Alan Gadian

Global temperature stabilization via controlled albedo enhancement of low-level maritime clouds

An assessment is made herein of the proposal that controlled global cooling sufficient to balance global warming resulting from increasing atmospheric CO2 concentrations might be achieved by seeding low-level, extensive maritime clouds with seawater particles that act as cloud condensation nuclei, thereby activating new droplets and increasing cloud albedo (and possibly longevity). This paper focuses on scientific and meteorological aspects of the scheme. Associated technological issues are addressed in a companion paper. Analytical calculations, cloud modelling and (particularly) GCM computations suggest that, if outstanding questions are satisfactorily resolved, the controllable, globally averaged negative forcing resulting from deployment of this scheme might be sufficient to balance the positive forcing associated with a doubling of CO2 concentration. This statement is supported quantitatively by recent observational evidence from three disparate sources. We conclude that this technique could thus be adequate to hold the Earths temperature constant for many decades. More work—especially assessments of possible meteorological and climatological ramifications—is required on several components of the scheme, which possesses the advantages that (i) it is ecologically benign—the only raw materials being wind and seawater, (ii) the degree of cooling could be controlled, and (iii) if unforeseen adverse effects occur, the system could be immediately switched off, with the forcing returning to normal within a few days (although the response would take a much longer time).

The possible influence of urban surfaces on rainfall development : a sensitivity study in 2D in the meso-γ-scale

Urban areas can represent a considerable part of the model domain in meso-scale numerical simulations with typical horizontal domain lengths of 20–200 km. This paper addresses the question of the extent of the influence of urban surfaces on the development of convective precipitation in meso-γ-scale numerical models. For this purpose, a spatially variable parameterization scheme for surface sensible heat flux, surface latent heat flux, and roughness is introduced into a meso-scale numerical model. A sensitivity study in 2D is performed to assess the impact of variations of the individual parameters on the development of precipitation. The results indicate that surface conditions should not be neglected and can have considerable influence on convective rainfall. It appears that within a time frame of 4 h it is particularly the sensible heat flux variations that have the most significant impact.

Marine Cloud Brightening

The idea behind the marine cloud-brightening (MCB) geoengineering technique is that seeding marine stratocumulus clouds with copious quantities of roughly monodisperse sub-micrometre sea water particles might significantly enhance the cloud droplet number concentration, and thereby the cloud albedo and possibly longevity. This would produce a cooling, which general circulation model (GCM) computations suggest could—subject to satisfactory resolution of technical and scientific problems identified herein—have the capacity to balance global warming up to the carbon dioxide-doubling point. We describe herein an account of our recent research on a number of critical issues associated with MCB. This involves (i) GCM studies, which are our primary tools for evaluating globally the effectiveness of MCB, and assessing its climate impacts on rainfall amounts and distribution, and also polar sea-ice cover and thickness; (ii) high-resolution modelling of the effects of seeding on marine stratocumulus, which are required to understand the complex array of interacting processes involved in cloud brightening; (iii) microphysical modelling sensitivity studies, examining the influence of seeding amount, seed-particle salt-mass, air-mass characteristics, updraught speed and other parameters on cloud–albedo change; (iv) sea water spray-production techniques; (v) computational fluid dynamics studies of possible large-scale periodicities in Flettner rotors; and (vi) the planning of a three-stage limited-area field research experiment, with the primary objectives of technology testing and determining to what extent, if any, cloud albedo might be enhanced by seeding marine stratocumulus clouds on a spatial scale of around 100×100 km. We stress that there would be no justification for deployment of MCB unless it was clearly established that no significant adverse consequences would result. There would also need to be an international agreement firmly in favour of such action.

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.

Determination of ice precipitation rates and thunderstorm anvil ice contents from satellite observations of lightning

Abstract The continuous satisfactory functioning of satellite-borne devices for the detection of global lightning offers the opportunity to explore relationships between lightning frequency f and other thundercloud parameters, notably, in this paper, the precipitating and non-precipitating contents and fluxes of ice. Simple calculations predict that the lightning frequency f is proportional to the product of the downward flux of solid precipitation through the body of the thundercloud and the upward flux of ice crystals into its anvil. This prediction is reinforced by computations performed using the multiple lightning model of Baker et al. [Q. J. R. Meteorol. Soc. 121 (1995) 1525; Atmos. Res. 51 (1999) 221]. Calculations indicate that the separation of charge and associated field development in thunderclouds are not significantly limited by charge saturation of the interacting hydrometeors: and that the mutual interactions of graupel pellets in the charging zones of thunderstorms can significantly enhance electric field development, culminating in lightning. An examination of data from the satellite-borne Lightning Imaging Sensor (LIS) and TRMM Microwave Imager (TMI) suggests that thunderstorms with the highest frequency of total lightning also possess the most pronounced microwave scattering signatures at 37 and 85 GHz. A total of 292 individual thunderstorms were examined, and a log-linear relationship was shown to exist (one for each frequency) between the number of optical lightning pulses produced by each storm and the corresponding microwave brightness temperatures. These relationships are consistent throughout the seasons in a variety of regimes (12 sites encompassing five continents, as well as oceanic measurements), suggesting that global relationships may be found to exist between lightning activity and cloud ice content.

The development of ice in a cumulus cloud over southwest England

An experiment involving the FAAM BAe 146 aircraft, called the ICE and Precipitation Initiation in Cumulus (ICEPIC) project, was conducted in order to measure the microphysical properties of UK summertime cumulus clouds. A line of clouds was penetrated near the ascending tops. Higher concentrations of ice particles than expected from activation on typical ice nuclei using the Meyers formula were observed at relatively high temperatures (T>-10 °C). The observations of numerous ice particles and the coexistence of both small and large cloud droplets, pristine ice columns and graupel pellets within the temperature zone of -3 to -9 °C strongly suggested the Hallett–Mossop (HM) process of splintering during riming. Agreement between the calculated and observed rates of splinter production supported this suggestion. The Model of Aerosols and Chemistry in Convective Clouds (MAC3) was utilized to establish a quantitative understanding of the observed development of glaciation of this cloud. The results of the model confirmed the important role of the HM process. They also showed that the warm rain process was fundamental to the production of graupel in the cloud studied, and hence the HM ice particles. A sensitivity test with double the concentration of aerosol particles showed that the concentration of supercooled raindrops decreased as expected, which resulted in fewer graupel particles and a smaller quantity of precipitation, which was delayed by about 5 min. However, the production rate of secondary ice particles generated by the HM process increased due to the increased concentration of small cloud droplets.

Validation of the summertime surface energy budget of Larsen C Ice Shelf (Antarctica) as represented in three high-resolution atmospheric models

We compare measurements of the turbulent and radiative surface energy fluxes from an automatic weather station (AWS) on Larsen C Ice Shelf, Antarctica with corresponding fluxes from three high-resolution atmospheric models over a 1 month period during austral summer. All three models produce a reasonable simulation of the (relatively small) turbulent energy fluxes at the AWS site. However, biases in the modeled radiative fluxes, which dominate the surface energy budget, are significant. There is a significant positive bias in net shortwave radiation in all three models, together with a corresponding negative bias in net longwave radiation. In two of the models, the longwave bias only partially offsets the positive shortwave bias, leading to an excessive amount of energy available for heating and melting the surface, while, in the third, the negative longwave bias exceeds the positive shortwave bias, leading to a deficiency in calculated surface melt. Biases in shortwave and longwave radiation are anticorrelated, suggesting that they both result from the models simulating too little cloud (or clouds that are too optically thin). We conclude that, while these models may be able to provide some useful information on surface energy fluxes, absolute values of modeled melt rate are significantly biased and should be used with caution. Efforts to improve model simulation of melt should initially focus on the radiative fluxes and, in particular, on the simulation of the clouds that control these fluxes.

Influence of topography and urban heat island effects on the outbreak of convective storms under unstable meteorological conditions: a numerical study

Analysis of observational data of convective storms in Northern England suggests that the particular combination of effects such as sea breezes, elevated terrain and the presence of large cities has an influence on the initiation and development of convective storms. To study the importance of these effects for the development of convection, a cloud physics model has been initialised with the topography and surface characteristics of Northern England. The model enabled inclusion of three principal influences: sea breezes, topography and urban heat islands. The results suggested that the presence of the Pennines, a north–south orientated ridge, could influence the initiation of convection due to its long sun-facing slopes, and to a lesser degree forced lifting along the slopes. Inclusion of urban heat island effects focused convection downwind of the principal urban areas. Copyright

Marine cloud brightening: regional applications

The general principle behind the marine cloud brightening (MCB) climate engineering technique is that seeding marine stratocumulus clouds with substantial concentrations of roughly monodisperse sub-micrometre-sized seawater particles might significantly enhance cloud albedo and longevity, thereby producing a cooling effect. This paper is concerned with preliminary studies of the possible beneficial application of MCB to three regional issues: (1) recovery of polar ice loss, (2) weakening of developing hurricanes and (3) elimination or reduction of coral bleaching. The primary focus is on Item 1. We focus discussion herein on advantages associated with engaging in limited-area seeding, regional effects rather than global; and the levels of seeding that may be required to address changing current and near-term conditions in the Arctic. We also mention the possibility that MCB might be capable of producing a localized cooling to help stabilize the West Antarctic Ice Sheet.

Demonstration of a Cut-Cell Representation of 3D Orography for Studies of Atmospheric Flows over Very Steep Hills

AbstractAdvances in computing are enabling atmospheric models to operate at increasingly fine resolution, giving rise to more variations in the underlying orography being captured by the model grid. Consequently, high-resolution models must overcome the problems associated with traditional terrain-following approaches of spurious winds and instabilities generated in the vicinity of steep and complex terrain.Cut-cell representations of orography present atmospheric models with an alternative to terrain-following vertical coordinates. This work explores the capabilities of a cut-cell representation of orography for idealized orographically forced flows. The orographic surface is represented within the model by continuous piecewise bilinear surfaces that intersect the regular Cartesian grid creating cut cells. An approximate finite-volume method for use with advection-form governing equations is implemented to solve flows through the resulting irregularly shaped grid boxes.Comparison with a benchmark orograp...