Das Wetter und der Klimawandel

Beiträge mit Schlagwort ‘Hadley-Circulation’

Full Disk Earth on 27th May, 2012

On May 27th 2012 european weather satellite MeteoSat took a nice picture of full disk earth displaying some intersting weather action. Meteosat circles Earth in a geostationary orbit (36.000 km altitude) delivering daily current views frome same fixed position above surface of the planet.

Jetstream and dynamical weather systems: A band of cloud across Northern Atlantic (from Greenland to Scandinavia) indicates behaviour of jetstream, driven by gradient in air-temperature (and gradient in air pressure arising thereby, respectively) between polar regions and mid latitudes.

Jetstream is crucial to weather forecast:

Like the vortexes in a raging river high and low pressure systems – weather-determining in mid latitudes – arise from turbulences in jetstream and are moved by flow of jetstream after that.

Weather on 27th May, 2012, 16:00 UTC . Innertropical Convergence Zone, the deserts of Subtropical High Pressure Belt and the weather systems (cyclones and anticyclones) of the mid – latidudes are easily dercernable. Natural Color RGB images makes use of three solar channels: red, green and blue. In this color scheme vegetation appears greenish because of its large reflectance in the green beam channel compared to the red and blue beam channels. Water clouds with small droplets have large reflectance at all three channels and hence appear whitish, while snow and ice clouds appears cyan because ice strongly absorbs in red. Bare ground appears brown because of the larger reflectance in the red beam channel than at the blue one, and the ocean appears black because of the low reflectance in all three channels. Source: Meteosat, EUMETSAT

The two vortexes over middle Northern Atlantic, one of them in the southwest of British Isles, another one further westward, are cut-off  lows. They have separated from jetstream some time ago, triggered by a large blocking high pressure system widespread over parts of Northern, Western and Central Europe and German Sea. Blocking highs occur if the flow of  jetstream slows down or even breaks so that a moving high pressure systeme comes to a deadlock. The two Cut-off lows over middle Northern Atlantic are following a pathway southward blocking high.

Inside high pressure systems (anticyclones), spinning arount clockwise, air sinks and nearly all clouds decay, because water in the condensed form tends to evaporate into water vapor. Thus high pressure systems lead to fair weather at most times.

Inside low pressure systems (cyclones), spinning around counterclockwise, air rises and cools, so that water vapor condenses, forming clouds made of tiny water droplets or ice crystals. Latent heat (thermal energy of condensation) released thereby, powers cloud formation for her part by warming the rising air. Low pressure systems imply bad weather with rainfall an thunderstorms many a time. 

Tropical-subtropical Hadley-Circulation:  Away from almost cloud-free subtropical belt of high pressure systems air flows to equator going along surface. These tradewinds are turning westward due to Earth´s rotation. Throughout the region of equator there´s a buildup of low pressure, called Innertropical Convergence Zone (ITCZ). Heated by the sun, equatorial air rises and cools, forcing whatever water vapor it holds to condense into clouds. The ascended air moves poleward, turning eastward by Earth´s rotation soon after. As moving poleward, air flow comes closer to the axis of earth’s rotation. That´s why it goes faster, often forming a subtropical jetstream that rotates more rapidly than the Earth itself. In addition air descends closing the circulation in this way. This so called Hadley-Circulation is breaking up in a row of cloud-rich convective cells around the whole planet Earth.

Jens Christian Heuer

A Satellite Picture explaining our Weather

The European weather satellite Meteosat, circles the Earth on a geostationary orbit (36.000 km altitude) providing daily current views of our planet. On this color infrared recording of  November 22th 2011, you can see some important phenomena of global weather patterns.  

Dynamical Weather Systems: Weather on earth-like planets is driven by temperature differences between equator and poles, caused by different sun´s irradiance. In mid – latitudes, where warm tropical and cool polar air masses encounter each other, gradient of temperature (and thereby gradient of pressure) is sufficient to generate a high altitude air current (called tropospheric polar jetstream) on both hemispheres, turning eastward under influence of earth’s rotation.

Breaking a critical speed limit, the jetstream forms Rossby waves with troughs and ridges(wave peaks). A lot of shear forces emerge. The waves break and roll up to vortices. These are the high pressure und low pressure systems, enabled to intermix the warm tropical and cool polar air masses.

The high pressure vortices (anticyclones) are spinning downward and clockwise (counterclockwise) on northern (southern) hemisphere, whereas the low pressure vortices (cyclones) are spinning upward and counterclockwise (clockwise) on northern (southern) hemisphere.

Weather at November 22th, 2011, 12:00 UTC . The ITCZ, the deserts in the Subtropical High Presure Belt and the Low Pressure Systems (Cyclones) of the mid – latidudes are easily dercernable. Natural Color RGB images makes use of three solar channels: red, green and blue. In this color scheme vegetation appears greenish because of its large reflectance in the green beam channel compared to the red and blue beam channels. Water clouds with small droplets have large reflectance at all three channels and hence appear whitish, while snow and ice clouds appears cyan because ice strongly absorbs in red. Bare ground appears brown because of the larger reflectance in the red beam channel than at the blue one, and the ocean appears black because of the low reflectance in all three channels. Source: Meteosat, EUMETSAT

Inside low pressure systems the air rises and cools, so that water vapor condenses, forming clouds made of tiny water droplets or ice crystals (bad weather). Latent heat (thermal energy of condensation) thereby released powers cloud formation on her part warming the rising air.

Inside high pressure systems the air sinks and clouds decay, because water in the condensed form tends to evaporate into water vapor (fair weather).

Cyclones derive their energy not only from the jetstream, but also from latent heat liberated during formation of clouds. In turn they transmitted back a portion of their energy to jetstream.

The pathways of cyclones are affected by the behaviour of the jetstream.But sometimes the high air current slow down or breaks actually, so that the cyclones are able to seperate from jetstream. These cut off lows move slowly and won’t exit a region until they are captured by a trough of a new jetstream, which meanwhile has usually formed.

Low Pressure Systeme (Cyclone) Source: Bjerknes (1922)

Tropical Hadley – Circulation: Away from this areas of high pressure the air masses move equatorially along the surface (tradewinds), where´s a buildup of low pressure (Innertropical Convergence Zone, ITCZ) : These tradewinds turn westward due to earth´s  rotation. Heated by the sun,  equatorial air rises and cools, forcing whatever water vapor it holds to condense into clouds. The ascended air moves poleward , but it is turned eastward by the earth´s rotation. As moving  polewards, the air current contracts closer to the axis of earth’s rotation. So it must spin faster, creating subtropical jetstreams that rotate more rapidly than the Earth itself..In parts however, the air descends in the belt of subtropical pressure, closing the air circulation. This so called Hadley-Circulation.partions in a row of convective cells around the whole planet.

Stratosphere and Polar Vortex: The stratosphere is the next layer of atmosphere above the troposphere, in which most weather processes play. The stratosphere contains little water vapor, but larger quantities of ozone, protecting life by absorption of dangerous solar ultraviolet radiation. Therefore the stratosphere is much warmer than the upper troposphere.

If the stratosphere over the poles is cold enough during the polar night, a polar vortex forms due to a sufficient gradient of temperature to build up an eastward stratospheric jetstream, which is a propulsion engine of tropospheric polar jetstream (see above).

A strong polar vortex favors a poleward, zonal circulation (along the lines of latitude), a weak, often divided polar vortex, however, favors a meridional circulation with pronounced troughs and ridges (along the lines of longitude).

Jens Christian Heuer

Schlagwörter-Wolke