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Archive - GFS - Theta-W 850hPa P

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ven 29.03 21 UTC

Theta-W 850hPa GFS Modèle

Modèle:

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

Mise à jour:
4 times per day, from 3:30, 09:30, 15:30 and 21:30 UTC
Greenwich Mean Time:
12:00 UTC = 13:00 CET
Résolution:
0.5° x 0.5° for forecast time <= 384 hrs
Paramètre:
Wet bulb potential temperature (θw) in C
Description:
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.
Summer
Winter
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)
GFS:
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.
NWP:
La prévision numérique du temps (PNT) est une application de la météorologie et de l'informatique. Elle repose sur le choix d'équations mathématiques offrant une proche approximation du comportement de l'atmosphère réelle. Ces équations sont ensuite résolues, à l'aide d'un ordinateur, pour obtenir une simulation accélérée des états futurs de l'atmosphère. Le logiciel mettant en œuvre cette simulation est appelé un modèle de prévision numérique du temps.


Prévision numérique du temps. (2009, décembre 12). Wikipédia, l'encyclopédie libre. Page consultée le 20:48, février 9, 2010 à partir de http://fr.wikipedia.org/w/index.php?title=Pr%C3%A9vision_num%C3%A9rique_du_temps&oldid=47652746.
Theta-W 850hPa GFS ven 29.03.2024 21 UTC
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