5 Clouds and Precipitation Extreme Rainfall and Rain from Shallow Clouds

This chapter reviews present knowledge on extreme precipitation and moderate rainfall from low-level clouds. Primary focus is on the statistics of precipitation characteristics rather than on a detailed description of individual case studies. First, observed variability of precipitation from low-level clouds and the existing techniques to separate different microphysical stages from remote-sensing measurements are reviewed. Over the tropical areas of Pacifi c and Atlantic oceans, the global distribution of shallow rainfall exhibits a “butterfl y” pattern. This feature encompasses heavily precipitating regions such as the intertropical, south Pacifi c, and south Atlantic convergence zones (ITCZ, SPCZ, and SACZ, respectively); the northern hemispheric counterpart of SPCZ and SACZ emerges only when shallow rain is isolated. The nature of extreme precipitation varies temporally. On a timescale of about a day, extreme precipitation is associated with synoptic-scale disturbances, including a notable example known as tropical plumes or moist conveyer belt, which could give rise to extreme daily precipitation in downstream arid regions. On an hourly timescale, extreme precipitation is caused by mesoscale moisture convergence, which is so intense that it maintains a continuous overturning of saturated air. Satellite observations imply that the global distribution of extreme precipitation shows a systematic difference from the total rainfall map in terms of, for example, the contrast between land and ocean. The distribution of low-level, precipitation-related latent heating associated with warm rain coincides with the butterfl y pattern. Its cohabitation and separation with the deep heating suggests that warm rain plays a role in providing a thick layer of moist static energy source to the convection, and that it is also related to the tropical plumes which cause extreme precipitation in the semiarid west coasts of continents. 108 Y. N. Takayabu and H. Masunaga Precipitation from Low-level Clouds Low-level clouds are typically non-precipitating, or just lightly precipitating, and are thus unlikely to cause hazardous weather events. Nevertheless, precipitation from low-level clouds is important meteorologically and climatologically for the following reasons: Precipitation from stratocumulus has a direct impact on the dynamic maintenance of its parent cloud (Nicholls 1984). In terms of the so-called second indirect effect of aerosols, an increase in aerosol concentration is hypothesized to suppress cloud droplet growth, leading to a longer cloud lifetime as the cloud drizzles out less effi ciently (Albrecht 1989). Precipitation from stratocumulus clouds can thus be a controlling factor of the Earth’s radiation budget, although they do not necessarily make a crucial contribution to global water cycle. Tropical and subtropical shallow cumulus yield signifi cant precipitation through warm rain processes. Short and Nakamura (2000) estimated that shallow rainfall accounts for more than 20% of the total rainfall over tropical oceans. Precipitation from low-level clouds comes generally in the form of warm rain, except in cases where a very low surface temperature does not allow cloud droplets to exist in the liquid phase. The microphysical processes involved in warm rain are relatively well understood (e.g., Mason 1952). Cloud droplets begin to grow in size, initially through vapor condensation to liquid water in supersaturated air. Condensational droplet growth is followed by the drizzling stage, onset by collision and coalescence with distinctly large droplets (20–100 μm) in a turbulent cloud layer. Drizzle turns to warm rain as the droplet coalescence proceeds further and precipitation rate increases. A number of fi eld campaigns with airborne probes have provided opportunities to study the microphysical properties of clouds in detail. In-situ measurements need to be complemented, however, by wide-area observations, such as satellite remote sensing, to obtain robust climatological evidence of large-scale variability. Yet how can we diagnose a microphysical condition from miles away, relying only on information carried by radiation emitted or scattered by cloud particles? To address this concern, let us look at some of the current remote-sensing methodologies used to detect cloud microphysical characteristics and discuss the global climatology of drizzle and shallow rain. Regional and Global Observations of Drizzle and Warm Rain Cloud Optical Thickness versus Effective Droplet Radius Cloud droplets are distributed around 5–10 μm in radius, so that the geometrical optics approximation is valid for cloud droplets at visible wavelengths. Cloud optical thickness, τc, measured remotely in the visible (0.65 μm wavelength) spectrum is thus a practical proxy of the second moment of the cloud droplet size distribution integrated over the whole cloud layer: Extreme Rainfall and Rain from Shallow Clouds 109 2 2 ( ) , c dz dr n r r (5.1) where z, r, and n(r) denote the cloud layer depth, cloud droplet radius, and droplet size distribution, respectively. To translate τc into a parameter of meteorological relevance, such as liquid water path (LWP): 3 4 LWP ( ) , 3 w dz dr n r r (5.2) where ρw =1g cm–3 is the liquid water density, conversion from observed τc to LWP requires knowledge on the relation between the second and third moments of n(r). Combining Equations 5.1 and 5.2: 2 LWP , 3 w c e r (5.3)