Gradient Winds

Gradient Wind (or flow) develops only in the absence of friction, when considering curved flow and flows perpendicular to the contours, for the same reason as in geostrophic flow. However, gradient wind is not truly geostrophic because it is constantly moving, thus undergoing an acceleration. Nonetheless, this time, in order for the air to follow parallel to the contours there must consider the effects of the centrifugal force as well as the pressure gradient force and the Coriolis force.

Subgeostrophic Flow is when V < V_g, air curves cyclonically (counter-clockwise), and the CF needs greater than in the geostrophic case in order to balance the PGF.

Supergeostrophic Flow is when V > V_g, air curves anti-cyclonically (clockwise), and the CF does not need to be as great as in the geostrophic case in order to balance the PGF.

Putting it all together…

The Three-Cell Model

            According to the three-cell model, the circulation of each hemisphere is composed of three distinct cells: the Hadley cell, a Ferrel cell, and a polar cell. Thought more realistic than the single cell model, the three-cell model is so general that only fragments of it actually appear in the real world. Nonetheless the names for many of its wind and pressure belts have become well established in our modern terminology, and it is important that we undertint where these hypothesized belts are located.

            The Hadley cell is a thermally direct (hot air rises, cool air sinks) circulation along the equator where strong solar heating causes air to expand upward and diverge toward the poles, creating a zone of low pressure at the equator. This zone of low pressure is known as the equatorial low or the intercontinental convergence zone (ITCZ), it is the rainiest latitude in the entire world where winds can become light or nonexistent for extended periods of time (doldrums). Nonetheless, air in the upper troposphere moves poleward toward the subtropics at about 20° to 30° latitude. Upon reaching about 20° to 30° latitude, air in the cell sinks towards the surface to from the subtropical highs (large bands of high surface pressure). The pressure gradient force (PGF) directs surface air from the subtropical highs to the ITCZ where the weak Coriolis force deflects the air slight to the right (left in the southern hemisphere), forming the northwest trade winds (southeast trade winds in the southern hemisphere).

            Immediately flanking the Hadley cell in each hemisphere is the Ferrel cell, which circulates air between the subtropical highs and the subpolar lows. On the equatorial side of the cell air flows poleward, the subtropical high then undergoes a deflection to the right (left in the southern hemisphere) due to the Coriolis force, creating the westerlies (easterlies in the southern hemisphere) wind belt. The Ferrel cell is considered a thermally indirect circulation (cool air rises, hot air sinks) meaning that, unlike the Hadley cell, this cell does not arise from differential heating but, instead, is caused by the turning of the polar cell and the Hadley cell.

            Finally, the polar cell’s surface air moves from the polar highs toward the subpolar lows. At the subpolar location air is slightly warmer, resulting in low surface pressure and rising air. The very cold conditions create high surface pressure and low-level motion towards the equator. The Coriolis force, in both hemispheres, deflects the air to form a zone known as the polar easterlies in the lower atmosphere. Like the Hadley cell, this cell is also considered to be a thermally direct circulation (hot air rises, cool air sinks).

The Bottom Line:

─     The three-cell model is not realistic at all.

─     ITCZ is real enough to observed from space—many deserts exist in their predicted locations

─     Trade winds are the most persistent winds on Earth.

─     The Hadley circulation provides a good account of low-latitude motions.

─     The Ferrel and Polar cells are not quite as well represented in reality—though they do have some manifestation in the actual climate.

─     It is difficult to observe a persistent pattern of polar easterlies—they emerge in long-term averages, but are not a prevailing wind belt.

Clouds

            The first widely accepted system for cloud classification was devised by English naturalist Luke Howard in 1803. It divided clouds into four basic categories:

1.    Cirrus—thin, wispy clouds of ice

2.    Stratus—layered clouds

3.    Cumulus—clouds having vertical development

4.    Nimbus—rain-producing clouds

Generalized Cloud Chart:

Our current classification scheme is a modified version of Howard’s, retaining his four categories and also allows new combinations. The ten principle types of clouds that result are then grouped according to their height and form:

High Clouds (greater than 19,000 ft)	Cirrus (Ci)	Thin, white, wispy clouds of ice resembling mares’ tails.
	Cirrostratus (Cs)	Extensive, shallow clouds somewhat transparent to sunlight, producing a halo around the Sun or Moon.
	Cirrocumulus (Cc)	High, layered cloud with billows or parallel rolls.
Middle Clouds (6,000 ft to 19,000 ft)	Altostratus (As)	Extensive, watery, layer clouds composed of water droplets. Allows some penetration of sunlight but Moon or Sun appears as bright spot within cloud.
	Altocumulus (Ac)	Shallow, mid-level cloud containing patches or rolls, often arranged in bands. Generally more opaque and having less distinct margins than cirrocumulus.
Low Clouds (below 6,000 ft)	Stratus (St)	Uniform layer of low cloud ranging from whitish to gray.
	Nimbostratus (Ns)	Low cloud producing light precipitation. Produces darker skies than altostratus.
	Stratocumulus (Sc)	Low-level equivalent to altocumulus with some vertical development.
Clouds with Extensive Vertical Development (may extend through mmm moo atmosphere)	Cumulus (Cu)	Detached billowy clouds with flat bases and moderate vertical development. Sharply defined boundaries.
	Cumulonimbus (Cb)	Clouds with intense vertical development with characteristic anvil. May be tens of thousands of meters thick. Appear very dark when viewed from below. Can create violent weather.

Unusual Clouds:

─     Lenticular Clouds—waves formed by the passage of air over a topographic barrier.

─     Banner Clouds—isolated atop mountain peaks

─     Mammatus—found in margins of cumulonimbus clouds, formed by downdrafts, and sometimes are distorted by complex motions.

─     Nacreous Clouds—stratus clouds only observed at high latitudes.

─     Noctilucent Clouds—in the mesosphere, can illuminate the sky at high latitudes during the twilight hours. Noctilucent clouds

Other:

─   Aircraft Contrails—A type of ice cloud, know as contrails, is frequently caused by jet aircraft. The very hot engine exhaust contains considerable water vapor as a result of fuel combustion, and turbulence in the wake of the aircraft rapidly mixes the exhaust with the cold, ambient air. The mixing of warm, moist air with cold air can lead to saturation and, in this case, the rapid formation of ice crystals.

Focus on the Environment and Social Impacts: High Temperatures and Human Health

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Supercell Thunderstorms

Ø  Forward and rear flank downdrafts are rapping around the updraft which is occlusion

Ø  Occlusion (pg. 162 – Figure 9.7)

Ø  New updraft will form on the triple point of the occlusion

Ø  V-Notch

̶        Strong updraft

̶        Goes around the updraft

̶        Inflow notch

Ø  Beaver’s Tail

̶        Flat cloud that lines up on the forward flank gust front

̶        Connected to the rain free base

Ø  Tail Cloud

̶        Attached to wall cloud

Ø  SCUD – Scattered Cumulus Under Deck

̶        Cooler air from the forward flank is drawn into the storm and reaches saturation below the cloud’s LCL

̶        Often the wall will lean toward the precipitation

̶        Strong motions are often apparent to the spotter

Ø  HP (High Precipitation)

̶        These storms are difficult to spot for two primary reasons:

1.    Obscuration and misplacement of important features and safety. The best place to spot the tornado is usually in its path

2.    Originally, HP supercells were considered rare. In reality, perhaps half of all supercells are HP

̶        HP supercells are also prolific tornado producers, much more than originally thought.

̶        These are usually big and scary storms.

̶        Great deal more precipitation in the RFD with rain actually falling through the rain free base (not really a rain free base)

̶        2 reasons they are EASY to find

1.    Visual Vault

2.    Beaver’s Tail

̶        Beaver’s tail is INFLOW into the storm

Ø  LP (Low Precipitation)

̶        Storm without a rear flank downdraft

̶        These storms are easy to identify marked by light precipitation in the main downdraft

̶        LP’s tornadic potential is somewhat limited

̶        They do produce damaging hail and can change modes throughout their lifetime

̶        Prolific producers of large hail

̶        IF tornadoes do form the storm may be becoming more like a supercell