Bài giảng khí hậu học chương 6

6.2 Static stability and the enviromental lapse rate cont.. When a parcel of unsaturated or saturated air is liftedand the Environmental Lapse Rate ELR is greater than the dry adiabatic

Chapter 6

Clouds and precipitation

G304 – Physical Meteorology and Climatology

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 lowlevels

- Localized convective lifting due to buoyancy

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.)

A cold front moved into western Texas from New Mexico

West Texas Roll Cloud

Provided by: Cindy

Cassidy

Date: Dec 28, 2006

6.1 Mechanisms that lift air (cont.)

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

Approaching warm front

Provided by: Val Vannet

Date: May 31, 2005

• 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

• 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.

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 unsaturated or saturated air is lifted

and the Environmental Lapse Rate (ELR) is less than

the saturated adiabatic lapse rate (SALR), the result

is absolutely stable air and the parcel will resist lifting

6.2 Static stability and the enviromental

lapse rate (cont.)

• When the ELR is between the dry and saturated adiabatic lapse rates the air is said to be conditionally unstable , and the tendency for a lifted parcel to sink or continue rising depends

on whether or not it becomes saturated and how far it is lifted.

• The level of free convection (LFC) is the height to 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 but cools to the LCL,

where it is cooler than the surrounding air Further lifting

cools the parcel at the SALR At the 200-m level, it is

still cooler than the surrounding air, but if taken to 300 m,

it is warmer and buoyant

6.2 Static stability and the enviromental

lapse rate (cont.)

6.3 Cloud types

• Clouds can assume a variety of shapes and sizes and can occur near surface or at high altitude.

• Most cloud types occur in the troposphere.

• Clouds can contain liquid droplets, ice crystals, or a mixture

of the two.

• Clouds can be thick or thin and have high or low liquid water

or ice contents.

• Four basic categories:

- Cirrus : thin, wispy clouds of ice

- Stratus : layered cloud

- Cumulus : clouds having vertical development

- Nimbus : rain-producing clouds

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 6000 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 clouds.

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 rows They are often gray incolor, although one part of the cloud may be darker than the restand 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 are called

nimbostratus These 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 thicknessvaries 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 vertical

development 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

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.

6.3 Cloud types (cont.)

• An important characteristic of clouds is their breadth or

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.)

A visible image

An infrared image

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

Dendrite ice crystals Plate ice crystal

Rain is precipitation arriving at the surface in the form

of liquid drops, usually between 0.5 and 5 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 flattenout 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 (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 0 °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

6.5 Forms of precipitation (cont.)

Sleet formation involves

a mid-level inversion

Freezing rain coats objects

Global average annual 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

Radar can measure the intensity of precipitation by emitting microwaveradiation 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

6.6 Measuring precipitation (cont.)