Barotropic SystemA weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone.
Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.
As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.BarotropyThe state of a fluid in which surfaces of constant density (or temperature) are coincident with surfaces of constant pressure; it is the state of zero baroclinity.EntropyThe amount of energy that is not available for work during a certain process.ExtratropicalA term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the
cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air
masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.Extratropical CycloneA cyclone in the middle and high latitudes often being 2000 kilometers in diameter and usually containing a cold front that extends toward the equator for hundreds of kilometers.Extratropical LowA low pressure center which refers to a migratory frontal cyclone of middle and higher latitudes. Tropical cyclones occasionally evolve into extratropical lows losing tropical characteristics and become associated with frontal discontinuity. Geostrophic WindA wind that is affected by coriolis force, blows parallel to isobars and whose strength is related to the pressure gradient (i.e., spacing of the isobars).Intertropical Convergence Zone(ITCZ) The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator. Isentropic AnalysisA way in the forecaster can look at the atmosphere in 3-dimensions instead of looking at constant pressure surfaces (such as the 850 mb, 700 mb, 500 mb, etc.) which are in 2-dimensions. In this analysis method, the forecaster looks at constant potential temperature (the temperature that it would take if we compressed or expanded it adiabatically to the pressure of 1000 mb) surfaces. Air parcels move up and down these surfaces; therefore, the forecaster can see where the moisture is located and how much moisture is available.Isentropic LiftLifting of air that is traveling along an upward-sloping isentropic surface.
Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms.Isentropic SurfaceA two-dimensional surface containing points of equal potential temperature.IsotropicHaving the same characteristics in all directions, as with isotropic antennas. Directional or focused antennas are not isotropic.Maritime Tropical Air MassAn air mass characterized by warm, moist air. Abbreviated mT.Probability of Tropical Cyclone ConditioThe probability, in percent, that the cyclone center will pass within 50 miles to the
right or 75 miles to the left of the listed location within the indicated time period
when looking at the coast in the direction of the cyclone's movement.Special Tropical Disturbance StatementThis statement issued by the National Hurricane Center furnishes information on strong and formative non-depression systems. This statement focuses on the major threat(s) of the disturbance, such as the potential for torrential rainfall on an island or inland area. The statement is coordinated with the appropriate forecast office(s).Subtropical CycloneA non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.Subtropical DepressionA subtropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less. Subtropical Jet(Abbrev. STJ) - this jet stream is usually found between 20° and 30° latitude at altitudes between 12 and 14 km.Subtropical StormA subtropical cyclone in which the maximum 1-minute sustained surface wind is 34 knots (39 mph) or more. TROPTropopauseTropical AdvisoryOfficial information issued by tropical cyclone warning centers describing all tropical
cyclone watches and warnings in effect along with details concerning tropical
cyclone locations, intensity and movement, and precautions that should be taken.
Advisories are also issued to describe: (a) tropical cyclones prior to issuance of
watches and warnings and (b) subtropical cyclones.Tropical Analysis and Forecast BranchOne of three branches of the Tropical Prediction Center (TPC). It provides year-round products involving marine forecasting, aviation forecasts and warnings (SIGMETs), and surface analyses. The unit also provides satellite interpretation and satellite rainfall estimates for the international community. In addition, TAFB provides support to NHC through manpower and tropical cyclone intensity estimates from the Dvorak technique.Tropical CycloneA warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters with organized deep convection and a closed surface wind circulation about a well-defined center.Tropical Cyclone Plan of the DayA coordinated mission plan that tasks operational weather reconnaissance requirements during the next 1100 to 1100 UTC day or as required, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance requirements for the succeeding 24-hour period.Tropical Cyclone Position EstimateThe National Hurricane Center issues a position estimate between scheduled advisories whenever the storm center is within 200 nautical miles of U.S. land-based weather radar and if sufficient and regular radar reports are available to the hurricane center. As far as is possible, the position estimate is issued hourly near the beginning of the hour. The location of the eye or storm center is given in map coordinates and distance and direction from a well-known point. Tropical Cyclone UpdateThis brief statement is issued by the National Hurricane Center in lieu of or preceding special advisories to inform of significant changes in a tropical cyclone or the posting or cancellation of watches and warnings.Tropical DepressionA tropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.Tropical DisturbanceA discrete tropical weather system of apparently organized convection--generally 100 to 300 mi in diameter--originating in the tropics or subtropics, having a nonfrontal migratory character and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.Tropical StormA tropical cyclone in which the maximum 1-minute sustained surface wind ranges from 34 to 63 knots (39 to 73 mph) inclusive.Tropical Storm SummaryWritten by the Hydrometeorological Prediction Center* (HPC) after subtropical and names tropical cyclones have moved inland and advisories have been discontinued. These advisories will be terminated when the threat of flash flooding has ended or when the remnants of these storms can no longer be distinguished from other synoptic features capable of producing flash floods. Storm summaries will not be issued for storms that enter the coast of Mexico and do not pose an immediate flash flood threat to the coterminous United States. They will be initiated when and if flash flood watches are posted in the United States because of an approaching system. Storm summaries will continue to be numbered in sequence with tropical cyclone advisories and will reference the former storm's name in the text. Summaries will be issued at 0100, 0700, 1300, and 1900 Eastern Daylight Time (EDT). The only exception will be the first one in the series may be issued at a nonscheduled time.Tropical Storm WarningAn announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are expected somewhere within the specified coastal area within 36 hours.Tropical Storm WatchAn announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are possible within the specified coastal area within 48 hours.Tropical Wave(formerly known as inverted trough) - A trough or cyclonic curvature maximum in the trade wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere or may be the reflection of an upper tropospheric cold low or an equatorward extension of a mid-latitude trough.Tropical Weather DiscussionThese messages are issued 4 times daily by the Tropical Analysis and Forecast Branch (TAFB) to describe significant synoptic weather features in the tropics. One message will cover the Gulf of Mexico, the Caribbean, and the Atlantic between the equator and 32 degrees North and east of 140 degrees West. Plain language is used in these discussions.Tropical Weather OutlookThis outlook normally covers the tropical and subtropical waters, discussing the weather conditions, emphasizing any disturbed and suspicious areas which may become favorable for tropical cyclone development within the next day to two. In the Atlantic, the outlook is transmitted daily at 0530, 1130, 1730, and 2230 Eastern local time. In the eastern Pacific, it is transmitted daily at 0100, 0700, 1300, and 1900 Eastern local time. For the Central Pacific, transmission times are 1000 and 2200 UTC. Existing tropical and subtropical cyclones are mentioned, as are depressions not threatening land. Given for each system are its location, size, intensity, and movement. For the first 24 hours of a depression or tropical cyclone, the outlook includes a statement identifying the AFOS and World Meteorological Organization (WMO) headers for the advisory on it.Tropical Weather SummaryThe National Hurricane Center issues a monthly summary of tropical weather is included at the end of the month or as soon as feasible thereafter, to describe briefly the past activity or lack thereof and the reasons why.TropicsAreas of the Earth within 20° North and South of the equator.TropopauseThe upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).Tropopause JetType of jet stream found near the tropopause. Examples of this type of jet are the subtropical and polar fronts.TroposphereThe layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers - see inversion, cap), vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).
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