Clouds 3. Chapter INTRODUCTION 17 18

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1 IGV99-B0 PII: S (0) ISBN: PAGE: ( ) Chapter 2 Clouds 3 Clouds are pictures in the sky 4 They stir the soul, they please the eye 5 They bless the thirsty earth with rain, 6 which nurtures life from cell to brain 7 But no! They re demons, dark and dire, 8 hurling hail, wind, flood, and fire 9 Killing, scarring, cruel masters 0 Of destruction and disasters Clouds have such diversity 2 Now blessed, now cursed, 3 the best, the worst 4 But where would life without them be? 5 Vollie Cotton 6.. INTRODUCTION 7 8 Since the late 940s, when the experiments by Langmuir (948) and Schaefer 9 (948) suggested that seeding of certain types of clouds could release additional 20 precipitation, there has been intensive investigation into the physics of clouds. 2 The major focus of these studies has been on the microphysical processes 22 involved in cloud formation and the production of precipitation. As the studies 23 have unraveled much about the detailed microphysics of clouds, it has become 24 increasingly apparent that these processes are affected greatly by macroscale 25 dynamics and thermodynamics of the cloud systems. We have also learned to 26 appreciate that the microphysical processes can alter the macroscale dynamic 27 and thermodynamic structure of clouds. Thus, while the focus of this book is 28 on the dynamics of clouds, we cannot neglect cloud microphysical phenomena. 29 The title of this book implies a perspective from which we view the cloud or 30 cloud system as a whole. From this perspective, cloud microphysical processes 3 can be seen as a swarm or ensemble of particles that contribute collectively, and 32 in an integrated way, to the macroscale dynamics and thermodynamics of the 33 cloud. 34 We take a similar perspective of small-scale air motions in clouds. Again, 35 we will not use our highest power magnifying lens to view the smallest scale 36 motions or turbulent eddies in clouds. We will instead examine the collective 37

2 IGV99-B0 PII: S (0) ISBN: PAGE: 2 ( ) 2 CHAPTER Clouds behavior or statistical contributions of the smallest cloud eddies (i.e. those less than a few hundred meters or so) to the energetics of clouds and to transport processes in clouds. Following the same analogy, we view the meso-β-scale and meso-α-scale with a wide-angle lens, thus encompassing their contributions to the energetics and transport processes of a particular cloud as well as to neighboring clouds or cloud systems. The meso-β-scale and meso-α-scale can, for the most part, be considered the environment of the cloud scale that is generally the meso-γ - scale THE CLASSIFICATION OF CLOUDS Cloud types are generally defined according to the phases of water present and the temperature of cloud top [AMS Glossary]. Clouds are referred to as warm clouds, or as liquid phase clouds if all portions of a cloud have temperatures greater than 0 C. For clouds extending above the 0 C level, precipitation formation can be either by ice phase or droplet coalescence processes. Clouds consisting entirely of ice crystals are called ice-crystal clouds. Analogously, a cloud composed entirely of liquid water drops is called a water cloud and a mixed-phase cloud contains both water drops (supercooled at temperatures below 0 C) and ice crystals, without regard to their actual spatial distributions (coexisting or not) within the cloud. Convective clouds extending into air colder than about 0 C are generally mixed clouds. Supercooled droplets may coexist with ice particles until temperatures are cold enough to support homogeneous freezing or below about 40 C. Since this text emphasizes the dynamics of clouds, it would seem appropriate that we adopt a classification of clouds that is based on the dynamic characteristics of clouds rather than on the physical appearance of clouds from the perspective of a ground observer. In fact, several scientists (Scorer, 963; Howell, 95; Scorer and Wexler, 967) have attempted to design such a classification scheme based on cloud motions. However, since we wish to label the various cloud forms for later discussion, we shall generally adhere to the classifications given in the International Cloud Atlas (World Meteorological Society, 956). This classification is based on ten main groups called genera, and most of the genera are subdivided into species. Each subdivision is based on the shape of the clouds or their internal structure. The species is sometimes further divided into varieties, which define special characteristics of the clouds related to their transparency and the arrangements of the macroscopic cloud elements. The definitions of the ten genera are as follows: Cirrus Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous (hairlike) appearance, or a silky sheen, or both.

3 IGV99-B0 PII: S (0) ISBN: PAGE: 3 ( ) Chapter Clouds 3 Cirrocumulus Thin, white patches, sheets, or layers of cloud without shading, composed of very small elements in the form of grains or ripples, 2 merged or separate, and more or less regularly arranged; most of the elements 3 have an apparent width of less than. 4 Cirrostratus Transparent, whitish cloud veil of fibrous or smooth 5 appearance, totally or partially covering the sky, and generally producing 6 halo phenomena. 7 Altocumulus White or grey, or both white and grey, patches, sheets, or 8 layers of cloud, generally with shading, composed of laminae, rounded 9 masses, or rolls, which are sometimes partially fibrous or diffuse and which 0 may or may not be merged; most of the regularly arranged small elements usually have an apparent width of Altostratus Greyish or bluish cloud sheet or layer of striated, fibrous, or 3 uniform appearance, totally or partially covering the sky, and having parts 4 thin enough to reveal the sun at least dimly, as through ground glass. 5 Altostratus does not produce halos. 6 Nimbostratus Grey cloud layer, often dark, the appearance of which is 7 rendered diffuse by more or less continuously falling rain or snow, which 8 in most cases reaches the ground. It is thick enough to completely obscure 9 the sun. Low, ragged clouds frequently occur below the nimbostratus layer. 20 Stratocumulus Grey or whitish, or both grey and whitish, patches, sheets, 2 or layers of cloud which almost always have dark parts, composed of 22 crenellations, rounded masses, or rolls, which are nonfibrous (except when 23 virga-inclined trails of precipitation-are present) and which may or may not 24 be merged; most of the regularly arranged small elements have an apparent 25 width of more than Stratus Generally grey clouds with a fairly uniform base, which may 27 produce drizzle, ice prisms, or snow grains. If the sun is visible through the 28 cloud, its outline is clearly discernible. Stratus clouds do not produce halo 29 phenomena except, possibly, at very low temperatures. Sometimes stratus 30 clouds appear in the form of ragged patches. 3 Cumulus Detached clouds, generally dense and with sharp outlines 32 developing vertically in the form of rising mounds, domes, or towers, of 33 which the bulging upper part often resembles a cauliflower. The sunlit parts 34 of these clouds are mostly brilliant white; their base is relatively dark and 35 nearly horizontal. Sometimes cumulus clouds are ragged. 36 Cumulonimbus Heavy, dense clouds, with a considerable vertical extent, in 37 the form of a mountain or huge tower. At least part of their upper portion is 38 usually smooth, fibrous, or striated and is nearly always flattened; this part 39 often spreads out in the shape of an anvil or vast plume. Under the base of 40 these clouds, which is generally very dark, there are frequently low ragged 4 clouds and precipitation, sometimes in the form of virga. 42

4 IGV99-B0 PII: S (0) ISBN: PAGE: 4 ( ) 4 CHAPTER Clouds In general we will not have to refer to the definitions of the clouds species or varieties used in the International Cloud Atlas. The exceptions mainly concern cumulus clouds, which we refer to as follows: Cumulus humilis Cumulus clouds of only a slight vertical extent; they generally appear flattened. Cumulus mediocris Cumulus clouds of moderate vertical extent, the tops of which show fairly small protuberances. Cumulus congestus Cumulus clouds which exhibit markedly vertical development and are often of great vertical extent; their bulging upper part frequently resembles a cauliflower. We also may have occasion to refer to the following supplementary features and accessories of clouds: Mamma Hanging protuberances, like udders, on the under surface of a cloud. Virga Vertical or inclined trails of precipitation (fall streaks) falling from the base but reaching the earth s surface. Pileus An accessory cloud of small horizontal extent, in the form of a cap or hood above the top or attached to the upper part of a cumuliform cloud which often penetrates it. Fog is not treated as a separate cloud genus in the International Cloud Atlas. Instead it is defined in terms of its microstructure, visibility, and proximity to the earth s surface as follows: Fog Composed of very small water droplets (sometimes ice crystals) in suspension in the atmosphere; it reduces the visibility at the earth s surface generally to less than 000 m. The vertical extent of fog ranges between a few meters and several hundred meters. We include the discussion of fog in the chapter on stratocumulus clouds, since we shall see there is not always a clear distinction between the formative mechanisms of a marine stratocumulus cloud whose base is elevated from the surface, and a fog which reaches the surface. Another cloud form discussed in this text that is not treated in the International Cloud Atlas as a separate cloud genus is the orographic cloud. According to the Glossary of Meteorology (Huschke, 959), an orographic cloud is a cloud whose form and extent is determined by the disturbing effects of orography upon the passing flow of air. Since orography can also initiate convective clouds, we shall often refer to a stable orographic cloud as the cloud form typically encountered in the wintertime during periods when the atmosphere is stably stratified. The cap cloud is the least complicated form of the orographic cloud and refers to a nearly stationary cloud that hovers over an isolated peak. The crest cloud is like the cap cloud with the exception that it hovers over a mountain ridge. The chinook arch or foehn wall cloud refers to a bank or wall of clouds associated with a chinook or foehn wind storm. Finally, the lenticular cloud, or lenticularis, is a lens-shaped cloud that forms over, or

5 IGV99-B0 PII: S (0) ISBN: PAGE: 5 ( ) Chapter Clouds 5 to the lee of, orographic barriers as a result of mountain waves. As the name implies, lenticular clouds generally have a smooth shape with sharp outlines, 2 sometimes vertically-stacked with clear air separating each lenslike element CLOUD TIME SCALES, VERTICAL VELOCITIES, AND 4 LIQUID-WATER CONTENTS 56 In this section we examine certain cloud characteristics that have a major 7 controlling influence upon whether or not precipitation processes are important 8 and whether diabatic processes such as condensational heating and radiative 9 transfer dominate the cloud energetics. Because these physical processes affect 0 the dynamics of the cloud, it is important to recognize under what conditions and in which cloud types these processes are important. 2 Saturation vapor pressure decreases when the temperature decreases (the 3 Clausius-Clapeyron relation). Cloud formation occurs when the saturation vapor 4 pressure becomes smaller than the actual partial pressure of the water vapor in 5 the air. The greater the difference, the stronger the forcing for liquid droplets or 6 ice particles to grow by vapor deposition. Clouds generally form when a buoyant 7 parcel of air is lifted (convective ascent) and cooled by adiabatic expansion. 8 As a parcel of air inside a cloud ascends, temperature decreases following a 9 moist adiabatic lapse rate which is slightly less (0.65 C per 00 m) than in 20 clear air adiabatic ascent ( C per 00 m), because of the latent heat released by 2 condensation. The rate of condensation depends on the temperature and pressure 22 of the cloud. At 900 m and 20 C, for example, it is approximately 2 g kg 23 per km of ascent. Assuming no mixing, the mixing ratio of condensed water at 24 any level above cloud base can be derived as the difference between the water 25 vapor mixing ratio at cloud base and the saturation water vapor mixing ratio 26 at that level. This is referred to as the adiabatic water-mixing ratio. Owing 27 to the mixing processes, the actual condensed water-mixing ratio is generally 28 lower than the adiabatic value. At the cloud base, the condensation of the 29 available water vapor is not instantaneous and the actual water vapor partial 30 pressure is higher than the saturation vapor pressure leading to supersaturation. 3 Supersaturation plays a critical role near cloud base for the activation of cloud 32 condensation nuclei (CCN) and ice nuclei (IN) that initiate cloud droplets or ice 33 crystals. The amount of condensed water content, either liquid or ice, is a key 34 parameter for precipitation formation. Precipitation is most likely to form in the 35 regions of largest condensed mixing ratio, i.e. in the least diluted cloud cells. 36 The macroscopic parameters of clouds that characterize precipitation and 37 diabatic process are () cloud time scales, (2) cloud vertical velocities, (3) 38 cloud liquid-water contents, (4) cloud temperature, and (5) cloud turbulence. 39 Time scales are important because precipitation processes are time dependent. 40 Therefore, if the cloud lifetime is too short for the time it takes to form 4 precipitation, the cloud will not precipitate even though other properties, such 42 as liquid-water content, are sufficient to support precipitation. Two time scales 43

6 IGV99-B0 PII: S (0) ISBN: PAGE: 6 ( ) 6 CHAPTER Clouds are critical. One is the cloud lifetime, which we shall label T c. The other, called the parcel lifetime, represents the time it takes a parcel to enter the cloud and exit its top or sides. We shall label this time scale as T P. Cloud vertical velocities are important because the updrafts control the time scale T P and determine the cloud s ability to suspend precipitation particles. The magnitude of vertical velocity also provides an estimate of the wet (saturated) adiabatic cooling rate. For example, in the middle troposphere the wet adiabatic lapse rate γ m is approximately 0.5 C/00 m. Thus the wet adiabatic cooling rate CR γ is CR γ (0.5 C/00 m) W, (.) where W is the cloud vertical velocity in meters per second. Both the potential for precipitation formation and the cooling rates of clouds depend on the liquid-water content (LWC) in a cloud, for two reasons. First, the LWC determines the ultimate potential for a cloud to produce precipitation. Generally speaking, unless a cloud generates a liquid-water content in excess of 0.5 g m 3, it is unlikely to precipitate. Of course, other factors, such as aerosol concentrations (see Chapter 4) and whether the cloud is supercooled, also control the critical LWC for initiating precipitation. Second, the LWC is important because it determines the rates of shortwave or longwave radiational heating and cooling. Cloud temperature also represents an important parameter in precipitation potential. The cloud-base temperature indicates the liquid-water producing potential of the cloud. For example, a cloud with a base temperature of +20 C has a cloud-base saturation mixing ratio of 5 g kg, while a cloud with a base temperature of +4 C has a cloud-base saturation mixing ratio of only 5 g kg. Thus, if these two clouds have equal depths, the one with the warmer cloud base has a much greater potential for producing rainfall. Cloud-top temperature is important for similar reasons, because the greater the difference between the cloud-base temperature and cloud-top temperature, the greater the potential for rainfall. Furthermore, if the cloud-top temperature is below 0 C, then ice is possible, which greatly affects precipitation and radiation processes. Turbulence, the last consideration in our discussion of macroscopic cloud parameters, is important because it mixes properties of the cloud and interacts closely with the other parameters. When we speak of characteristic time scales, vertical velocities, liquid-water contents, and temperatures, the level of turbulence determines how representative these characteristic scales really are. For example, in some convective clouds the average updraft velocity may be m s, while the standard deviation of the vertical velocity may be as large as 3 m s. The level of turbulence also affects the precipitation processes, due to the formation of higher peak supersaturations and to increased interactions among cloud particles of different types and sizes. Turbulence is also likely to affect the radiative properties of a cloud. In a turbulent cloud the cloud top is

7 IGV99-B0 PII: S (0) ISBN: PAGE: 7 ( ) Chapter Clouds 7 likely to be very lumpy, and large fluctuations in liquid water will exist. As a consequence, the cloud-top radiative emittance and absorptance will differ 2 significantly from that found in a more homogeneous cloud. 3 Let us next consider how the characteristics just introduced differ in several 4 cloud forms that we will study in this book Fog 67 Fog may be considered the least dynamic of clouds. Fogs typically have 8 lifetimes (T c ) of 2 to 6 h. The mean vertical velocity in fog is usually quite 9 small. If we assume a mean updraft of 0.0 m s for a 00-m-deep fog, 0 the time scale for a parcel entering the cloud base and exiting the cloud top would be 2 T P = 00 m/0.0 m s = 0 4 s, (.2) 3 that is, T P is on the order of 3 h. This represents the time scale in which cloud 4 microphysical processes must operate in order to generate precipitation. 5 However, the liquid-water content in fog typically ranges from 0.05 to g m 3. Thus, precipitation is unlikely in all but the deepest, wettest, and 7 most maritime fogs, even though the mean vertical velocity might indicate a 8 potential for precipitation. If we use our estimate of W, of 0.0 m s, we 9 determine that the cooling rate due to wet adiabatic cooling is of the order of 20 CR γ = (0.5 C/00 m)0.0 m s = C s, (.3) 2 which is approximately 0.2 C h. By comparison, the rate of cooling by 22 longwave radiation flux divergence at the top of the fog can easily range from 23 to 4 C h. Thus, we see that fogs can be dominated by radiative cooling. 24 The absolute magnitude of turbulence in fogs is usually small, although 25 there have been reports of vertical velocity fluctuations in some valley fogs 26 as large as m s. However, if we consider turbulence in terms of fluctuations 27 from the mean motions, it appears that because both horizontal and vertical 28 mean velocities are typically small in fogs, a fog is dominated by turbulence. 29 Thus, turbulence affects transport and nearly all physical processes in fogs, even 30 though its absolute magnitude is generally small Stratus and Stratocumulus Clouds Stratus clouds and stratocumulus clouds do not differ markedly from fogs in 34 terms of time scales, liquid-water contents, or turbulence levels. The lifetimes 35 of stratus and stratocumulus clouds are longer, ranging from 6 to 2 h. As in 36 fog, the time scale for a parcel to enter a stratus having a mean vertical velocity 37 of 0. m s and rising through a depth of, say, 000 m may be 3 h. Typical 38 liquid-water contents in stratus clouds range from 0.05 to 0.25 g m 3, with 39

8 IGV99-B0 PII: S (0) ISBN: PAGE: 8 ( ) 8 CHAPTER Clouds some maxima of over 0.6 g m 3 reported. This combination of time scales and liquid-water contents results in precipitation in the deepest, wettest stratus and stratocumulus clouds in the form of drizzle. Again, assuming vertical velocities of 0. m s, the wet adiabatic cooling rates are of the order of 2 C h. Thus, radiation and wet adiabatic cooling are approximately equal contributors to the destabilization of stratus and stratocumulus clouds. The turbulence level in stratus clouds is low in absolute magnitude, just as it is in fog. However, since mean vertical velocities are also small, turbulence is a significant contributor to vertical transport processes, energetics, and the physics of stratus clouds Cumulus (Humilis and Mediocris) Clouds Cumulus clouds whose vertical extent may be 500 m have a lifetime (T c ) of 0-30 min, which is shorter than that for the preceding two types of clouds. If we consider an average vertical velocity of 3 m s, the time scale for a parcel to enter the cloud base and exit the cloud top is of the order of T P = 500 m/3 m s = 500 s 0 min. (.4) The liquid-water content of small cumuli rarely exceeds.0 g m 3 and is typically approximately 0.3 g m 3. Thus, for such short time scales and low liquid-water contents, precipitation is unlikely in all but the most maritime or cleanest airmass, wettest cumuli. Comparing wet adiabatic cooling rates to cloud-top radiation cooling, we estimate CR γ (0.5 C/00 m) 3 m s = C s 50 C h, (.5) which is considerably greater than the cloud-top radiation cooling rates for clouds of such liquid-water contents (CR IR 4 C h ). Thus, wet adiabatic cooling dominates radiative effects in such clouds. The turbulence levels in small cumuli is relatively moderate, with root-meansquare (RMS) velocities ranging from to 3 m s. Thus, turbulence plays an important role in such clouds Cumulus Congestus Clouds The lifetime of cumulus congestus clouds exceeds that of cumuli, from 20 to 45 min. However, the transit time T P for a parcel entering the cloud base, rising at 0 m s, and exiting the cloud top is similar to that of small cumuli, since T P = 5000 m/0 m s = 500 s 0 min. (.6)

9 IGV99-B0 PII: S (0) ISBN: PAGE: 9 ( ) Chapter Clouds 9 That is, higher updraft velocities in cumulus congestus clouds offset their greater depth in determining T P. Because of the small T P, precipitation would be 2 unlikely if it were not for the higher liquid-water content of cumulus congestus 3 clouds, which ranges from 0.5 to 2.5 g m 3. Because the turbulence level in such 4 clouds is often quite strong, it is possible for air parcels to spend a considerably 5 longer residence time in the cloud than would be implied by T P. 6 As in the smaller cumuli, radiative effects are secondary to wet adiabatic 7 processes in the energetics of cumulus congestus clouds Cumulonimbus Clouds 9 0 Cumulonimbi are the longest living convective clouds. They have lifetimes from 45 min to several hours. However, the time scale for a parcel of air to 2 enter the cloud base and commence forming precipitation before exiting the 3 top remains relatively short. Let us take, as an example, a cumulonimbus cloud 4 that is 2,000 m deep and has an average updraft velocity of 30 m s. The 5 Lagrangian time scale is only 6 T P = 2,000 m/30 m s = 400 s, (.7) 7 which is actually less than that in the smaller cumuli. Because of the enormous 8 cooling of air parcels rising through the great depths of the cloud, typical liquid- 9 water contents in cumulonimbi range from.5 to 4.5 g m 3 and often greater. 20 These high liquid-water contents compensate, to some extent, for the short time 2 scale. The short time scale sometimes limits the formation of precipitation, 22 which accounts for the weak echo regions (WERs) that are often observed by 23 radars. It should be noted that such intense updrafts as those present in WERs 24 are not characteristic of the entire convective storm. Because turbulence levels 25 can be so intense, there is considerable opportunity for air parcels to experience 26 much longer lifetimes than are encountered rising in the main updraft. 27 With the exception of the anvil outflow region of cumulonimbi, wet adiabatic 28 processes dominate over radiative cooling. However, radiative cooling may 29 contribute significantly to the destabilization and maintenance of the weak 30 updraft regions of cumulonimbus anvils Stable Orographic Clouds Let us consider now a wintertime stable orographic cloud that is above a m-high mountain with a half-width of 8 km (Fig..). For this type of cloud, 35 the cloud lifetime could be many hours or even days. However, the time scale 36 for precipitation processes to operate if the winds are about 5 m s is only 37 T P = 8,000 m/5 m s = 200 s = 20 min. (.8) 38

10 IGV99-B0 PII: S (0) ISBN: PAGE: 0 ( ) 0 CHAPTER Clouds 5 m s 400 m 8 km FIGURE. Schematic diagram of a stable orographic cloud Thus, the time scale T p is longer than that for cumuli but considerably shorter than that for stratus clouds. The liquid-water contents of wintertime stable orographic clouds do not differ substantially from those of stratocumuli; they are typically less than 0.2 g m 3. It is only in highly efficient maritime clouds or colder and efficient ice-phase-dominated clouds that precipitate occurs. If we consider typical updraft speeds near the mountain barrier to be about m s, we have an estimate of wet adiabatic cooling rates of CR γ = 8 C h, (.9) which is greater by an order of magnitude than radiative cooling rates. Thus, near the barrier crest, wet adiabatic processes remain dominant. At distances removed from the barrier crest, however, where a blanket cloud may reside, or in weaker wind situations, one can anticipate that radiative processes become more significant in such clouds. There has been very little characterization of the levels of turbulence in wintertime stable orographic clouds. At cloud levels near the barrier crest, surface-generated turbulence could be quite significant. At higher cloud levels, however, turbulence levels can be expected to be relatively weaker under the typically stable conditions. We can see from these simple comparisons and contrasts that clouds form in a broad range of conditions that control the ultimate destiny of the cloud. Depending on the vertical velocity, liquid-water content, and cloud time scale, precipitation processes may or may not affect significantly the dynamics of the cloud. Similarly, radiation processes may or may not be an important destabilizing influence on the cloud. It should be remembered that these are only rough estimates and that one can expect considerable variability within a given cloud category. To account for such variability we must construct sophisticated models of each of the cloud types. In the following chapters, we present the foundation for constructing such models of the dynamics and physics of various cloud systems.

11 IGV99-B0 PII: S (0) ISBN: PAGE: ( ) Chapter Clouds REFERENCES Howell, W. E. (95). The classification of cloud forms. Clouds, fogs and 2 aircraft icing. In Compendium of Meteorology (T. F. Malone, Ed.), 3 pp Am. Meteorol. Soc., Boston, Massachusetts. 4 Huschke, R. E., (Ed.) (959). In Glossary of Meteorology Am. Meteorol. 5 Soc., Boston, Massachusetts. 6 Langmuir, I. (948). The growth of particles in smokes and clouds and the 7 production of snow from supercooled clouds. Proc. Am. Philos. Soc. 92, Schaefer, V. J. (948). The production of clouds containing supercooled water 9 droplets or ice crystals under laboratory conditions. Bull. Am. Meteorol. Soc. 0 29, 75. Scorer, R. S. (963). Cloud nomenclature. Q. J. R. Meteorol. Soc. 89, Scorer, R. S., and Wexler, H. (967). Cloud Studies in Colour. Pergamon, 3 Oxford. 4 World Meteorological Society (956). International Cloud Atlas, Vol., 5 Geneva. 6

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