<!--Copyright (C) 1988-2005 by the Institute of Global Environment and Society (IGES). See file COPYRIGHT for more information.--> <html> <head> <title>GrADS Function: tmave</title> </head> <body bgcolor="e0f0ff" text="#000000"> <h2><b>tmave()</b></h2> <p> This function does time averaging while applying a mask. The syntax is: <p> <ul><code>tmave(<i>maskexpr,expr,timexpr1,timexpr2</i>)</code></ul> <p> where: <p> <ul> <code><i>maskexpr</i> </code> - the mask expression; when evaluated at a fixed time, it must give a single value<br> <code><i>expr</i> </code> - the expression to be averaged<br> <code><i>timexpr1,2</i>  </code> - the limits of the time averaging domain<br> </ul> <p> <h3>Usage Notes</h3> <ol> <li>This function works similarly to the <a href="gradfuncave.html"><code>ave</code></a> function, except for the masking. Using <code>tmave</code> is much more efficient than using <a href="gradfuncmaskout.html"><code>maskout</code></a> with <a href="gradfuncave.html"><code>ave</code></a>). <p> <li>The function loops through the specified time steps, and evaluates <code><i>maskexpr</i></code> at each fixed time. <code><i>maskexpr</i></code> must yeild a single value. If this value is the undefined/missing data value, then <code><i>expr</i></code> for that time is not included in the average. <p> <li>If <code><i>maskexpr</i></code> is not the undefined data value, it is used as the weight for <code><i>expr</i></code> in the average. So if you define <code><i>maskexpr</i></code> accordingly, you can use the <code>tmave</code> function to do weighted time averaging. <p> <li>The tricky aspect of using <code>tmave</code> is setting up <code><i>maskexpr</i></code>. If <code><i>expr</i></code> is a grid with X and/or Y and/or Z varying, then <code><i>maskexpr</i></code> *MUST* refer to either a defined variable or a file with only time varying. In general, you have to set up <code><i>maskexpr</i></code> in advance. </ol> <p> <h3>Examples</h3> <ol> <li> Say you want to average <code>slp</code> over some time range but only when <code>sst</code> over some region is above some value. You can do this by: <p> <ul> <pre> set x 1 set y 1 set t 1 last define sstmask = aave(sst,lon=-180,lon=-90,lat=-20,lat=20) define sstmask = const(maskout(sstmask,sstmask-25.0),1) </pre> </ul> <p> Now <code>sstmask</code> is a time series where the value is 1 when the <code>sst</code> areal average is above 25 and undefined when the value is below 25. <a href="gradfuncmaskout.html"><code>maskout</code></a> set the values below 25 to missing; <a href="gradfuncconst.html"><code>const</code></a> set the non-missing values to 1. We can now do our <code>tmave</code>: <p> <ul> <pre> set lon -180 -90 set lat -20 20 set t 1 d tmave(sstmask,slp,t=1,t=last) </pre> </ul> <p> <li>The mask could also be written to a file with all dimensions nonvarying except for time. Here is what some of the records in the data descriptor file might look like: <p> <ul> <pre> dset <i>maskfilename</i> xdef 1 linear 1 1 ydef 1 linear 1 1 zdef 1 linear 1 1 tdef 100 linear .... </pre> </ul> </ol> </body> </html>
