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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Hi Giorgio<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">I don’t know the specific LAPACK code too well, but this could happen because of weak/zero covariances. There are a couple of things you may need to check with your data (if you haven’t already done
so) ;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">Is it possible you have too many data points with no variance at all (e.g. land masked, or too many repeating values)<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">The data is just white/red noise, which again will give very low covariances<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">As you say, you’re masking out those weak/zero covariances, but have you checked to see how many actual values are left?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Will <o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">ncl-talk <ncl-talk-bounces@mailman.ucar.edu> on behalf of Giorgio Graffino via ncl-talk <ncl-talk@mailman.ucar.edu><br>
<b>Reply to: </b>Giorgio Graffino <g.graffino@tim.it><br>
<b>Date: </b>Wednesday, 31 March 2021 at 2:34 am<br>
<b>To: </b>"ncl-talk@ucar.edu" <ncl-talk@ucar.edu><br>
<b>Subject: </b>[ncl-talk] eofunc giving empty eigenvalues<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Dear NCL people,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I'm struggling at computing the EOFs of a zonal wind stress field. I'm always getting this warning from the eofunc functions:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New";color:black;background:white">Warning! LAPACK routine returned eigenvalue <= 0.
</span><span style="font-family:"Courier New""><br>
Setting it to zero...<br>
<br>
</span>I've seen that the original LAPACK's dspevx function sets a 1.e-6 threshold for computing EOFs; if there is a value below that threshold, zero EOFs are returned (<a href="https://github.com/yyr/ncl/blob/master/ni/src/lib/nfpfort/eof_scripps.f">https://github.com/yyr/ncl/blob/master/ni/src/lib/nfpfort/eof_scripps.f</a>).
I'm using the where function to set all values below 1.e-6 as fill values (something like taux = where(taux.le.1.e-6,taux@_FillValue,taux)) before the eofunc function is called, yet I'm getting the same warning and the same empty EOFs. Is there anything else
I'm missing?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Giorgio<o:p></o:p></p>
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