<div class="eI0">
  <div class="eI1">Mod&egrave;le:</div>
  <div class="eI2"><h2>CFS: The NCEP Climate Forecast System (CFS)</h2></div>
 </div>
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  <div class="eI1">Mise &agrave; jour:</div>
  <div class="eI2">1 times per day, at 17:00 UTC</div>
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 <div class="eI0">
  <div class="eI1">Greenwich Mean Time:</div>
  <div class="eI2">12:00 UTC = 13:00 CET</div>
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 <div class="eI0">
  <div class="eI1">R&eacute;solution:</div>
  <div class="eI2">1.0&deg; x 1.0&deg;</div>
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 <div class="eI0">
  <div class="eI1">Param&egrave;tre:</div>
  <div class="eI2">Wet bulb potential temperature (&#952;w) in C</div>
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  <div class="eI1">Description:</div>
  <div class="eI2">

The ThetaW map - updated every 6 hours - shows the modelled wet bulb potential temperature at the 850hPa level. 
The theta w (&#952;w) 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: &#952;w.
As long as an air parcel undergoes an adiabatisch process, be it either dry or saturated, and
in both descending and ascending motions &#952;w does not change. Even when precipitation is
evaporating adiabatically &#952;w does not change, therefore &#952;w is "conservative".<BR>
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 &#952;w has
approximately the same value in the entire air mass both horizontally and vertically, Hence &#952;w
can be used to characterise an air mass, with both sensible and latent heat are accounted
for.<BR>
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).<BR>
<BR>
Table 1: Characteristic values for &#952;w at 850 hPa (in &deg;C) for various air masses.
<TABLE CELLSPACING=0 COLS=4 BORDER=1>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT><B>Summer</B></TD>
			<TD ALIGN=LEFT><BR></TD>
			<TD ALIGN=LEFT><B>Winter</B></TD>
			<TD ALIGN=LEFT><BR></TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>cA &lt; 7 </TD>
			<TD ALIGN=LEFT>mA &lt; 9</TD>
			<TD ALIGN=LEFT>cA &lt; -5 </TD>
			<TD ALIGN=LEFT>mA &lt; -7</TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>cP 7 - 12 </TD>
			<TD ALIGN=LEFT>mP 6 - 12</TD>
			<TD ALIGN=LEFT>CP -6 &ndash; 2</TD>
			<TD ALIGN=LEFT>mP -3 - 5</TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>CML 11 &ndash; 16</TD>
			<TD ALIGN=LEFT>mML 11 - 16</TD>
			<TD ALIGN=LEFT>CML 1 &ndash; 8</TD>
			<TD ALIGN=LEFT>mML 3 - 9</TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>cT 15 - 19 </TD>
			<TD ALIGN=LEFT>mT 14 - 19</TD>
			<TD ALIGN=LEFT>CT 8 &ndash; 14</TD>
			<TD ALIGN=LEFT>mT 8 - 16</TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>cE &gt; 17 </TD>
			<TD ALIGN=LEFT>mE &gt; 18</TD>
			<TD ALIGN=LEFT>cE &gt; 14 </TD>
			<TD ALIGN=LEFT>mE &gt; 16</TD>
		</TR>
	</TBODY>
</TABLE>
<BR>
If the &#952;w 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 &#952;w 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 &#952;w gradient is
present.(Source: <a href="http://www.maq.wur.nl/UK/" target="_blank">Wageningen University</a>)

    
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  <div class="eI1">CFS:</div>
  <div class="eI2">The CFS model is different to any other operational weather forecasting model you will see on Weatheronline.
<br>
Developed at the Environmental Modelling Center at NCEP (National Centers for Environment Prediction) in the USA, 
the CFS became operational in August 2004.
<br>
The systems works by taking reanalysis data (NCEP Reanalysis 2) and ocean conditions from GODAS 
(Global Ocean data Assimilation).  Both of these data sets are for the previous day, and so you 
should be aware that before initialisation the data is already one day old.
<br>
Four runs of the model are then made, each with slightly differing starting conditions, and from 
these a prediction is made.
<br>
Caution should be employed when using the forecasts made by the CFS. However, it is useful when 
monitored daily in assessing forecasts for the coming months, the confidence levels in these 
forecasts and in an assessment of how such long range models perform.
<br>
A description of the CFS is given in the following manuscript.<br>
S. Saha, S. Nadiga, C. Thiaw, J. Wang, W. Wang, Q. Zhang, H. M. van den Dool, H.-L. Pan, S. Moorthi, D. Behringer, D. Stokes, M. Pena, S. Lord, G. White, W. Ebisuzaki, P. Peng, P. Xie , 2006 : The NCEP Climate Forecast System. Journal of Climate, Vol. 19, No. 15, pages 3483.3517.<br>
<a href="http://cfs.ncep.noaa.gov/" target="_blank">http://cfs.ncep.noaa.gov/</a><br>
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 <div class="eI0">
  <div class="eI1">NWP:</div>
  <div class="eI2">La pr&eacute;vision num&eacute;rique du temps (PNT) est une application de la m&eacute;t&eacute;orologie et de l'informatique. Elle repose sur le choix d'&eacute;quations math&eacute;matiques offrant une proche approximation du comportement de l'atmosph&egrave;re r&eacute;elle. Ces &eacute;quations sont ensuite r&eacute;solues, &agrave; l'aide d'un ordinateur, pour obtenir une simulation acc&eacute;l&eacute;r&eacute;e des &eacute;tats futurs de l'atmosph&egrave;re. Le logiciel mettant en &oelig;uvre cette simulation est appel&eacute; un mod&egrave;le de pr&eacute;vision num&eacute;rique du temps.<br><br>
<br>Pr&eacute;vision num&eacute;rique du temps. (2009, d&eacute;cembre 12). Wikip&eacute;dia, l'encyclop&eacute;die libre. Page consult&eacute;e le 20:48, f&eacute;vrier 9, 2010 &agrave; partir de <a href="http://fr.wikipedia.org/w/index.php?title=Pr%C3%A9vision_num%C3%A9rique_du_temps&oldid=47652746" target="_blank">http://fr.wikipedia.org/w/index.php?title=Pr%C3%A9vision_num%C3%A9rique_du_temps&oldid=47652746</a>.<br>
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