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

Cloud base or condensation level

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.

What is cloud?

A cloud is a visible mass of condensed droplet, frozen crystals suspended in the atmosphere above the surface of the Earth

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 layeredclouds that form when extensive areas of stable air are lifted.

Usually the rate of uplift producing a stratus cloud is only a fewtens 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 orexclusively as diffuse radiation. When viewing the Sun or Moon behindaltostratus, one sees a bright spot behind the clouds instead of a halo.

Altocumulus are layered clouds that form long bands or containa series of puffy clouds arranged in rows. They are often gray incolor, 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 entirelyof ice but tend to be more extensive

horizontally and have a lower concentrationof crystals. When viewed through a layer

of cirrostratus, the Moon or Sun has awhitish, milky appearance but a clear

outline. A characteristic feature ofcirrostratus clouds is the halo,

a circular arc around the Sun or Moonformed by the refraction (bending) of

light as it passes through the ice crystals.

Cirrocumulus are composed of ice crystals that arrangethemselves into long rows of individual, puffy clouds.Cirrocumulus form during episodes of wind shear, a conditionin 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, cirrocumulusclouds 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 verticaldevelopment and occur when the air is absolutely or conditionallyunstable. 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: OverviewClouds

Liquid droplets Droplets & ice crystals

Ice crystals

Aggregations of

Low clouds Middle clouds High clouds

Formed when air becomes saturatedDue to

Modify from: Danielson, et al, 2003, Meteorology

Mechanism of Cloud Formation: Overview

Cooling Moisture gain Mixing

Adiabatic ascent

Buoyant (unstable)

ascentOrogr-aphic

Fron-tal

Forces (stable) ascent

Contact with

earth’s surface

Earth’s surface Evaporating precipitation

Parcel Vertical

Saturation Process

Modify from: Danielson, et al, 2003, Meteorology

More than one mechanism may be archive in the same cloud

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

Time

P1, T1, 1

P4, T4, 4

Hei

ght

P2, T2, 2

P3, T3, 3

Pres

surePe = P4

Assume that during this displacement, no exchange of heat energy occur between air parcel and environment

Change in density, temperature and pressure

Pe

Adiabatic Changes in Rising Air Parcel

Tp1

Tp2

Changes in Density

Density = massvolume

Density decrease due to greater volume air parcel

Changes in Temperature

Assume adiabatic process, there is no external source of energy for air parcel.

Air parcel require energy for expansion, so energy come from parcel’s component molecular.

1mv22

21mv1

2

2 = + work

Air parcel expansion

Rate of temperature change is known as dry adiabatic lapse rate

10C/km

Adiabatic Changes in Rising Air Parcel Change in Pressure

According to gas law;Pressure = constant density temperature

P = RTPressure 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

Temperature-pressure plotting chart

Variables of State

Graphing Dry Adiabatic Processes

Dry adiabatic temperature change

ascentdecent

Dry adiabatic lapse rate10C/km

Graphing Dry Adiabatic ProcessesDewpoint Changes

Saturated Mixed ratio line

Moisture change can be accounted. The saturation mixing ratio value corresponding to the dewpointgives the actual amount of water vapor present in the air.

There is water vapor 2 grams within each kilogram of dry 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

Lifting Condensation Level

Determining Relative HumidityRH = Mixing ratio

Saturation mixing ratio 100

= 2 g/kg8 g/kg

100%

= 25%

Determining the Lifting Condensation Level

LCL is level where dewpointand rising air temperature become equal.

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 ratioSaturation mixing ratio

100

= 2 g/kg0.5 g/kg

100%

= 400%

Moist Adiabatic Processes Relative humidity in the cloud typically is never far

from 100% that means temperature and dewpointtemperature 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 tocondense 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

Radiosonde data

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.

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 temperature

Environment temperature

Tp < Te

From P = RT

= PRT

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.New Mexico, Late afternoon

Stability of Unsaturated air Tp > Te

From P = RT = P

RT

Air parcel temperature

Environmental temperature

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 The front rises steeplyrises steeply, about1 km over 50 km distance, with , about1 km over 50 km distance, with showersshowers and and thunderstormsthunderstorms at the front.at the front.

Lifting in Fronts and Low Pressure Ceneter

The crossThe cross--sectional view shows the gentle slope of overrunning sectional view shows the gentle slope of overrunning warm air warm air (shallow slope 1:300) (shallow slope 1:300) , a typical temperature inversion , a typical temperature inversion (32(32ooF isotherm bends back), and the shifting winds.F isotherm bends back), and the shifting winds.

Occluded FrontsOccluded Fronts

Fast moving cold fronts may overtake the slower Fast moving cold fronts may overtake the slower moving warm front, particularly when they are moving warm front, particularly when they are influenced by cyclonic winds.influenced by cyclonic winds.

Cold occlusion describes this scenario with very cold Cold occlusion describes this scenario with very cold air, as compared with the warm occlusion.air, as compared with the warm occlusion.

Figure 12.19Figure 12.19Formation of a cold occluded Formation of a cold occluded front: The faster moving cold front: The faster moving cold front (a) catches up to the front (a) catches up to the slower moving warm front (b), slower moving warm front (b), and forces it and the warm air and forces it and the warm air mass to rise off the ground (c).mass to rise off the ground (c).[Green shading represents [Green shading represents precipitation]precipitation]

(a)

(b)

(c)

Idealized life cycle Idealized life cycle of a wave cycloneof a wave cyclone

Tropical Cyclone (TC).A tropical cyclone is a warm-core, low pressuresystem 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 tropicalcyclone 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. Sea surface temperature at least 80?F (27?C)

2. Calm air over the sea3. 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. DecayTropical 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

1

2

3

4

63 km/h or less "tropical depressions". 63 - 118 km/h "tropical storm"119 km/h or more 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)

Tropical Cyclone Classification

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 Cloud condensation nuclei

r = 0.1 mm,

N = 106 number per liter,

and v = 0.0001 cm/s

Average rain drop

r = 50 mm,

N = 103 number per liter,

and v = 27 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

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.....

Collision-Coalescence process

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 readilyfrom liquid water than from solid ice, at subfreezing temperatures

thus a vapour pressure that is saturated for water droplets is supersaturatedfor 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