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Weather Glossary - E

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East Coast Low (ECL)

An intense low pressure systems that often develop very rapidly (sometimes explosively) in the Tasman Sea close to (within 5° usually) the east coast of Australia. It often brings gale-force or storm-force winds, phenomenal seas and localised heavy rainfall, and may persist for a few days.

ECLs are responsible for many of the extreme rainfall maxima along the NSW coastline.

ECLs are generally "cut off" from the usual westerly airflow.

Easterly wave

A migratory wave like trough or disturbance in tropical regions that moves from east to west. Normally it moves slower then the winds in which it is embedded. It is often associated with tropical cyclone development.

Echo

The energy return of a radar signal after it has hit the target.

ECMWF

European Centre for Medium-Range Weather Forecasting. Operational references in forecast discussions typically refer to the ECMWF's global forecast model, which provides forecasts out to 7 days.

Eddy

A small disturbance of wind in a large wind flow which can produce turbulence.

Ekman spiral

An idealised description of the way wind-driven ocean currents vary with depth. In the atmosphere, it represents the way the wind varies from the surface upwards in the planetary boundary layer.

Ekman transport is the net water transport due to the Ekman spiral. In the southern hemisphere, net mass transport is 90° to the left of the surface wind direction. Ekman transport away from the eastern Australian coast (caused by prolonged northerly winds) results in upwelling near the coast.

El Niņo

The extensive warming of the central and eastern Pacific Ocean that leads to a major shift in weather patterns across the Pacific. In Australia (particularly eastern Australia), El Niņo events are associated with cooler sea surface temperatures and an increased probability of drier conditions. See La Niņa, ENSO.

Elevated convection

Convection occurring within an elevated layer, i.e. a layer in which the lowest portion is based above the earth's surface. Elevated convection often occurs when air near the ground is relatively cool and stable, e.g. during periods of isentropic lift, when an unstable layer of air is present aloft. In cases of elevated convection, stability indices based on near-surface measurements (such as the lifted index) typically will underestimate the amount of instability present. Severe weather is possible from elevated convection, but is less likely than it is with surface-based convection. Also called high-based convection.

Elevation

The measure of height with respect to a point on the earth?s surface above sea level.

Energy Helicity Index (EHI)

An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms. It is related directly to storm-relative helicity in the lowest 2 km (SRH, in mē/sē) and CAPE (in J/kg) as follows:

EHI=(CAPEx SRH)/160,000.

Thus, higher values indicate unstable conditions and/or strong vertical shear. Since both parameters are important for severe weather development, higher values generally indicate a greater potential for severe weather. Vaues of 1 or more are said to indicate a heightened threat of tornadoes; values of 5 or more are rarely observed, and are said to indicate potential for violent tornadoes. However, as with most indices, there are no magic numbers or critical threshold values to confirm or predit the occurence of tornadoes of a particular intensity.

Enhanced V

A pattern seen on satellite infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a V-shaped region of colder cloud tops extending downwind from the thunderstorm core. The enhanced V indicates a very strong updraft, and therefore a higher potential for severe weather.

Enhanced V should not be confused with V notch, which is a radar signature.

ENSO

El Niņo/Southern Oscillation. The condition in the tropical Pacific Ocean where the reversal of surface air pressure at opposite ends of the Pacific induces westerly winds, a strengthening in the equatorial countercurrent and extensive ocean surface warming.

Entrance region

The region upstream from a wind speed maximum in a jetstream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the left half of the entrance region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the Left Rear Quadrant (or Left EntranceRegion) of the jet max. Severe weather potential sometimes increases in this area as a result. See also Exit Region, Right Exit Region.

Equatorial trough

The zone of relatively low pressure, which lies between the subtropical anticyclones of the two hemispheres.

Equilibrium level

(EL) On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment.

The height of the equilibrium level is the height at which thunderstorm updrafts no longer acelerate upward. Thus, to a close approximation, it represents the height of expected (or ongoing) thunderstorm tops. However, strong updrafts will continue to rise past the equilibrium level before stopping, resulting in storm tops that are higher than the equilibrium level. This process sometimes can be seen visually as an overshooting top or anvil dome.

The equilibrium level typically is higher than the tropopause, and is a more accurate reference for storm tops.

Equivalent Potential Temperature

The temperature a parcel of air would have if

  • it is lifted until it became saturated,
  • all water vapour is condensed out, and
  • returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars.

Equivalent Potential Temperature, typically expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.

Same as Theta e.

Evaporate

The phase change between a liquid and a gas.

Evapotranspiration

The total amount of water that is transferred from the earth?s surface to the atmosphere. It is made up of evaporation and transpiration.

Exit region

The region downstream from a wind speed maximum in a jetstream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the right half of the exit region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the right front quadrant (or right exit region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also entrance region, left entrance region.

Extratropical cyclone

A cyclone (low pressure system) that possesses a cold core. Any low pressure system that forms outside the tropics is extratropical. A tropical cyclone will become extratropical if it drifts into temperature regions and becomes cold cored.

Eye

The centre of a tropical cyclone, characterised by a circular area of light winds and rain free skies. They can range in size from about 10km to 90km.

Eye wall

The wall of dense thunderstorms that surrounds the eye of a Tropical Cyclone. Generally around 50-100km in diameter and the region of strongest winds and heaviest rainfall.

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