G304 – Physical Meteorology and Climatology Chapter Clouds and precipitation By Vu Thanh Hang, Department of Meteorology, HUS 6.1 Mechanisms that lift air • Four mechanisms lift air so that condensation and cloud formation can occur: - Orographic lifting, the forcing of air above a mountain barrier - Frontal lifting, the displacement of one air mass over another - Convergence, the horizontal movement of air into an area at low levels - Localized convective lifting due to buoyancy 6.1 Mechanisms that lift air (cont.) The upward displacement of air that leads to adiabatic cooling is called orographic uplift When air approaches a topographic barrier, it can be lifted upward or deflected around the barrier Downwind of a mountain ridge, on its leeward side, air descends the slope and warms by compression to create a rain shadow effect, an area of lower precipitation 6.1 Mechanisms that lift air (cont.) - Fronts are transition zones in which great temperature differences occur across relatively short distances - When cold air advances toward warmer air Æ cold front (a) - When warm air flows toward a wedge of cold air Æ warm front (b) 6.1 Mechanisms that lift air (cont.) West Texas Roll Cloud Provided by: Cindy Cassidy Date: Dec 28, 2006 A cold front moved into western Texas from New Mexico 6.1 Mechanisms that lift air (cont.) Approaching warm front Provided by: Val Vannet Date: May 31, 2005 The view looks towards Dodd Hill from beneath the pylons which cross the minor road in the east of the grid box The high cirrus cloud heralds the approach of a front and rain tomorrow 6.1 Mechanisms that lift air (cont.) • Pressure differences set the air in motion in the effect we call wind • When a low-pressure cell is near the surface, winds in the lower atmosphere tend to converge on the center of the low from all directions • Horizontal movement toward a common location implies an accumulation of mass called horizontal convergence 6.1 Mechanisms that lift air (cont.) • Free convection is lifting that results from heating the air near the surface • It is often accompanied by updrafts strong enough to form clouds and precipitation • Free convection arises from buoyancy 6.2 Static stability and the enviromental lapse rate • Statically unstable air becomes buoyant when lifted and continues to rise if given an initial upward push; • Statically stable air resists upward displacement and sinks back to its original level when the lifting mechanism ceases • Statically neutral air neither rises on its own following an initial lift nor sinks back to its original level; it simply comes to rest at the height to which it was displaced • Static stability is closely related to buoyancy • When an air parcel is less dense than surrounding air Æ positive buoyancy Æ floats upward 6.2 Static stability and the enviromental lapse rate (cont.) • Temperatures in the parcel are governed by either the dry or saturated adiabatic lapse rate, whereas the surroundings are governed by the environmental lapse rate (ELR) • Different types of air with regard to their static stability: absolutely unstable, absolutely stable, conditionally unstable 6.3 Cloud types (cont.) 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, not yield precipitation and they evaporate soon after formation 6.3 Cloud types (cont.) Intensely developed clouds are cumulus congestus They consist of multiple towers, and each tower has several cells of uplift This gives them a fortress-like appearance with numerous columns of varying heights Their strong vertical development implies that these clouds form in unstable air 6.3 Cloud types (cont.) 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 6.4 Cloud coverage and observation • An important characteristic of clouds is their breadth or coverage • When clouds occupy more than nine-tenths of the sky, conditions are said to be overcast • When coverage is between six-tenths and nine-tenths, it is called broken • Scattered clouds occupy between one-tenth and one-half of the sky, and less than one-tenth cloud cover is classified as a clear-sky condition 6.4 Cloud coverage and observation (cont.) 6.4 Cloud coverage and observation (cont.) • Cloud heights and coverages up to a height of 3650m are determined by automated devices called laser ceilometers • The laser units emit a brief pulse of energy upward that gets reflected downward by cloud droplets or ice crystals • The laser beam travels at a known speed, so the amount of time it takes for the pulse to make its round trip can easily be translated to the height of the cloud base 6.4 Cloud coverage and observation (cont.) • The distribution and movement of cloud cover can be observed by satellite • The satellite view the cloud cover by sensing reflected visible radiation • Visible images are not very good for distinguishing between high, medium, low level clouds, unable to provide any information at night • Infrared images sense the amount of electromagnetic energy emitted by clouds Æ can define cloud base, cloud top, thickness 6.4 Cloud coverage and observation (cont.) An infrared image A visible image 6.5 Forms of precipitation Dendrite ice crystals Plate ice crystal Snow results from the growth of ice crystals through deposition, riming, and aggregation Ice crystals in clouds can have a wide variety of shapes, including six-sided plates, columns, solid or hollow needles, and complex dendrites with numerous long, narrow extensions 6.5 Forms of precipitation (cont.) Rain is precipitation arriving at the surface in the form of liquid drops, usually between 0.5 and mm Episodic precipitation from rapidly developing cumuliform clouds is called showers and can occur as either rain or snow Raindrops are not teardrop-shaped They are initially spherical (a) but flatten out on the bottom as they fall (b) As they flatten, the greater surface area on the bottom causes greater resistance and further flattening (c) Eventually, the droplet breaks apart (d) 6.5 Forms of precipitation (cont.) • Graupel occurs when an ice crystal takes on additional mass by riming and contains very small air bubbles that give it a spongy texture and milky-white appearance • Hail consists of ice pellets formed in roughly concentric layers • Updrafts carry a particle into the colder reaches of a cloud, and the liquid water coating the ice freezes • When the stone exits the updraft and falls, it becomes wet from its collisions with liquid droplets • The hailstone can be captured once again by an updraft, and the coating of water freezes • This process, when it occurs repeatedly, forms large hail 6.5 Forms of precipitation (cont.) Sleet formation involves a mid-level inversion Freezing rain coats objects Sleet (above) occurs as rain falling from a cloud, passes through a cold layer, and freezes into ice pellets This is most common along warm fronts Freezing rain begins when a light rain or drizzle of supercooled drops falls through air with a temperature at or slightly below °C When the raindrops hit the surface, they form a thin film of water, but only for a moment Soon afterward the water freezes to form a coating of ice Global average annual precipitation 6.6 Measuring precipitation - Rainfall is usually measured by a rain gauge - Standard gauges have collecting surfaces with diameters of 20.3 cm - The precipitation funnels into a tube with one-tenth the surface area of the collector, so that the depth of water undergoes a tenfold increase - The precipitation level is measured by inserting a calibrated stick into the water, removing it, and noting the depth of the wet portion - The water equivalent of the snow, which is the depth of water that would result if all the snow were melted, can be roughly estimated using a conversion ratio of 10:1 6.6 Measuring precipitation (cont.) Radar can measure the intensity of precipitation by emitting microwave radiation with wavelengths of several centimeters Precipitating droplets, ice crystals, and hailstones can scatter the emitted radiation back to the radar unit The more intense the backscattered radiation, the more intense the precipitation [...]... clouds 6. 3 Cloud types (cont.) The ten principal types of clouds that result are then grouped according to their height and form: - High clouds: cirrus, cirrostratus, and cirrocumulus - Middle clouds: altostratus and altocumulus - Low clouds: stratus, stratocumulus, and nimbostratus - Clouds with vertical development: cumulus and cumulonimbus 6. 3 Cloud types (cont.) High clouds are generally above 60 00... clouds occupy between one-tenth and one-half of the sky, and less than one-tenth cloud cover is classified as a clear-sky condition 6. 4 Cloud coverage and observation (cont.) 6. 4 Cloud coverage and observation (cont.) • Cloud heights and coverages up to a height of 365 0m are determined by automated devices called laser ceilometers • The laser units emit a brief pulse of energy upward that gets reflected.. .6. 2 Static stability and the enviromental lapse rate (cont.) When a parcel of unsaturated or saturated air is lifted and the Environmental Lapse Rate (ELR) is greater than the dry adiabatic lapse rate (DALR), the result is absolutely unstable air 6. 2 Static stability and the enviromental lapse rate (cont.) When a parcel of... generally above 60 00 m (19,000 ft) The simplest of the high clouds are cirrus, which are wispy aggregations of ice crystals 6. 3 Cloud types (cont.) Cirrostratus clouds are composed entirely of ice but tend to be more extensive horizontally and have a lower concentration of crystals 6. 3 Cloud types (cont.) Cirrocumulus are composed of ice crystals that arrange themselves into long rows of individual, puffy... Cirrocumulus form during episodes of wind shear, a condition in which the wind speed and/or direction changes with height 6. 3 Cloud types (cont.) Altostratus clouds are the middle-level counterparts to cirrostratus They are more extensive and composed primarily of liquid water 6. 3 Cloud types (cont.) Altocumulus are layered clouds that form long bands or contain a series of puffy clouds arranged in... rest and consist mainly of liquid droplets rather than ice crystals 6. 3 Cloud types (cont.) 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 6. 3 Cloud types (cont.) Low, layered clouds that yield light precipitation... clouds look very much like stratus, except for the presence of precipitation 6. 3 Cloud types (cont.) 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 6. 3 Cloud types (cont.) Cumuliform clouds are those that have substantial... precipitation and they evaporate soon after formation 6. 3 Cloud types (cont.) Intensely developed clouds are cumulus congestus They consist of multiple towers, and each tower has several cells of uplift This gives them a fortress-like appearance with numerous columns of varying heights Their strong vertical development implies that these clouds form in unstable air 6. 3 Cloud types (cont.) Cumulonimbus are the... which a parcel of air must be lifted for it to become buoyant and to rise on its own 6. 2 Static stability and the enviromental lapse rate (cont.) Assume the ELR is 0.7 °C/100 m and the air is unsaturated As a parcel of air is lifted, its temperature is less than that of the surrounding air, so it has negative buoyancy 6. 2 Static stability and the enviromental lapse rate (cont.) A parcel starts off unsaturated... to provide any information at night • Infrared images sense the amount of electromagnetic energy emitted by clouds Æ can define cloud base, cloud top, thickness 6. 4 Cloud coverage and observation (cont.) An infrared image A visible image 6. 5 Forms of precipitation Dendrite ice crystals Plate ice crystal Snow results from the growth of ice crystals through deposition, riming, and aggregation Ice crystals