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100<li class="toctree-l2 current"><a class="current reference internal" href="#">Vertical wind</a><ul>
101<li class="toctree-l3"><a class="reference internal" href="#calculation-of-vertical-velocity-and-preparation-of-output-files">Calculation of vertical velocity and preparation of output files</a></li>
102<li class="toctree-l3"><a class="reference internal" href="#calculation-of-the-vertical-velocity-from-the-horizontal-wind-using-the-continuity-equation">Calculation of the vertical velocity from the horizontal wind using the continuity equation</a></li>
103<li class="toctree-l3"><a class="reference internal" href="#calculation-of-the-vertical-velocity-from-the-pre-calculated-mars-parameter-77">Calculation of the vertical velocity from the pre-calculated MARS parameter 77</a><ul class="simple">
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183  <div class="section" id="vertical-wind">
184<h1>Vertical wind<a class="headerlink" href="#vertical-wind" title="Permalink to this headline"></a></h1>
185<div class="section" id="calculation-of-vertical-velocity-and-preparation-of-output-files">
186<h2>Calculation of vertical velocity and preparation of output files<a class="headerlink" href="#calculation-of-vertical-velocity-and-preparation-of-output-files" title="Permalink to this headline"></a></h2>
187<p>Two methods are provided in <code class="docutils literal notranslate"><span class="pre">flex_extract</span></code> for the calculation of the vertical velocity for <code class="docutils literal notranslate"><span class="pre">FLEXTRA</span></code>/<code class="docutils literal notranslate"><span class="pre">FLEXPART</span></code>:</p>
188<ol class="lowerroman simple">
189<li><p>from the horizontal wind field,</p></li>
190<li><p>from the MARS parameter 77, which is available for operational forecasts and analyses since September 2008 and for reanalysis datasets <strong>ERA5</strong> and <strong>CERA-20C</strong>, which contains the vertical velocity directly in the eta coordinate system of the ECMWF model.</p></li>
191</ol>
192<p>Especially for high resolution data, use of the <code class="docutils literal notranslate"><span class="pre">MARS</span></code> parameter 77 is recommended,
193since the computational cost (measured in ECMWF HPC units) is reduced by 90-95% at
194T799. The extraction time, which depends heavily also on the performance of <code class="docutils literal notranslate"><span class="pre">MARS</span></code>, is
195generally reduced by 50% as well. The <code class="docutils literal notranslate"><span class="pre">MARS</span></code> parameter 77 is then multiplied by <code class="docutils literal notranslate"><span class="pre">dp/deta</span></code> to
196give a vertical velocity in Pa/s as needed by <code class="docutils literal notranslate"><span class="pre">FLEXPART</span></code>.</p>
197<p>Calculation from the horizontal wind field is still required for historical case studies using
198<strong>ERA-40</strong>, <strong>ERA-Interim</strong> or operational data prior to September 2008.</p>
199</div>
200<div class="section" id="calculation-of-the-vertical-velocity-from-the-horizontal-wind-using-the-continuity-equation">
201<h2>Calculation of the vertical velocity from the horizontal wind using the continuity equation<a class="headerlink" href="#calculation-of-the-vertical-velocity-from-the-horizontal-wind-using-the-continuity-equation" title="Permalink to this headline"></a></h2>
202<p>The vertical velocity in the ECMWF’s eta vertical coordinate system is computed by the Fortran program <code class="docutils literal notranslate"><span class="pre">calc_etadot</span></code>, using the continuity equation and thereby ensuring mass-consistent 3D wind fields. A detailed description of <code class="docutils literal notranslate"><span class="pre">calc_etadot</span></code> can be found in the
203documents v20_update_protocol.pdf, V30_update_protocol.pdf and
204V40_update_protocol.pdf. The computational demand and accuracy of <code class="docutils literal notranslate"><span class="pre">calc_etadot</span></code> is highly
205dependent on the specification of parameters <code class="docutils literal notranslate"><span class="pre">GAUSS</span></code>, <code class="docutils literal notranslate"><span class="pre">RESOL</span></code> and <code class="docutils literal notranslate"><span class="pre">SMOOTH</span></code>. The
206following guidance can be given for choosing the right parameters:</p>
207<blockquote>
208<div><ul>
209<li><p>For very fine output grids (0.25 degree or finer), the full resolution T799 or even T1279 of the operational model is required (<code class="docutils literal notranslate"><span class="pre">RESOL=799</span></code>, <code class="docutils literal notranslate"><span class="pre">SMOOTH=0</span></code>). The highest available resolution (and the calculation of vertical velocity on the Gaussian grid (<code class="docutils literal notranslate"><span class="pre">GAUSS=1</span></code>) is, however, rather demanding and feasible only for resolutions up to T799. Higher resolutions are achievable on the HPC. If data retrieval at T1279  needs to be performed on <em>ecgate</em>, the computation of the vertical velocity is feasible only on the lat/lon grid (<code class="docutils literal notranslate"><span class="pre">GAUSS=0</span></code>), which also yields very good results. Please read document v20_update_protocol.pdf-v60_update_protocol.pdf to see if the errors incurred are acceptable for the planned application.</p></li>
210<li><p>For lower resolution (often global) output grids, calculation of vertical velocities with lower than operational spectral resolution is recommended. For global grids the following settings appear optimal:</p>
211<blockquote>
212<div><ul class="simple">
213<li><p>For 1.0 degree grids: <code class="docutils literal notranslate"><span class="pre">GAUSS=1</span></code>, <code class="docutils literal notranslate"><span class="pre">RESOL=255</span></code>, <code class="docutils literal notranslate"><span class="pre">SMOOTH=179</span></code></p></li>
214<li><p>For 0.5 degree grids: <code class="docutils literal notranslate"><span class="pre">GAUSS=1</span></code>, <code class="docutils literal notranslate"><span class="pre">RESOL=399</span></code>, <code class="docutils literal notranslate"><span class="pre">SMOOTH=359</span></code></p></li>
215<li><p>Calculation on the lat/lon grid is not recommended for less than the operational (T1279) resolution.</p></li>
216<li><p>If <code class="docutils literal notranslate"><span class="pre">GAUSS</span></code> is set to 1, only the following choices are possible for <code class="docutils literal notranslate"><span class="pre">RESOL</span></code> on <em>ecgate</em>: 159,255,319,399,511,799, (on the HPC also 1279; 2047 in future model versions). This choice is restricted because a reduced Gaussian grid is defined in the ECMWF EMOSLIB only for these spectral resolutions. For <code class="docutils literal notranslate"><span class="pre">GAUSS=0</span></code>, <code class="docutils literal notranslate"><span class="pre">RESOL</span></code> can be any value below the operational resolution.</p></li>
217<li><p>For <code class="docutils literal notranslate"><span class="pre">SMOOTH</span></code>, any resolution lower than <code class="docutils literal notranslate"><span class="pre">RESOL</span></code> is possible. If no smoothing is desired, <code class="docutils literal notranslate"><span class="pre">SMOOTH=0</span></code> should be chosen. <code class="docutils literal notranslate"><span class="pre">SMOOTH</span></code> has no effect if the vertical velocity is calculated on a lat/lon grid (<code class="docutils literal notranslate"><span class="pre">GAUSS=0</span></code>).</p></li>
218</ul>
219</div></blockquote>
220</li>
221<li><p>The on-demand scripts send an error message for settings where <code class="docutils literal notranslate"><span class="pre">SMOOTH</span></code> (if set) and <code class="docutils literal notranslate"><span class="pre">RESOL</span></code> are larger than 360./<code class="docutils literal notranslate"><span class="pre">GRID</span></code>/2, since in this case, the output grid cannot resolve the highest wave numbers. The scripts continue operations, however.</p></li>
222<li><p>Regional grids are not cyclic in zonal directions, but global grids are. The software assumes a cyclic grid if <code class="docutils literal notranslate"><span class="pre">RIGHT</span></code>-<code class="docutils literal notranslate"><span class="pre">LEFT</span></code> is equal to <code class="docutils literal notranslate"><span class="pre">GRID</span></code> or is equal to <code class="docutils literal notranslate"><span class="pre">GRID</span></code>-360.</p></li>
223<li><p>Finally, model and flux data as well as the vertical velocity computed are written to files <code class="docutils literal notranslate"><span class="pre">&lt;prefix&gt;yymmddhh</span></code> (the standard <code class="docutils literal notranslate"><span class="pre">flex_extract</span></code> output files) If the parameters <code class="docutils literal notranslate"><span class="pre">OMEGA</span></code> or <code class="docutils literal notranslate"><span class="pre">OMEGADIFF</span></code> are set, also files <code class="docutils literal notranslate"><span class="pre">OMEGAyymmddhh</span></code> are created, containing the pressure vertical velocity (omega) and the difference between omega from <code class="docutils literal notranslate"><span class="pre">MARS</span></code> and from the surface pressure tendency. <code class="docutils literal notranslate"><span class="pre">OMEGADIFF</span></code> should be set to zero except for debugging, since it triggers expensive calculations on the Gaussian grid.</p></li>
224</ul>
225</div></blockquote>
226</div>
227<div class="section" id="calculation-of-the-vertical-velocity-from-the-pre-calculated-mars-parameter-77">
228<h2>Calculation of the vertical velocity from the pre-calculated MARS parameter 77<a class="headerlink" href="#calculation-of-the-vertical-velocity-from-the-pre-calculated-mars-parameter-77" title="Permalink to this headline"></a></h2>
229<p>Since November 2008, the parameter 77 (deta/dt) is stored in <code class="docutils literal notranslate"><span class="pre">MARS</span></code> on full model levels. <code class="docutils literal notranslate"><span class="pre">FLEXTRA</span></code>/<code class="docutils literal notranslate"><span class="pre">FLEXPART</span></code> in its current version requires <code class="docutils literal notranslate"><span class="pre">deta/dt</span></code> on model half levels, multiplied by <code class="docutils literal notranslate"><span class="pre">dp/deta</span></code>. In <code class="docutils literal notranslate"><span class="pre">flex_extract</span></code>, the program <code class="docutils literal notranslate"><span class="pre">calc_etadot</span></code> assumes that parameter 77 is available if the <code class="docutils literal notranslate"><span class="pre">CONTROL</span></code> parameter <code class="docutils literal notranslate"><span class="pre">ETA</span></code> is set to 1.</p>
230<p>It is recommended to use the pre-calculated parameter 77 by setting <code class="docutils literal notranslate"><span class="pre">ETA</span></code> to 1 whenever possible.</p>
231<p>Setting the parameter <code class="docutils literal notranslate"><span class="pre">ETA</span></code> to 1 disables calculation of vertical velocity from the horizontal wind field, which saves a lot of computational time.</p>
232<div class="admonition note">
233<p class="admonition-title">Note</p>
234<p>However, the calculations on the Gaussian grid are avoided only if both <code class="docutils literal notranslate"><span class="pre">GAUSS</span></code> and <code class="docutils literal notranslate"><span class="pre">ETADIFF</span></code> are set to 0. Please set <code class="docutils literal notranslate"><span class="pre">ETADIFF</span></code> to 1 only if you are really need it for debugging since this is a very expensive option. In this case, <code class="docutils literal notranslate"><span class="pre">ETAyymmddhh</span></code> files are produced that contain the vertical velocity from horizontal winds and the difference to the pre-calculated vertical velocity.</p>
235</div>
236<p>The parameters <code class="docutils literal notranslate"><span class="pre">RESOL</span></code>, <code class="docutils literal notranslate"><span class="pre">GRID</span></code>, <code class="docutils literal notranslate"><span class="pre">UPPER</span></code>, <code class="docutils literal notranslate"><span class="pre">LOWER</span></code>, <code class="docutils literal notranslate"><span class="pre">LEFT</span></code>, <code class="docutils literal notranslate"><span class="pre">RIGHT</span></code> still apply. As for calculations on the Gaussian grid, the spectral resolution parameter <code class="docutils literal notranslate"><span class="pre">RESOL</span></code> should be compatible with the grid resolution (see previous subsection).</p>
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