Dr. Piotr J. Flatau pcirrus@gmail Based on notes by W. R. Cotton Chapter 1

1. SIO209 Overview of Cloud Dynamics Scripps Institution of Oceanography University of California Dr. Piotr J. Flatau pcirrus@gmail.com Based on notes by W. R. Cotton Chapter 1

2. SIO209 Cloud dynamics - definition Cloud Dynamics is the study of the evolution of clouds including their formation and dissipation mechanismscloud air motions forces creating those motions. Cloud dynamics also includes the interaction of cloud air motions with precipitation processessolar and terrestrial radiationsmaller and larger scales of atmospheric motion. Generally, cloud dynamics involve a macroscopic view of clouds in which cloud particles are described from an ensemble perspective rather than a detailed examination of individual cloud particle physics. The detailed examination of individual cloud particle physics is referred to as cloud microphysics. Cloud physics - physics related to clouds

3. SIO209 Forms and importance Depending on the larger scale environment, clouds take on a variety of forms layer-type clouds such as boundary layer stratocumulus and middle tropospheric stratus to deep convective clouds and thunderstorms. Clouds may have a major impact on the general circulation of the atmosphere the earth's radiation and hydrological budget and the chemistry of the atmosphere and precipitation.

4. SIO209 Role of Convective Clouds Convective clouds: Vertically transport heat, moisture, gases, aerosols, and momentum from the earth's surface to the low, middle, and upper troposphere and even the lower stratosphere. Convective clouds also transport particles and gases, such as air pollutants, into the upper troposphere and lower stratosphere where they may reside for long periods of time and undergo photochemical transformations. Convective clouds can also function as 'wet' chemical reactors transforming particles and gases into acid precipitation.

5. SIO209 Layered clouds Layer clouds vertically mix constituents through shallow layers transport air in slow slantwise ascent over large horizontal distances. large horizontal extent stratus and cirrus clouds absorb and reflect solar radiation absorb longwave or terrestrial radiation emitted by the earth's surface emajor impact on the global heat budget.

6. SIO209 Clouds – Cumulus clouds Cumulus clouds primarily buoyancy-driven cloudsthe air parcels rise due to their buoyancy expand as pressure decreases and cool adiabatically at a constant rate of 10°C/km.

7. SIO209 Cloud convection As a parcel ascends and cools adiabatically relative humidity of the parcel increasesat approximately 100% relative humidity hygroscopic aerosol particles (i.e. salt particles) take on water vapor to form cloud droplets. air parcel becomes saturated at the Lifting Condensation Level (LCL).condensation of vapor onto cloud drops releases latent heat of condensation and the heat is diffused to the surrounding airparcel cools at a lesser rate (approximately 6°C/km in the lower troposphere in middle latitude sum­mertime) - the wet adiabatic lapse rate.

8. SIO209 Cloud convection As shown in Figure 1.2, an air parcel rising through the LCL will become warmer than it would, had it cooled dry adiabatically. In fact if the parcel can ascend to the level of free convection (LFC) it will become warmer than its surroundings and rise as a buoyant convective cloud. The environmental lapse rate shown in Figure 1.2 is said to be conditionally unstable since the instability depends upon there being sufficient moisture in the ascending parcel to reach saturation.

9. SIO209 Convection in real clouds cloudy air parcels can mix with surrounding cloud-free air parcelsevaporative cooling - reducing the temperature of the resul­tant mixture. One of the entrainment processes impact cloud buoyancy and on the growth of precipitation particles 3D cloud models vs. parcel models

10. SIO209 Diagrams – skew T shallow, small diameter cumulus clouds - tradewind cumulus and fair weather cumulus. form from buoyant thermals (i.e., blobs of warm air) that develop in the atmospheric boundary layer (ABL). sounding is unstable to dry air motions near the earth's surfacenearly neutral up to a height Z the ABL top this stable layer of air is called a capping inversion.

11. SIO209 Clouds – liquid water content thermals form in the ABL small and only weakly buoyantlarger and more buoyant. more buoyant thermals may ascend to the LCL where they become saturated, thus forming a cloudlose their buoyancy slightly above the LCL - slowly evaporate (“forced cumuli”) as shown in Figure 1.3. level of free convection - latent heat of condensation is liberated to allow the cloud to ascend to greater heights in the atmosphere “active cumuli” environmental stability mixing (environmental stability and vertical shear of the horizontal wind) capping inversion, the cloud quickly loses its buoyancy and becomes a nonbuoyant “passive” How Much Water mass is in a Cloud?

12. SIO209 Cu Cumulus clouds transport heat, moisture, momentum, and passive materials (i.e. gases, particulates, pollutants) from the ABL into the lower troposphere. Aalter the thermodynamic stability in the cloud layer by heating the lower parts of the layer by condensational latent heat release and cooling the upper part of the cloud layer by evaporation of droplets and by longwave radiation cooling From a cloud microphysics point of view two time scales are important: (1) the Lagrangian time scale (Tp) or time it takes a parcel of air to enter the base of a cloud and exit the top and (2) the total lifetime of the cloud (TL). Ordinary cumuli have typical depths of the order of 1500 m and characteristic updraft speeds of 3 m/s, therefore Tp = 1500/3= 500s ~ 10 minutes This represents the time available for initiation of precipitation formation. Once initiated, precipitation can continue over the remaining lifetime of the cloud. Typical ordinary cumulus cloud lifetimes are on the order of 10-30 min.

13. SIO209 Cu climate role Cu in tropical tradewind regions; effect the earth radiation budgetlow clouds, tops are only slightly cooler than the earth's surface they radiate much of the infrared radiation emitted from the earth's surfaceliquid water content in cumulus clouds is relatively large, they reflect much of the sun's radiation that is incident at their tops ordinary cumulus clouds contribute to a net cooling of the atmosphere. The amount of net cooling, however, is proportional to the cloud coverage; their impact on global cooling is thus less than that of stratocumulus clouds.

14. SIO209 Towering cumulus clouds cumulus congestus cloudsresemble Cumore condensed liquid wa­ter, have greater updraft and downdraft speeds, and live longer1) a more unstable subcloud and cloud layer 2) the absence of a pronounced capping inversion3) the presence of some form of sub-cloud horizontal convergence.

15. a Cold Front Figure 1.4: Examples of atmospheric circulations that produce sub-cloud moisture convergence, (a) Sea breeze convergence in coastal areas, (b) Rising motions over a heated hill, (c) Forced ascent over a mountain, which can be enhanced by organized slope flow due to heating of the higher terrain, (d) Convergence along cold frontal boundaries associated with extra-tropical cyclonic storms or cold pools associated with thunderstorms or mesoscale convective systems. SIO209 Towering cumulus clouds examples of atmospheric circulations that produce subcloud convergence. Because the atmosphere behaves almost as an incompressible fluid, horizontal convergence near the ground causes upward motion which can trigger larger-scale clouds and supply the clouds with energy in the form of heat and moisture

16. SIO209 Towering cumulus clouds lifetime of towering cumulus clouds is 20 min to 45 min typical parcel lifetimes 5000m updrafts on the order of 10m/s = 500sec more condensed liquid water: greater potential of producing precipitation particles non-freezing clouds raindrops collision and subsequent coalescence between larger and smaller drops. faster falling raindrops unleash the upper part of the cloud from its burden of condensed water giving it extra buoyancy

17. SIO209 Towering cumulus clouds settling drops into the lower part of the cloud accumulate condensed water there and causes greater loading of the updraft, sometimes weakening it to form a downdraft. rain falls into the subcloud layer: droplets begin to evaporate: cooling the air enhancing the strength of the downdraft. as the downdraft slows down near the ground, the cool diverging air can lift the low-level moist air to resupply the cloud updraft with moisture thus sustaining the cloud. If vertical shear of the horizontal wind is present, precipitation can fall away from the updraft air, thus a diverging downdraft can form without destroying or weakening the updraft

18. SIO209 Towering cumulus clouds Towering cloud have tops colder than 0°C freezing of supercooled raindrops, activation of ice nuclei, and vapor deposition growth of ice crystals is possible latent heat of freezing and sublimation released during ice particle growth contributes to cloud buoyancy. this boost in buoyancy leads to explosive growth of towering cumuli with some towers penetrating into the lower stratosphere.

19. SIO209 Cu congestus - climate sweep larger quantities of pollutantsvertically transport more heat, moisture and momentum into the middle and upper troposphere than do ordinary cumuli latent heat released from an ensemble of towering cumulus clouds in a region can contribute to large scale tropospheric circulations, especially in equatorial regionstowering cumulus clouds precipitate, they contribute albeit weakly to the global hydrological budget their contribution to the earth's radiation budget is relatively small partly due to the smaller area coverage of towering cumuli than ordinary cumuli, and to the fact that they penetrate into the middle troposphere, where the cooling effects of reflecting shortwave radiation is nearly balanced by warming due to absorption and emission of longwave radiation.

20. SIO209 Cumulonimbus convective clouds of significant vertical extent (often the entire depth of the troposphere) precipitation processes play a major role in their lifecycle, organization, and energetics. grow in an environment which is very unstable to wet convection mechanism for producing low-level convergence often aids in producing them fundamental unit of a cumulonimbus is called a cell which is defined by radar as a region of concentrated precipitation and is also characterized by a region of coherent updraft and downdraft. cumulonimbus clouds are classified by the particular organization and lifecycle of their cell(s).

21. SIO209 Cumulonimbus convective clouds of significant vertical extent (often the entire depth of the troposphere) precipitation processes play a major role in their lifecycle, organization, and energetics. grow in an environment which is very unstable to wet convection mechanism for producing low-level convergence often aids in producing them fundamental unit of a cumulonimbus is called a cell which is defined by radar as a region of concentrated precipitation and is also characterized by a region of coherent updraft and downdraft. cumulonimbus clouds are classified by the particular organization and lifecycle of their cell(s).

22. SIO209 Cumulonimbus • The lifecycle of an ordinary thunderstorm • The cumulus stage is characterized by one or more towers fed by low-level convergence of moist air. Air motions are primarily upward with some lateral and cloud top entrainment depicted • the mature stage is characterized by both updrafts and downdrafts and rainfall. Evaporative cooling at low-levels forms a cold pool and gust front which advances, lifting warm-moist, unstable air. An anvil at upper levels begins to form • The dissipating stage is characterized by downdrafts and diminishing convective rainfall. Stratiform rainfall from the anvil cloud is also common. The gust front advances ahead of the storm preventing air from being lifted at the gust front into the convective storm.

23. SIO209 Cumulonimbus cumulonimbus cloud - lifecycle 45 minutes to one hourtowering cumulus cloudsregion of low-level convergence of warm, moist airtowering cumulus clouds merge to form a larger, precipitating cell updrafts dominate the system during the growth stage and precipitation forms in the upper levels of the towers mature stage commences with rain settling in the sub-cloud layerdowndraft air spreads horizontallyat the interface between the cool, dense downdraft air and the warm, moist air, a gust front forms. the warm, moist air lifted by the gust front provides the fuel for maintaining the vigorous updrafts water loading and the entrainment of dry environmental air in the storm generate downdrafts in the cloud interior, which rapidly transport precipitation particles to the sub-cloud air where they partially evaporate. The evaporatively chilled air strengthens the low-level outflow and gust front.

24. SIO209 Cumulonimbus warming occurs aloft by condensation and freezing in updrafts and cooling in downdrafts at low-levels sustains the vigorous, convective cycle. The stronger the vertical shear of the horizontal wind, the more likely the downdraft air will not weaken or destroy the updrafts, and the efficiency of the machine increases. The intensity of precipitation from the storm reaches a maximum during its mature stage. Once the gust front advances too far ahead of the storm system, warm, moist air lifted at the gust front does not enter the updraft of the storm. This marks the beginning of the dissipation stage of the storm in which the updrafts weaken and the downdrafts predominate. Rainfall intensity subsides, often turning into a period of light steady rainfall. 10,000 m depth having updraft speeds on the order of 15 m/s, a Lagrangian Tp = 10, 000m/15ms^1 = 660sec

25. SIO209 Cumulonimbus major contributors to rainfall in many regions, especially semi-arid regions Latent heating associated with ensembles of ordinary cumulonimbi can play an important role in driving planetary circulations in the tropics cumulonimbi play an important role in vertically redistributing gases and particulates in the atmosphere, spewing boundary layer material into the upper troposphere and lower stratosphere and bringing higher-level atmospheric constituents down to the surface. They also function as wet chemical reactors in which atmospheric gases and aerosol become embedded in droplets and undergo chemical reactions. lightning provides a major natural source for NOx's and ozone. efficient scrubbers of the atmosphere in which the nucleation and scavenging of aerosol particles by numerous small droplets, followed by the collection of those droplets by raindrops focuses or concentrates the particulates onto a few big droplets. In this way, cumulonimbi contribute substantially to acid precipitation

26. SIO209 Multicell thundestorms composed of a number of cells, each undergo­ing a lifecycle of 45 to 60 minutes may have lifetimes of several hours favored in regions of strong conditional instability and moderate wind shear produce hailstones sporadic episodes of tornadoes, flash floodsupdrafts can be so strong that there isn't sufficient time to produce precipitation. Observed by radar, these storms exhibit regions of very low radar reflectivity partially surrounded by higher reflective cores (weak echo regions WERs) produce more total lightning strikes and often with a higher frequency, thus being more active producers of NOx's and ozone. Under weak wind conditions, multicell storms can produce locally heavy convective rainfall. If such a persistent heavy raining storm is in a polluted environment, such a storm can not only produce flash floods but also produce “hotspots” of acidic precipitation.

27. SIO209 Supercell thundestorms Supercell Storms environmental conditional instability is large vertical shear of the horizontal wind is also large thunderstorms tend to organize into a single cell storm (two to six hours) updraft in supercell storms is quite strong, often exceeding 40 m/s, and rotating. the rotation of supercells can be discerned with the naked eyepersistent, weak echo region completely surrounded by heavy precipitation (bounded weak echo region (BWER), or echo-free-vault) is a result of the very strong updrafts which do not provide sufficient time for precipitation-sized particles to form and to the centrifugal action of the rotating updraft which can thrust particles laterally from it. produce large hailstones, sometimes in swaths as long as 300 kmspawning large, persistent tornadoes Istrong updrafts (~ 40 m/s), a 12,000 m deep supercell has a Lagrangian time scale of only Tp = 12, 000m/40m/s^1 = 300s,

28. SIO209 Supercell thundestorms not responsible for producing heavy rainfall events since they move rapidly in the environment characterized by strong vertical shear of the horizontal wind they are not the major producers of acid precipitation events do not produce as high a frequency of lightning flashes and associated chemical changes as multicell thunderstorms strong updrafts can inject large quantities of lower tropospheric pollutants into the upper troposphere and lower stratosphere.

29. SIO209 MCS Occasionally thunderstorms organize into systems on the scale of several hundred kilometers and have durations of six to twelve hours or more. We call these systems mesoscale convective systems (MCSs) A characteristic of MCSs is that because they are so large and have long lifetimes, the air flowing into and out of these thunderstorm systems is rather strongly effected by the earth's rotation. As a result lower and middle tropospheric air flowing into MCSs normally turns cyclonically (or counterclockwise in the northern hemisphere), while air flowing out of MCSs in the upper troposphere turns anticyclonically (or clockwise in the northern hemisphere). In contrast, smaller rotating thunderstorms such as supercells acquire their rotation from tilting of environmental vertical shear rather than the earth's rotation. The fact that MCSs respond to the earth's rotation has a major impact on their organization, structure and lifecycle.

30. SIO209 MCS Systems that respond more strongly to the earth's rotation (i.e. last longer, have a larger horizontal extent, and are at higher latitude) are more typified by slow slantwise ascent (as contrasted with vertically erect convective updrafts and downdrafts) of moist, low-level air and slow slantwise descent of dry middle-level air. In addition to convective showers, precipitation usually occurs as steady, stratiform rainfall.

31. SIO209 Squall Lines • Best known form of MCS organization is the squall line • occur at almost any latitude from the tropics to near the poles • a sharp roll-like line of clouds followed by a sudden wind squall or gust of 12 to 25 m/s. Immediately behind the surface squall a heavy downpour starts, which may produce as much as 30mm of rain in 30 minutes in the tropics. • Often the heavy downpour is followed by several hours of steady rainfall from the stratiform-anvil cloud that trails the squall line. • Occasionally, squall lines exhibit both a trailing and a leading stratiform-anvil region. The squall line is composed of two scales of motion: • the cumulus scale, having a horizontal dimension on the order of 2 to 25 km and • the mesoscale, characterized by air motions on a scale of 20 to 200 km. • The precipitation from squall lines reflects the presence of these two scales of motion, as about 60% of the precipitation is in the form of intense showers and 40% is in the form of steady, stratiform rainfall. • The largest and most violent squall lines are the pre-frontal lines that form in middle latitudes. Typically they form along, or ahead of a cold front associated with a vigorous, mid-latitude cyclonic storm.

32. SIO209 Cloud clusters Many mesoscale convective systems do not exhibit a well-defined line organization of the convective cells convective cells are organized in a more or less random patternsometimes parallel to the upper level winds rather than perpendicular as in squall lines, or with wind-parallel and -perpendicular bands coexisting. cloud clusters in the tropics mesoscale convective complexes (MCC's) in middle latitudes. Cloud clusters range in size from slightly larger than a multicellular thunderstorm to large aggregates of thunderstorms that may be nearly 1000 km in width. MCC's reside at the larger end of the spectrum of cloud clusters and as such are more strongly influenced by the earth's rotation. As a result MCC's tend to be longer lived and more inertially stable than smaller scale mesoscale convective systems.

33. SIO209 Cloud clusters Coud clusters and MCCs produce rainfall over a large areas exceeding 100,000 km2. Like squall lines, most of the rainfall early in the MCC lifecycle is in convective showers. As the systems mature, however, the rainfall transforms into primarily stratiform, steady rain which can last for 6 to 12 hours. If the systems move slowly, they can produce such a large volume of rainfall in a given watershed that major, catastrophic floods occur. Severe weather in the form of hail and tornadoes is often sporadic in MCC's, usually occurring during the early, intense convective phase of the storm. What is surprising is that as many as 25% of all MCC's produce severe, straight-line, damaging winds in swaths 100 km or more in width and 500-1000 km in length. Such severe straight-line wind events are called derechos.

34. SIO209 Cloud clusters - climate giant vacuum cleaners sweeping often polluted boundary layer air into the middle and upper troposphere and replacing it with clean middle tropospheric air. MCC's have been found to remove boundary layer air over nearly one half a million square kilometers Scavenging by cloud particles and settle out in precipitation particles creating major acid rain events. Other polllutants can be exported into the upper troposphere and lower stratosphere. Remove large quantities of water vapor from the lower troposphere and injecting it into the middle and upper troposphere and lower stratosphere, and precipitating large amounts to the surface. Latent heat deposited in the atmosphere is roughly proportional to surface precipitation, these systems likewise play a major role in the global energy budget, specially in the tropics where MCSs are frequent and cover a large fraction of tropical latitudes.

35. SIO209 Marine stratocumulus occupy large portions of the eastern Pacific and eastern Atlantic oceans and small portions of the western Indian ocean. cover 34% of the world's oceans at any given time; play an important role in the global radiation budget. well-mixed subcloud and cloud layer, capped by a strong temperature inversion and drop in dew point temperature. Large-scale sinking motion maintains the capping inversion which serves as a lid preventing convective circulations in the stratocumulus cloud layer from penetrating very far into the overlying stable airmass. Typical lifetimes of stratus and stratocumulus clouds are long; being 6 to 12 h. The parcel lifetimes for 1000 m deep clouds having vertical velocities 0.1 m/s. Lagrangian timescale 10,000 sec.

36. SIO209 Marine stratocumulus liquid water contents ranging from 0.05 to 0.25 g m^3 and long Lagrangian time-scales, drizzle can form in the deepest, wettest stratus and stratocumulus clouds The liquid water content, depth and strength of vertical motions vary considerably in stratocumulus clouds. Some stratocumulus clouds are driven primarily by the transport of heat and moisture from the sea surface. Latent heat release during the condensation of vapor to form cloud drops further invigorates the updrafts in the cloud layer, deepening the entire layer beneath the capping inversion. Radiative cooling near the top of the cloud layer is important to destabilization of the cloud layer and to the intensity of convective overturning. In other stratocumulus layers, heat and moisture fluxes from the ocean surface are weak, and the intensity of convective overturning is regulated mainly by cloud top radiative cooling and by evaporation of cloud drops near cloud top.

37. SIO209 Marine stratocumulus Vertical shear of the horizontal wind also contributes to stratocumulus cloud formation and to vertical mixing in stratocumulus clouds. In some instances strong winds in the ABL can generate stratocumulus clouds even where there is little or no temperature difference between the sea surface and overlying air. It appears that vertical wind shear can also trigger sporadic episodes of vertical mixing rather than continuous, homogeneous mixing. Other factors affecting the intensity and bulk properties of stratocumulus are: the strength of large-scale sinking motion, the occurrence of drizzle, and the presence of middle and high clouds above the stratocumulus deck.

38. SIO209 Marine stratocumulus stratocumulus clouds are low clouds and their tops are only slightly cooler than the earth's surface. They therefore radiate much of the infrared radiation emitted from the earth's surface. Likewise since the liquid water content in stratocumulus clouds is relatively large, they reflect much of the sun's radiation that is incident at their tops. As a consequence, stratocumulus clouds contribute to a net cooling of the atmosphere. Because stratocumulus clouds are nearly solid cloud decks, their contribution to a net cooling is greater than that by shallow cumulus clouds.

39. SIO209 Marine stratocumulus predictions with general circulation models of climatic changes due to greenhouse gases must contain better algorithms for predicting stratocumulus cloud amounts and optical depths. general circulation models must predict the transition from solid stratocumulus decks to broken cumulus layers. The reduced cloud coverage of a cumulus layer changes the amount of net cooling produced by low-level clouds. Because the transition from solid to broken cloud cover is a function of the fluxes of heat and moisture from the sea surface as well as large scale sinking motions, which are in turn related to sea surface temperatures and other factors, the general circulation model must also predict sea surface temperatures accurately

40. SIO209 Middle and upper level clouds vast sheets of middle and high-level clouds in the troposphere are ubiquitous, covering between 30-40% of the earth at any one time Global coverage of very thin cirrus clouds in the upper tro­posphere may be as high as 80%. Unlike stratocumulus clouds, middle and high clouds may contribute to a net warming of the troposphere. Cirrus clouds, are relatively thin and as a result do not absorb or reflect much of the sun's energy. On the other hand, they are excellent absorbers of longwave radiation emitted by the earth's surface, thus inhibiting the escape of longwave radiation energy to space. The reduced loss of longwave radiation to space contributes to a net warming at the earth's surface much like so-called “greenhouse gases”.

41. SIO209 Middle and upper level clouds cirrus clouds are radiatively thin, much of the upward infrared radiation is absorbed uniformly through the cirrus layer contributing to a net warming of the cloud layer middle tropospheric clouds, however, produce no net warming or cooling since the opposing influence of reflection of solar radiation is balanced by absorption and emission of longwave radiation at warmer temperatures. Middle and high clouds can be produced by a variety of mechanisms. Slow, slantwise ascent of moist air in extra-tropical cyclones can give rise to widespread altostratus, altocumulus and cirrus clouds deep convective clouds can inject large quantities of moisture and cloud debris into the middle and upper troposphere. Organized convective systems such as tropical cyclones and mesoscale convective systems are prolific producers of middle and high clouds.

42. SIO209 Middle and upper level clouds Owing to the small difference between ice saturation vapor pressures and environmental vapor pressures at the cold temperatures in the upper troposphere, very small additions of moisture or weak vertical motions and adiabatic cooling can create cirrus clouds. Thus jet contrails in regions of dense air traffic can release sufficient moisture in the upper troposphere to create widespread thin cirrus cloud cover. If the contrails become sufficiently widespread, they can alter the global radiation budget averaged over the diurnal cycle, contributing, albeit slightly, to a “greenhouse-type” warming.

43. SIO209 Middle and upper level clouds Many middle and high clouds are multilayered in structure. The actual mechanisms responsible for multi-layering are still not well understood. Likewise, some middle and high clouds exhibit a well-mixed thermodynamic structure suggesting that radiative destabilization of the cloud layer may trigger convective overturning. Overall our knowledge of the dynamics of middle and high clouds remains rather primitive. Only a few models have been developed for theoretically exploring their structure. Because of their height above the ground, expensive high altitude aircraft are required to sample them. Sensitive remote sensing devices such as lidars and radars are now being developed and applied to the study of those clouds.

44. SIO209 Conclusions Clouds and cloud systems described above are associated with larger scale weather systems. Middle and upper tropospheric layer clouds are associated with the gentle rising motions in extra-tropical and tropical cyclones. Layer clouds also form as exhaust products of deep convection embedded in tropical and extra-tropical cyclones. the full spectrum of convective clouds we have discussed form in large scale cyclonic storms. Squall lines, severe convective storms, ordinary thunderstorms, and towering cumulus clouds form in the warm sector and along frontal bands of extratropical cyclones. The global climatology of cloud cover, precipitation, transports of pollutants and trace gases, and latent heating is, therefore, strongly affected by the presence of large mountain ranges, and the climatology of larger scale weather systems.