Houze Robert A., Jr. Cloud Dynamics

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time derivative in geostrophic coordinate system

List of Symbols xxix

height derivative in geostrophic coordinate system

an average; in discussions of atmospheric air motions, the

average is usually taken over a spatial volume or area; in

discussions of radar measurements (Chapter 4), the

overbar is used to indicate other types of averages (over

time, over a power spectrum); in discussions of laboratory

experiments (Chapter 7), it is used to indicate a time

average

()

average along the outer boundary of a cloud at a given

height

throughout the book, the subscript

0 indicates a property

of a hydrostatically balanced reference state; it is also

used in discussions of adjustment to geostrophic and

gradient wind balance to indicate the half-amplitude of an

initial discontinuity in fluid depth (Chapter 2); in

discussions of Doppler radar (Chapter 4) to indicate the

center of a circle centered on the radar; in discussions of

the cloud-topped boundary layer (Chapter 5) and flow over

topography (Chapter 12) to indicate conditions at the

surface of the earth; in describing a tornado vortex

(Chapter 8) to indicate the height of the lower tip of the

funnel cloud; and in discussing hurricanes (Chapter 10) to

indicate the temperature of the outflow from the eyewall

property of entrained air

property of detrained air

geostrophic value of a variable

conditions at cloud base

conditions at cloud top

conditions at

th~top

of the boundary layer

deviation from

deviation from nor ( )

deviation from a hydrostatically balanced reference state

in discussions of shallow layer clouds, these brackets

indicate integration with respect to height over the depth

of a mixed layer (Chapter 5); in discussions of cloud

modeling (Chapter 7), they indicate a horizontal average at

a given height over a circular region centered on the

central vertical axis of the cloud

) indicates evaluation at constant ( )

= i

(.i-)

I + j

(.i-)

I + k

(.i-)

y.p.t

x.p.t

x.y.t

horizontal gradient operator

three-dimensional gradient operator

()e

()s

()g

()t

( )"

()'

()*

()

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Part

Fundamentals

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Chapter

1 Identification

Clouds

"Here

lions threat, there elephants will range,

And camel-necks to vapoury dragons change

...

Clouds exhibit a wide variety of sizes and shapes, which reflect variations in the

dynamical processes producing clouds. In this chapter, we review the basic no-

menclature for identifying clouds. This nomenclature makes no attempt to explain

the observed clouds; it is based only on the sizes, heights, and physical appear-

ance of the cloud. This purely descriptive method of cloud identification is very

practical since

provides weather observers at locations all over the world a

simple, direct way to report clouds without having to make a physical interpreta-

tion of what they see.

For

our

purposes, it allows us to establish major categories

of cloud type based purely on observation. The classification of cloud phenomena

thus remains completely independent of our attempts to explain their structures

physically

and

dynamically in later chapters.

Before describing the system of cloud identification, we will briefly review the

nomenclature used to describe the vertical and horizontal dimensions of cloud

phenomena in Sec. 1.1. Since the vertical dimensions are often described in terms

of the nomenclature used to describe the vertical structure of the atmosphere, we

review first the terminology used to describe the vertical layering of the atmo-

sphere. Then, also in Sec. 1.1, we present the definitions of terms used to describe

the horizontal scales of phenomena in the atmosphere. Since the horizontal sizes

of clouds range from less than a kilometer to thousands of kilometers, this hori-

zontal-scale nomenclature is quite necessary to the description and classification

of clouds.

In Sec. 1.2, we examine the cloud identification scheme used by weather ob-

servers. This method has evolved

over

two centuries into an internationally ac-

cepted procedure.

is based solely on what clouds look like to a person observing

them visually.

The

basic cloud types identified according to this scheme will be

described with pictorial examples.

5 From

Goethe's

In Honour

Howard, a poem about clouds written in recognition of Luke

Howard, the early nineteenth-century British pharmacist, meteorologist, and devout Quaker who

devised the naming system of clouds in use today. Goethe, who was himself interested in clouds,

greatly admired Howard and was so touched upon corresponding with him that he wrote the poem

(translated from German by George Soane and Sir John Bowring) from which these lines referring to

the ephemeral forms of clouds are taken. Other lines from Goethe's poem appear at the headings of

later chapters.

For

more on Howard and his relationship to Goethe, see Scott (1976).

4 1 Identification of Clouds

The visual cloud identification scheme is inherently limited by the field of view

of the observer on the earth or in an aircraft. Larger cloud patterns associated

with organized storm systems can be seen only from a platform in space. In Sec.

1.3 we therefore extend the cloud identification scheme to include the systems of

storm clouds that are identifiable in satellite pictures.

1.1 Atmospheric Structure and Scales

Before proceeding with our discussion of cloud types and nomenclature, we first

take note of some basic terminology used in describing the structure of the atmo-

sphere and its phenomena. The atmosphere is usually divided into several layers,

based on the mean vertical profile oftemperature (Fig.

1.1). Most clouds occur in

the

troposphere, which is the lowest layer. In the troposphere, which contains

nearly all of the water in the atmosphere, the temperature decreases with height in

the mean. The top of the troposphere, called the

tropopause, occurs at about the

12-km level.

is lower than this over the poles and higher in equatorial regions.

Above the tropopause, the mean temperature profile is first isothermal and then

increases with height in the

stratosphere. At the bottom of the troposphere, the

atmosphere is affected by the presence of the earth's surface, through the trans-

fers of heat and momentum. The layer in which this influence is felt is called the

100

:d.~~

THERMOSPHERE

0.001

0.002

Q.OO5

0.01

0.02

0.05

0.1

EGO

MESOSPHERE

0.2

I-

0.5

STRATOPAUSE

l/l

Q..

STRATOSPHERE

100

200

500

1000

160

300

Figure 1.1 Vertical temperature profile for the U.S. Standard Atmosphere. (From Wallace and

Hobbs,

1977.)

1.2 Cloud Types Identified Visually 5

Figure 1.2 Division of the atmosphere into two layers: a boundary layer near the surface and

the free atmosphere above. The top of the boundary layer is often

-I

km but may be much less, e.g.,

-100

m, depending on wind and thermodynamic properties of the air near the surface. The tropopause

height is

-10-12

km at high altitudes and

-14-18

km in the tropics. (From Stull, 1988. Reprinted by

permission of Kluwer Academic Publishers.)

planetary boundary layer (Fig. 1.2). As will be discussed in Sec. 2.11.2, the depth

of the planetary boundary layer is highly variable, ranging from

-10

m to

2-3

km.

The lowest 10%

ofthe

planetary boundary layer is referred to as the surface layer.

The region lying above the boundary layer is referred to as the free atmosphere.

The scales of air motion encountered in cloud dynamics can be divided roughly

into three ranges. The

synoptic scale encompasses phenomena exceeding about

2000 km in horizontal scale; the

mesoscale covers phenomena between about 20

and 2000km in scale; and the

convective scale covers phenomena between 0.2 and

20 km." These definitions are loose and somewhat overlapping. As yet, a univer-

sally accepted physically (as opposed to phenomenologically) based distinction of

these scales has not been achieved." The discussions and interpretations in this

book are not strongly dependent on the distinctions among these three scales;

however, the above ranges are useful to keep in mind as a guide.

1.2 Cloud Types Identified Visually

1.2.1 Genera, Species, and Etages

Visual observation of clouds shows that they take on several distinctive forms

which are recognized and named internationally, so that weather observers can

record and report the local state of the sky in a way that is readily understandable

without the aid of pictures. Once every 6 hours, observers at weather stations

around the world identify the amount and types of clouds present, and this infor-

mation is transmitted for immediate use, as well as archived for climatological

purposes. Many stations make a more limited evaluation of the state of the sky

every hour.

The internationally agreed method of naming clouds serves as a convenient

way to begin to organize our discussion of cloud dynamics. The categories of

6 These ranges were suggested by Orlanski (1975). For an extension of Orlanski's scheme into the

domain of cloud and aerosol microphysical processes, see Hobbs (1981).

For

a discussion of the problem of defining the mesoscale physically, see Emanuel (l986b).

6 1 Identification of Clouds

clouds reported by observers are identified purely on the basis of their visual

appearance. Thus, the observer is not required to make a physical interpretation,

only a description. Our task then is to provide a dynamical explanation for each

type of cloud. Chapters

5-8

and Chapter 12 will be devoted to the dynamics of

those clouds that can be identified visually by a ground observer. Chapters

9-11

will be concerned with the dynamics of larger conglomerates of clouds, which are

too extensive spatially to be identified by a ground observer and must therefore be

viewed from a satellite perspective.

The method of visual identification and classification of clouds that we follow

here is basically that of the World Meteorological Organization's

International

Cloud Atlas,

which is the guidebook for official weather observers around the

world." According to this scheme, cloud types are given descriptive names based

on Latin root words.

10 CumuLus means heap or pile. Stratus is the past participle

of the verb meaning to flatten out or cover with a layer.

Cirrus means a lock of hair

or a tuft of horsehair.

Nimbus

refers to a precipitating cloud, and altum is the

word for height. These five Latin roots are used either separately or in combina-

tion to define 10 mutually exclusive cloud

genera, which are organized into three

groups, or

etages, corresponding to the typical height of the base of the cloud

above the local height of the

earth's

surface, as indicated in Table 1.1.

The

etages overlap and their limits vary with altitude. Each genus may take on several

different forms, which are designated as

species. Species are further subdivided

into

varieties. In this book, we will refer to only a few species and varieties;

however, we will consider all of the ten genera.

In addition to the genera in Table 1.1, we will consider

fog

as an eleventh cloud

type. Fog is generally any cloud whose base touches the ground.

does not

appear as a cloud genus in Table 1.1 because, according to the internationally

specified procedures for reporting and archiving meteorological data, fog is coded

by weather observers not as a cloud but rather as a "restriction to visibility." In

8 There have been several editions of this atlas. The first was the International Atlas

Clouds and

Study

the Sky, Volume I, General Atlas, published in 1932by the International Commission for the

Study of Clouds. The World Meteorological Organization (established in 1951) published the first

edition

ofthe

International Cloud Atlas, Volumes I and II, in 1956. The first volume contains descrip-

tive and explanatory text, while the second volume contains 224 illustrative plates. An abridged

version combining the essential information in the two original volumes was published in 1969. A

revised edition of Volume I was published under the title

Manual on the Observation

Clouds and

Other Meteors

in 1975. An extensively updated version of Volume II, containing 196 plates, was

published in 1987.

9 Other useful pictorial cloud guides include: Cloud Atlas, An Artist's View

Living Cloud (Itoh

and Ohta, 1967),

Clouds

the World: A Complete Color Encyclopedia (Scorer, 1972),A Field Guide

to the Atmosphere

(Schaefer and Day, 1981), and The Cloud Atlas

China (National Meteorological

Service of China, 1984), and

Spacious Skies (Scorer and Verkaik, 1989).

10 The Latin naming scheme was proposed in 1803 by Luke Howard. The Latin names quickly

caught on and have been used in meteorological textbooks since the mid-nineteenth century.

II The method of dividing the atmosphere vertically into three layers (etages) where clouds form

was introduced by the French naturalist Jean Babtiste Lamarck in 1802. He proposed a cloud classifi-

cation with French names, which were not universally adopted. Today's cloud classification scheme is

a combination of Howard's Latin names and Lamarck's organization into three

etages,

1.2 Cloud Types Identified Visually 7

Table 1.1

Genera and Etages of Clouds Identified Visually

Height of cloud base

Polar

Temperate Tropical

Genus

Etage

regions regions

regions

Cumulus }

Cumulonimbus

Stratus Low below 2 km

below 2 km

Stratocumulus

Nimbostratus

Altostratus

}

Middle

2-4

2-7

km 2-8 km

Altocumulus

Cirrus }

Cirrostratus High

3-8

5-13 km 6-18 km

Cirrocumulus

this book, we depart from this convention and consider fog to be a type of cloud.

is grouped with clouds

ofthe

lowest etage, since the cloud base is at the ground.

In Sees. 1.2.2-1.2.4, we will define, describe briefly, and show pictorial exam-

ples of each of the cloud types corresponding to the 10genera listed in Table 1.1

plus fog. We will examine these

cloud types in groups, according to etage,

considering first the low clouds, then middle clouds, and finally high clouds. This

order of discussion follows roughly the way in which a trained weather observer

normally proceeds, as the observer's

job

is to describe the entire state of the sky,

in terms of all of the clouds present. First the low clouds are identified. Then, to

the extent that they are not obscured by the low clouds, the middle cloud types

and amounts are determined, and finally the high clouds are evaluated, to the

extent that they are not obscured by low and middle clouds.

After discussing the 11 basic cloud types and their grouping according to the

three etages in Sees. 1.2.2-1.2.4, we will discuss the particular form taken by

certain orographically induced clouds in Sec.

1:2.5. Many of these orographic

clouds are designated as

lenticularis, 12 which is a species of stratocumulus, altocu-

mulus, or cirrocumulus. Lenticularis, however, is such a unique cloud form that it

could be considered a genus unto itself, and we will treat it separately in this text.

Chapters

5-9

are concerned with the dynamics of clouds of the

basic cloud

types, except for the lenticularis species, which is covered in Chapter

12 as part of

the separate treatment of the dynamics of orographic clouds.

1.2.2 Low Clouds

Clouds of the lowest etage include six types: the five low-cloud genera listed in

Table 1.1 plus fog. These six types may be divided into two subgroups:

cumuli-

12 Meaning the shape of a lentil in Latin, or, in more modern terms, lens shaped.

8 1 Identification of Clouds

form clouds (cumulus and cumulonimbus), which are composed

rapidly rising

air currents that give the clouds a bubbling and towering aspect, and

stratiform

clouds (fog, stratus, stratocumulus, and nimbostratus), which are broad sheets of

quiescent clouds characterized by little or no vertical movement of air. We will

describe the cumuliform clouds first, then the stratiform.

1.2.2.1 Cumuliform Clouds

Cumulus

clouds are

"detached

clouds, generally dense and with sharp outlines,

developing vertically in the form of rising mounds, domes or towers, of which the

bulging upper part often resembles a cauliflower. The sunlit parts of these clouds

are mostly brilliant white; their base is relatively dark and nearly horizontal.

Sometimes cumulus is ragged.

"13

Cumulus occur in a wide range of sizes. They

are less than a kilometer in horizontal and vertical extent in their early stages

development and often never become any larger (Fig. 1.3a), particularly when the

individual clouds are isolated. On other occasions, when there is a tendency for

the clouds to cluster, cumulus-may grow to larger size (Fig. 1.3b). A large cumulus

cloud (species

congestus) consists of a heap of rapidly fluctuating bulbous towers,

which give it its "cauliflower" appearance.

may have tops extending into the

second etage, However, it is always considered to be a low cloud because its base

is usually in the lowest layer.

Cumulonimbus is a

"heavy

and dense cloud, with a considerable vertical ex-

tent, in the form of a mountain or huge towers. At least part of its upper portion is

usually smooth, or fibrous or striated, and nearly always flattened; this part often

spreads out in the shape of an

anvil or vast plume. Under the base of this cloud,

which is often very dark, there are frequently low ragged clouds either merged

with it or not, and precipitation sometimes in the form of virga [precipitation not

reaching the ground]. " The cumulonimbus is an advanced stage of cumulus devel-

opment. As cumulus congestus continue to grow, they develop precipitation

(hence the nimbus designation), and the top usually turns to ice. An example of

cumulus congestus growing to the cumulonimbus stage is shown in Fig. 1.4a-d. In

its later stages the icy structure at the top has a fibrous appearance, and high

winds aloft can blow the top downwind, thus producing the anvil structure (Fig.

lAc

and d). The top of the anvil in the tallest clouds is usually very near the

tropopause level; it is flattened because the rising air in the cloud cannot signifi-

cantly penetrate the very stable stratosphere. As the glaciated anvil ages, large

quantities of icy cloud material are injected into the upper troposphere (Fig. 1.5).

From a satellite perspective, the anvil spreading downwind is the primary feature

identifying a cumulonimbus (Fig. 1.6). Like cumulus congestus, the cumulonim-

bus is classified as a low cloud because its base is within the lowest etage.

often

extends through all three layers, with its anvil occurring in the highest etage.

The anvil is not an essential feature of the cumulonimbus. In the tropics, a

towering cumulus, whose top is well below the

O°C

level, often produces a heavy

IJ Definitions of cloud genera given in quotation marks are from the International Cloud Atlas. (The

same definitions are used in all versions of the atlas.)