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Precipitation and Cloud Topic What is cloud Cloud Classification Cloud Formation Overview of Cloud Formation Cloud formation by Bounyant Lifting Cloud formation by Forced lifting Cloud formation due to Cooling by the Earth’s surface What is cloud? A cloud is a visible mass of condensed droplet, frozen crystals suspended in the atmosphere above the surface of the Earth The condensing substance is typically water vapor, which forms small droplets or ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystal they become visible as clouds. Cloud base or condensation level Cloud Classification Cloud Classification Low Clouds Middle Clouds High Clouds Cloud Vertical Development Low Clouds Low Clouds Stratus Stratocumulus Nimbostatus Low cloud base height are within 2000 m. from earth surface. Generally compose of water droplets or supercool droplets. Stratocumulus are low, layered clouds with some vertical development. Their darkness varies when seen from below because their thickness varies across the cloud. Thicker sections appear dark, and thinner areas appear as bright spots. Low clouds have bases below 2000 m. Stratus are layered clouds that form when extensive areas of stable air are lifted. Usually the rate of uplift producing a stratus cloud is only a few tens of centimeters per second, and its water content is low. Low, layered clouds that yield light precipitation are called nimbostratus. Seen from below, these clouds look very much like stratus, except for the presence of precipitation. Middle Clouds Middle Clouds Altostratus Altocumulus Middle-cloud base occur between 2000 to 7000 m. above ground. Cloud temperature below 0 C. As a result, middle-cloud compose of supercool droplets or ice crystals. Altostratus clouds are the middle-level counterparts to cirrostratus. They are more extensive and composed primarily of liquid water. Altostratus scatter a large proportion of incoming sunlight back to space. The insolation that does make its way to the surface consists primarily or exclusively as diffuse radiation. When viewing the Sun or Moon behind altostratus, one sees a bright spot behind the clouds instead of a halo. Altocumulus are layered clouds that form long bands or contain a series of puffy clouds arranged in rows. They are often gray in color, although one part of the cloud may be darker than the rest and consist mainly of liquid droplets rather than ice crystals. High Clouds High Clouds Cirrus Cirrostratus Cirrocumulus High cloud typically form between 6000 to 10000 m. above the ground depend of the season and the latitude (higher where warmer). High cloud compose of entirely ice crystals. High clouds are generally above 6000 m (19,000 ft). The simplest of the high clouds are cirrus, which are wispy aggregations of ice crystals. Cirrostratus clouds are composed entirely of ice but tend to be more extensive horizontally and have a lower concentration of crystals. When viewed through a layer of cirrostratus, the Moon or Sun has a whitish, milky appearance but a clear outline. A characteristic feature of cirrostratus clouds is the halo, a circular arc around the Sun or Moon formed by the refraction (bending) of light as it passes through the ice crystals. Cirrocumulus are composed of ice crystals that arrange themselves into long rows of individual, puffy clouds. Cirrocumulus form during episodes of wind shear, a condition in which the wind speed and/or direction changes with height. Wind shear often occurs ahead of advancing storm systems, so cirrocumulus clouds are often a precursor to precipitation. Because of their resemblance to fish scales, cirrocumulus clouds are associated with the term “mackerel sky.” Clouds vertical development Clouds vertical development Cumulus Cumulonimbus Cloud base height are lower than 2000 m. but their top may reach top of tropopause. Cumuliform clouds are those that have substantial vertical development and occur when the air is absolutely or conditionally unstable. Fair-weather cumulus (above) called cumulus humilis, do not yield precipitation and they evaporate soon after formation. Cumulonimbus are the most violent of all clouds and produce the most intense thunderstorms. In warm, humid, and unstable air, they can have bases just a few hundred meters above the surface and tops extending into the lower stratosphere. A cumulonimbus is distinguished by the presence of an anvil composed entirely of ice crystals formed by the high winds of the lower stratosphere that extend the cloud forward. Mechanism of Cloud Formation: Overview Clouds Aggregations of Liquid droplets Droplets & ice crystals Ice crystals Low clouds Middle clouds High clouds Formed when air becomes saturated Due to Modify from: Danielson, et al, 2003, Meteorology Mechanism of Cloud Formation: Overview Saturation Process Cooling Moisture gain Mixing Adiabatic ascent Contact with Forces (stable) earth’s surface ascent Buoyant (unstable) ascent Orogr Fron -aphic -tal Earth’s surface Evaporating precipitation Parcel Vertical More than one mechanism may be archive in the same cloud Modify from: Danielson, et al, 2003, Meteorology Mechanism of Cloud Formation: Overview More than one mechanism may be archive in the same cloud. Upward Motion as a Cooling Mechanism Adiabatic Changes in Rising Air Parcel Graphic Dry Adiabatic Processes Moist Adiabatic Processed Adiabatic Changes in Rising Air Parcel Change in density, temperature and pressure P4, T4, 4 Height P e = P4 P3, T3, 3 P2, T2, 2 Pe P1, T1, 1 Time Pressure Assume that during this displacement, no exchange of heat energy occur between air parcel and environment Adiabatic Changes in Rising Air Parcel Changes in Density Density = mass volume Density decrease due to greater volume air parcel Changes in Temperature Assume adiabatic process, there is no external source of energy for air parcel. 10C/km Air parcel require energy for expansion, so energy come from parcel’s component molecular. 2 1mv1 = 2 Tp1 Rate of temperature change is known as dry adiabatic lapse rate 2 1mv2 + work 2 Tp2 Air parcel expansion Adiabatic Changes in Rising Air Parcel Change in Pressure According to gas law; Pressure = constant density temperature P = RT Pressure will decrease until air parcel pressure equals the surrounding air pressure; otherwise, a pressure gradient will exist, and the parcel will continue to expand. Why cloud form in rising air Graphing Dry Adiabatic Processes Variables of State Temperature-pressure plotting chart Graphing Dry Adiabatic Processes Dry adiabatic lapse rate 10C/km Dry adiabatic temperature change ascent decent Graphing Dry Adiabatic Processes Dewpoint Changes Saturated Mixed ratio line There is water vapor 2 grams within each kilogram of dry air Moisture change can be accounted. The saturation mixing ratio value corresponding to the dewpoint gives the actual amount of water vapor present in the air. The dewpoint temperature follows a line of constant mixing ratio. Therefore, dewpoint temperature decrease much more slowly with height than the air temperature. Graphing Dry Adiabatic Processes Determining Relative Humidity RH = Mixing ratio 100 Saturation mixing ratio = 2 g/kg 100% 8 g/kg = 25% Determining the Lifting Condensation Level LCL is level where dewpoint and rising air temperature become equal. Lifting Condensation Level LCL commonly marks the cumulus cloud base. The first wisps of cloud beginning to form. Moist Adiabatic Processes Is it possible that air parcel continuously move follow dry adiabatic lapse rate? Moist Adiabatic Processes Is it possible that air parcel continuously move follow dry adiabatic lapse rate? Impossible because the air would be highly supersaturated, an occurrence not observed. RH = Mixing ratio 100 Saturation mixing ratio = 2 g/kg 100% 0.5 g/kg = 400% Moist Adiabatic Processes Relative humidity in the cloud typically is never far from 100% that means temperature and dewpoint temperature equal to each other as the saturated air continues to ascent. As water vapor condenses to droplet, it changes conditions in the parcel in two important ways; The water molecule condense from gas to liquid. The air’s parcel mixing ratio decrease. The water releases considerable latent heat, thus warming air parcel Moist Adiabatic Processes Mixing ratio = 1g/kg at “D” Mixing ratio of air parcel = 2g/kg The air parcel lost 2 – 1 = 1 gram of water vapor from each kilogram of dry air. The lost water vapor become droplet of ice crystal forming. Cloud formation by Buoyant Lifting Heating of the earth’s surface may lead to convection, and convection consists in part of rising air currents. Thus the rising parcel is part of a convective circulation. As the earth’s surface becomes warmer, more air is carried aloft convectively, cooling as it rise until its water vapor begins to condense to form cumulus clouds. However, the above explanation is incomplete. How height the rising air will go? Will it stop rising before reaching the LCL and form no cloud at all? Will it reach LCL and form a cumulus humilis, or will it grow to a giant cumulonimbus? Parcel versus the Environment Parcel = a specific group of gas molecules that does not mix with the surrounding air but changes temperature with altitude at either the dry or moist adiabatic rates. Environment = consist of different air molecules at each level whose temperature and humidity values may differ erratically from one level to the next. Radiosonde data Cloud Formation Due to Static Instability Stability of Unsaturated air Stability of Saturated Air Stability of Lapse Rate Changes in Stability Stability of Unsaturated air Air parcel Environment temperature temperature New Mexico, Late afternoon Tp < Te From P = RT = P RT Air parcel density less than environment. Therefore, being denser, the parcel will sink back toward the surface from which it was perturbed; air is stable. Stability of Unsaturated air Environmental temperature Air parcel temperature Tp > Te From P = RT = P RT Air parcel density greater than environment. Therefore, lower density, the parcel will rising from the surface; air is stable. New Mexico, Early spring morning Stability of Unsaturated air Cumulus clouds occur with greater frequently in the afternoon than in the morning. In late afternoon, the earth’s surface is at its warmest, the near-surface environment is at its most unstable, and convective currents leading to cumulus formation are therefore most likely to exist. Near sunrise, the ground is coldest, causing the near-surface atmosphere to be at its most stable, with a corresponding absence of deep convection and cumulus cloud development. Stability of Saturated Air At 800 mbar, air is saturated. Stability and Lapse Rate Absolutely unstable; any environment whose lapse rate is greater than dry adiabatic. A lifting air parcel, whether it cools dry or moist adiabatically, will remain warmer than the environment and hence will be buoyant and continue to rise. Stability and Lapse Rate Conditionally unstable; any environment with lapse rate between dry and moist adiabatic. Parcel stability is depend on the air’s humidity. If it is saturated, a parcel will unstable. If it is unsaturated, a parcel is stable. Stability and Lapse Rate Absolutely stable; environment lapse rate less than the moist adiabatic rate. Parcel dose not depend on the air’s humidity: whether the parcel rise dry or moist adiabatically. Cumulus Cloud Growth Cumulus formation results from buoyant forces acting on a perturbed air parcel. The parcel, being warmer and less dense than the environment, rises and cools at dry adiabatic rate. Eventually, condensation occurs; thereafter, cooling proceeds at the moist adiabatic rate as the parcel continues to rise. Cumulus Cloud Growth Cloud Formation by Forced Lifting Stratiform clouds Some external mechanism forces the rntire layer to ascend. Orographic Lifting Lifting in Fronts and Low Pressure Centers Orographic Lifting Unlike buoyant lifting, in which individual cells of air rise, the entire lowest layer of the atmosphere has ascended by moving uphill. The result is the uniform sheet of clouds. Orographic lifting of stable air causes stratiform and lenticular clouds. The air, perturbed upward orographically, receives no additional lift from buoyancy and settles back to lower altitude once past the perturbing influence if high terrain. If the orographic lifted air is unstable, however, then cumulus clouds result from the combined influences of buoyancy and orographic lifting. The high frequency of cumulus clouds and thunderstorms in the mountains is a result of orographic lifting and buoyancy working together. Lifting in Fronts and Low Pressure Ceneter The front rises steeply, about1 km over 50 km distance, with showers and thunderstorms at the front. Lifting in Fronts and Low Pressure Ceneter The cross-sectional view shows the gentle slope of overrunning warm air (shallow slope 1:300) , a typical temperature inversion (32oF isotherm bends back), and the shifting winds. Occluded Fronts Fast moving cold fronts may overtake the slower moving warm front, particularly when they are influenced by cyclonic winds. Cold occlusion describes this scenario with very cold air, as compared with the warm occlusion. Figure 12.19 Formation of a cold occluded front: The faster moving cold front (a) catches up to the slower moving warm front (b), and forces it and the warm air mass to rise off the ground (c). [Green shading represents precipitation] (c) (b) (a) Idealized life cycle of a wave cyclone Tropical Cyclone (TC). A tropical cyclone is a warm-core, low pressure system without any "front" attached, that develops over the tropical or subtropical waters, and has an organized circulation. Depending upon location, tropical cyclones have different names around the world. In the: Atlantic/Eastern Pacific Oceans - hurricanes Western Pacific - typhoons Indian Ocean - cyclones Regardless of what they are called, there are several favorable environmental conditions that must be in place before a tropical cyclone can form. They are: Warm ocean waters (27?C) throughout a depth of about 46 m. An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection. Relatively moist air near the mid-level of the troposphere (4,900 m). Generally a minimum distance of at least 480 km from the equator. A pre-existing near-surface disturbance. Low values (less than about 37 km/h) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the change in wind speed with height. 1. Formation 1. 2. 3. Sea surface temperature at least 80?F (27?C) Calm air over the sea Coriolis Force, the force that causes the cyclone to spin. 2.Prematurity As the tropical low becomes further organized and the surface winds reach gale force it is then declared a tropical cyclone according to international convention. Satellite and radar observations of the system show the distinctive spiral banding pattern. 3. Full Maturity If the ocean and atmosphere environment continues to be favorable the cyclone may continue to intensify as it moves poleward. The cloud system becomes more circular in shape and develops a distinct eye. This is the severe cyclone stage where the cyclone is at its most dangerous. Approximately half of the cyclones that form progress to full maturity. 4. Decay Tropical cyclones normally decay when they move into a less favourable environment, either over land or the cooler waters in higher latitudes. The rate of decay varies with the circumstances. A tropical cyclone moving into mid-latitude westerlies may be quickly sheared apart by strong upper winds, or it may react with a frontal system and persists for several more days. Similarly, a cyclone moving over land normally dissipates rapidly due to loss of its energy source, namely the warm ocean surface. However in northern Australia cyclones moving inland are frequently observed to persist as rain depressions for a number of days bringing widespread flood rains, and may even redevelop if they move over the ocean once more. Tropical Cyclone Structure 4 1 2 3 Tropical Cyclone Classification 63 km/h or less 63 - 118 km/h 119 km/h or more "tropical depressions". "tropical storm" the cyclone is called: •hurricane in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, and the South Pacific Ocean east of 160?E, (The word hurricane comes from the Carib Indians of the West Indies, who called this storm a huracan. Supposedly, the ancient Tainos tribe of Central America called their god of evil "Huracan". Spanish colonists modified the word to hurricane.), •typhoon in the Northwest Pacific Ocean west of the dateline (super typhoon if the maximum sustained winds are at least 150 mph / 241 km/h) Cloud Development How are cloud droplets with typical diameters of 10-100 mm formed? surface tension keeps the surface area of a liquid to a minimum, thus it is difficult for molecules to stick together when the droplets are small this makes it impossible to start growing, or nucleating droplets of pure water Average Cloud droplet r = 10 mm, N = 106 number per liter, and v = 1 cm/s Average rain drop r = 50 mm, N = 103 number per liter, and v = 27 cm/s Average Cloud condensation nuclei r = 0.1 mm, N = 106 number per liter, and v = 0.0001 cm/s Cloud Development Why cloud droplets are observed to form in the atmosphere when ascending air just reaches equilibrium saturation? Answer: Atmosphere contains significant concentrations of particles of micron or submicron size which have affinity for water and serve as center for condensation. These particles are called condensation nuclei Cloud Development condensation nuclei (tiny solid and liquid particles, which are impurities in the air) of diameters 0.1-10 mm provide relatively large surfaces on which condensation of water vapour can readily occur; frozen nuclei help in the growth of ice crystals hygroscopic (water attracting) nuclei also help in the formation of cloud droplets Two important processes to form raindrops..… Collision-Coalescence Process Bergeron Process Collision-Coalescence process a large cloud droplet falls at a larger terminal velocity than a small droplet the terminal velocity characterizes the fall speed of an object, such as a paratrooper, where the downward force of gravity is balanced by the upward force of air resistance Collision-Coalescence process as the large droplet falls through a cloud, it sweeps up and collects smaller droplets through collision and coalescence the process becomes effective only when the cloud droplets reach about 40 mm but..... cloud droplets do not usually grow larger than 20 mm in diameter through condensation nuclei alone thus we need the..... Bergeron process operates in clouds at temperatures below 0oC, and requires the co-existence of water vapour, ice crystals and supercooled liquid water droplets (i.e., unfrozen water droplets below 0oC) at the same temperature, the saturation vapour prssure over a liquid surface is greater than over an ice surface, i.e., water molecules vaporize more readily from liquid water than from solid ice, at subfreezing temperatures thus a vapour pressure that is saturated for water droplets is supersaturated for ice crystals Bergeron process the net result is that the ice crystals grow at the expense of the supercooled water droplets as the ice crystals grow larger, they become heavier and the terminal velocity increases; they fall out of the cloud base and the coalescence process takes over