratio, which is computed in the model, and the cloud droplet number concentration, N, which is prescribed. The autoconversion rate is set to zero for r < r0, a prescribed threshold mean droplet radius. We investigate the model sensitivity to r0 and to N, the cloud droplet concentration, which is linked to the concentration of cloud condensation nuclei and is likely to vary. We find that an increase in N leads to an increase in the amount of cloud water stored in the atmosphere. In further experiments the mean droplet radius used in the parameterization of cloud optical properties is calculated in the same way as in the precipitation parameterization in order to bring more consistency between the different schemes. We again investigate the model sensitivity to r0 and to N and we find that an increase in N significantly enhances cloud albedo.

A Parameterization of Warm Clouds for Use in Atmospheric General Circulation Models

Abstract Simple parameterizations of droplet effective radius in stratiform and convective clouds are presented for use in global climate models. Datasets from subtropical marine stratocumulus, continental and maritime convective clouds, and hill cap clouds in middle latitudes and a small amount of data from stratocumulus clouds in middle latitudes have been examined. The results suggest strongly that a simple relationship exists between droplet effective radius and liquid water content in layer clouds with the droplet effective radius proportional to the cube root of the liquid water content. The constant of proportionality is different over oceans and continents. In current global climate models liquid water content is not a predicted variable in convective clouds, and the data strongly suggest that a fixed value of droplet effective radius between 9 and 10 μm should be used for continental clouds more than 500 m deep and 16 μm for maritime cumulus more than 1.5 km deep.

Relationships between lightning activity and various thundercloud parameters: satellite and modelling studies

Abstract The lightning frequency model developed by Baker et al. [Baker, M.B., Christian, H.J., Latham, J., 1995. A computational study of the relationships linking lightning frequency and other thundercloud parameters, Q. J. R. Meteorol. Soc., 121, 1525–1548] has been refined and extended, in an effort to provide a more realistic framework from which to examine computationally the relationships that might exist between lightning frequency f (which is now being routinely measured from a satellite, using the NASA/MSFC Optical Transient Detector (OTD)) and a variety of cloud physical parameters. Specifically, superior or more comprehensive representations were utilised of: (1) glaciation via the Hallett–Mossop (H–M) process; (2) the updraught structure of the model cloud; (3) the liquid-water-content structure of the model cloud; (4) the role of the reversal temperature T rev in influencing lightning characteristics; (5) the critical breakdown field for lightning initiation; and (6) the electrical characteristics of the ice crystal anvil of the model cloud. Although our extended studies yielded some new insights into the problem, the basic pattern of relationships between f and the other parameters was very close to that reported by Baker et al. (1995). The more elaborate treatment of T rev restricted somewhat the range of conditions under which reverse-polarity lightning could be produced if the cloud glaciated via H–M, but confirmed the earlier conclusion that such lightning would not occur if the glaciation was of the Fletcher type. The computations yielded preliminary support for the hypothesis that satellite measurements of f might be used to determine values of the ice-content of cumulonimbus anvils: a parameter of climatological importance. The successful launch and continuing satisfactory functioning of the OTD [Christian, H.J., Goodman, S., 1992. Global observations of lightning from space, Proc. 9th Int. Conf. on Atmospheric Electricity, St. Petersburg, pp. 316–321; Christian, H.J., Blakesee, R.J., Goodman, S.J., 1992. Lightning imaging sensor (LIS) for the earth observing system. NASA Tech. Memorandum, 4350] make it possible—with a high degree of precision—to measure lightning location, occurrence time and frequency f over extensive areas of the Earths surface. Measured global distributions of lightning and associated lightning stroke radiance demonstrate that: lightning activity is particularly pronounced over the tropics, much greater over land than over the oceans, and exhibits great seasonal variability; lightning radiance tends to be greater over the oceans, less when lightning activity is high, and greater in the Northern Hemisphere winter than summer.

A Simple Model of Droplet Spectral Evolution during Turbulent Mixing

Abstract The mixing of cloudy, saturated air with cloud-free, subsaturated air is examined with a simple one-dimensional model of the mixing process. The model incorporates (i) a one-dimensional parameterization of turbulent deformation, (ii) molecular diffusion, (iii) sedimentation of droplets and (iv) droplet evaporation in the changing humidity field. The model predicts droplet spectral shapes after homogenization of the two air masses. The turbulent deformation parameterization is based on inertial subrange arguments. It yields a crude estimate of the time development of the vapor and temperature fields in a single characteristic eddy in one dimension rather than the full range of eddy scales in three dimensions. For the high turbulence levels commonly found in cumuli the evaporation process appears close to that described earlier as homogeneous, which may in part be due to the approximations in our treatment. For all other conditions the calculations show total evaporation of some drops accompanied b...