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Theta-W 850hPa GFS Modell


GFS (Global Forecast System) Global Model from the "National Centers for Environmental Prediction" (NCEP)

4 times per day, from 3:30, 09:30, 15:30 and 21:30 UTC
Greenwich Mean Time:
12:00 UTC = 14:00 MESZ
0.5° x 0.5° for forecast time <= 384 hrs
Wet bulb potential temperature (θw) in C
The ThetaW map - updated every 6 hours - shows the modelled wet bulb potential temperature at the 850hPa level. The theta w (θw) areas are encircled by isotherms - lines connecting locations with equal wet bulb potential temperature. When an air parcel, starting from a certain pressure level, is lifted dry adiabatically until saturation and subsequently is brought to a level of 1000 hPa along a saturated adiabat it reaches what is called the saturated potential wet-bulb temperature: θw. As long as an air parcel undergoes an adiabatisch process, be it either dry or saturated, and in both descending and ascending motions θw does not change. Even when precipitation is evaporating adiabatically θw does not change, therefore θw is "conservative".
An air mass is defined as a quantity of air with a horizontal extent of several hundred or thousand kilometres and a thickness of several kilometres, which is homogeneous in thermal characteristics. Such an air mass may form when air has been over an extensive and homogeneous part of the Earth's surface during a considerable amount of time. This is the so-called source area. In due time, by means of radiative exchange processes and contact with the Earth's surface, an equilibrium develops which is evident from the fact that θw has approximately the same value in the entire air mass both horizontally and vertically, Hence θw can be used to characterise an air mass, with both sensible and latent heat are accounted for.
Depending on possible source areas several main air mass types can be distinguished: polar air (P), midlatitude air (ML) and (sub)tropical air (T). Also, but these are less important arctic air (A) and equatorial air (E). These five main types can be subdivided in continental air (c) and maritime air (m).

Table 1: Characteristic values for θw at 850 hPa (in °C) for various air masses.
cA < 7 mA < 9 cA < -5 mA < -7
cP 7 - 12 mP 6 - 12 CP -6 – 2 mP -3 - 5
CML 11 – 16 mML 11 - 16 CML 1 – 8 mML 3 - 9
cT 15 - 19 mT 14 - 19 CT 8 – 14 mT 8 - 16
cE > 17 mE > 18 cE > 14 mE > 16

If the θw distribution is considered on a pressure surface, preferably 850 hPa, then extensive areas with a small or no gradient can be observed. These areas of homogeneous θw values may be associated with air masses. Often various homogeneous areas are separated from one another by relatively narrow transformation zones displaying a strong gradient. Here frontal zones intersect with the pressure surface. Generally speaking a surface front is located where at 850 hPa the 'warm boundary' of the zone with the large θw gradient is present.(Source: Wageningen University)
The Global Forecast System (GFS) is a global numerical weather prediction computer model run by NOAA. This mathematical model is run four times a day and produces forecasts up to 16 days in advance, but with decreasing spatial and temporal resolution over time it is widely accepted that beyond 7 days the forecast is very general and not very accurate.

The model is run in two parts: the first part has a higher resolution and goes out to 180 hours (7 days) in the future, the second part runs from 180 to 384 hours (16 days) at a lower resolution. The resolution of the model varies in each part of the model: horizontally, it divides the surface of the earth into 35 or 70 kilometre grid squares; vertically, it divides the atmosphere into 64 layers and temporally, it produces a forecast for every 3rd hour for the first 180 hours, after that they are produced for every 12th hour.
Numerische Wettervorhersagen sind rechnergestützte Wettervorhersagen. Aus dem Zustand der Atmosphäre zu einem gegebenen Anfangszeitpunkt wird durch numerische Lösung der relevanten Gleichungen der Zustand zu späteren Zeiten berechnet. Diese Berechnungen umfassen teilweise mehr als 14 Tage und sind die Basis aller heutigen Wettervorhersagen.

In einem solchen numerischen Vorhersagemodell wird das Rechengebiet mit Gitterzellen und/oder durch eine spektrale Darstellung diskretisiert, so dass die relevanten physikalischen Größen, wie vor allem Temperatur, Luftdruck, Windrichtung und Windstärke, im dreidimensionalen Raum und als Funktion der Zeit dargestellt werden können. Die physikalischen Beziehungen, die den Zustand der Atmosphäre und seine Veränderung beschreiben, werden als System partieller Differentialgleichungen modelliert. Dieses dynamische System wird mit Verfahren der Numerik, welche als Computerprogramme meist in Fortran implementiert sind, näherungsweise gelöst. Aufgrund des großen Aufwands werden hierfür häufig Supercomputer eingesetzt.

Seite „Numerische Wettervorhersage“. In: Wikipedia, Die freie Enzyklopädie. Bearbeitungsstand: 21. Oktober 2009, 21:11 UTC. URL: (Abgerufen: 9. Februar 2010, 20:46 UTC)
Theta-W 850hPa GFS Di 17.10.2017 06 GMT
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