Meteorology 534
Cloud Dynamics

Fall Semester 1996

Introduction: Classification of clouds (Houze, Cotton and Anthes chapters 1, International Cloud Atlas)

Identification:

Based on visual appearance -- distinctive forms

Names based on Latin root words --

Cumulus -- heap or pile -- detached clouds with vertical development

Stratus -- past participle of verb meaning to flatten, cover with layer -- grey clouds, fairly uniform base

Cirrus -- Lock of hair, tuft of horse-hair -- cirrus/cirro denotes high clouds

Nimbus -- Precipitating cloud

Altum -- Height, used to denote middle clouds

These rootwords used to define ten mutually exclusive genera
genus -- main characteristic form of cloud -- cirrus, cirrocumulus, cirrosstratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus and cumulonimbus -- an eleventh, fog, may be added as a distinct genus

These divided into etages (groups)
Three groups according to cloud-base height: High (6-12 km), middle (2-6 km) and low (0-2 km)

Each genus may be further divided into different species
Species -- perculiarities in shape and difference in internal structure of clouds -- fibratus, uncinus, spissatus, castellanus, floccus, stratiformis, nebulosus, lenticularis, fractus, humilis, mediocris, congestus, calvus, capillatus

which again can be further divided into varieties
Varieties -- special characteristics of arrangement and transperency -- intortus, vertebratus, undulatus, radiatus, lacunosis, duplicatus, translucidus, perlucidus, opacus

There are nine supplementary features and accessory clouds which are also used -- incus, mamma, virga, praecipitatio, arcus, tuba, pileus, velum, panus

Characteristics:

Important macroscopic parameters that characterize precipitation and diabatic processes
Cloud time scales, can identify two -- Time* -> time parcel spends in cloud, Time& -> cloud life-time -- precipitation formation require TIME

Vertical velocities -- control time scale (Time*) and the cloud's ability to suspend precipitation particles, provide measure of the wet adiabatic cooling rate (wet adiabatic lapse rate times vertical velocity) -- which physical processes important

Liquid water content (LWC) -- determine potential to form precipitation (typically > 0.5 g/m3 needed), long- and shortwave radiational heating and cooling

Cloud temperature -- cloud base temperature indicate LWC producing potential, cloud top indicative if ice processes possible

Turbulence -- mix cloud properties

Summary of characteristics of some common clouds

Cloud Type	W (m/s)	Depth (m)	Lapse rate (Deg/hour)	LWC (g/m3)	Time*	Time&
Fog	0.01	100	0.2	0.05	10 000 s	2-4 hr
Stratus	0.1	1 000	2	0.25	3 hr	3+ hr
Cumulus	3	1 500	50	0.5	10 min	10-30 min
Cu Cong	10	5 000	150	2	10 min	20 min
Cb	30	12 000	950	5	6 min	45 min
Orographic	1	1000+	18	0.2	20 min	1+ hr