Comparative Climatology of Terrestrial Planets

1. INTRODUCTION Clouds and aerosols play a key role in climate models, and uncertainties in the effects of clouds provide the largest error source for predicting global warming. Earth aerosols affect climate through direct radiative changes, aerosolcloud interactions (indirect effects), atmospheric chemistry, snow albedo, and ocean biogeochemistry (Mahowald et al., 2011). Clouds on the other planets expose us to alternate scenarios and examples of climate histories that are different from Earth (for a comparison of mesosphere clouds on Mars and Earth, see the chapter by Määttänen et al. in this volume). An early comparison of terrestrial clouds to those of Jupiter was made by Rossow (1978). Although Earth clouds are the best studied and known, we can still learn from the other planets. Kahn (1989) provides examples of how comparative planetology can illuminate our understanding of Earth by providing more extreme cases of more subtle effects on Earth, by providing better data that records early planet history now erased on the Earth, and providing inspiration leading to new insights. 2. EARTH CLOUDS AND AEROSOLS 2.1. Measurement Techniques As can be expected, we have a deeper understanding of Earth’s clouds and aerosols than other planets due to a larger array of measurements and more sophisticated climate models. Measurements of clouds and aerosols on Earth can be broadly categorized into in situ (instruments that locally measure properties) and remote sensing (instruments that remotely measure properties, typically using the electromagnetic spectrum). Common in situ techniques involve balloons, aircraft, and surface stations. Common remote sensing techniques involve active lidars and radars and passive radiometers and photometers onboard satellites , aircraft, and surface stations. Since the late 1970s, we have gained a near-global dataset of many cloud and aerosol properties from satellites. Additionally, the increase in computing power in recent years has enabled development of sophisticated climate models that include treatment of clouds and aerosols, although many of their processes 329 Clouds and Aerosols on the Terrestrial Planets L. W. Esposito University of Colorado A. Colaprete NASA Ames Research Center J. English National Center for Atmospheric Research R. M. Haberle and M. A. Kahre NASA Ames Research Center Clouds and aerosols are common on the terrestrial planets, highly variable on Earth and Mars, and completely covering Venus. Clouds form by condensation and photochemical processes. Nucleation of cloud droplets by certain aerosols provides an indirect linkage. Earth clouds cover over half of the planet, are composed of mainly liquid water or ice, and are a significant component of Earth’s surface and top of atmosphere energy balance. On Venus, H2SO4 is the dominant cloud constituent, produced by chemical cycles operating on SO2, likely produced from geologic activity. Martian water ice clouds generally have smaller particles than on Earth, although they form by the same processes. Mars clouds affect the deposition of radiation, drive photochemical reactions, and couple to the dust cycle. In the past, Mars clouds may have produced a significant greenhouse effect at times of high obliquity and early in its history. Mars atmospheric dust has both a seasonal cycle and great dust storms. Dust significantly influences the thermal and dynamical structure of the martian atmosphere. Mars CO2 clouds provide both latent heat and radiative effects on the atmosphere, possibly more important on the early, wet, and warmer Mars climate. Esposito L. W., Colaprete A., English J., Haberle R. M., and Kahre M. A. (2013) Clouds and aerosols on the terrestrial planets. In Comparative Climatology of Terrestrial Planets (S. J. Mackwell et al., eds.), pp. 329–353. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816530595-ch13. 330 Comparative Climatology of Terrestrial Planets are still parameterized as their spatial and temporal scales are smaller than the grid resolution allowed by current supercomputers. 2.2. Clouds Clouds are ubiquitous and regularly cover over half of Earth’s surface. They are almost entirely composed of liquid water droplets or ice crystals. Clouds form when air becomes sufficiently supersaturated with respect to liquid water or ice to overcome the energy barrier of changing the curvature of the cloud drop, which almost exclusively occurs in the troposphere (between Earth’s surface and 18 km above the surface). Supersaturation is commonly produced through the ascent of air parcels, which is usually caused by surface heating. Therefore clouds most often occur over water and between the surface and 2 km (Fig. 1). Clouds are also prevalent in the tropical upper troposphere from strong convection and polar middle troposphere from advection and radiative cooling in the winter. Zonally, cloud...