<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">Hi Dong,<div><br></div><div>I don’t have a good answer for this, but keep in mind that the bottom drag will be applied as a body force to the bottom-most grid cell, i.e. there is a scaling with the grid cell width implied, so that for thin cells the drag will be smaller. To overcome this, a logarithmic law of the wall is implemented for the drag, see <a href="https://mitgcm.readthedocs.io/en/latest/algorithm/algorithm.html#vertical-dissipation">https://mitgcm.readthedocs.io/en/latest/algorithm/algorithm.html#vertical-dissipation</a>, eq 2.121 and 2.122, maybe that can be useful for you. With the formulae you could also estimate the bottom drag as a function of roughness length, which may be the more universal parameter.</div><div><br></div><div>Martin<br id="lineBreakAtBeginningOfMessage"><div><br><blockquote type="cite"><div>On 28. May 2024, at 12:34, Dong Jian (ENV - Postgraduate Researcher) <D.Jian@uea.ac.uk> wrote:</div><br class="Apple-interchange-newline"><div><meta charset="UTF-8"><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><div style="margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif;"><span style="font-size: 11pt;">Hi</span><span lang="EN-US" style="font-size: 11pt;">, Dear MITgcm-ers<o:p></o:p></span></div><div style="margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif;"><span lang="EN-US" style="font-size: 11pt;"><o:p> </o:p></span></div><div style="margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif;"><span lang="EN-US" style="font-size: 11pt;">I am wondering if anyone has experience with the choice of bottomDragQuadratic coefficient? Typical values I can refer to are bottomDragQuadratic = 1.E-3, or bottomDragQuadratic=2.5E-3.<o:p></o:p></span></div><div style="margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif;"><span lang="EN-US" style="font-size: 11pt;"><o:p> </o:p></span></div><div style="margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif;"><span lang="EN-US" style="font-size: 11pt;">But I am testing a regional model over very rough topography, I do find larger bottomDragQuadratic (even much larger than 2.5E-3 ) gives a closer transport and more similar circulation pattern compared with reanalysis, is there a limit value on the choice of bottomDragQuadratic?<o:p></o:p></span></div><div style="margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif;"><span lang="EN-US" style="font-size: 11pt;"><o:p> </o:p></span></div><div style="margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif;"><span lang="EN-US" style="font-size: 11pt;">Thanks in advance!<o:p></o:p></span></div><div style="margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif;"><span lang="EN-US" style="font-size: 11pt;">Dong<o:p></o:p></span></div></div><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;">_______________________________________________</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;">MITgcm-support mailing list</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><a href="mailto:MITgcm-support@mitgcm.org" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">MITgcm-support@mitgcm.org</a><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><a href="http://mailman.mitgcm.org/mailman/listinfo/mitgcm-support" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">http://mailman.mitgcm.org/mailman/listinfo/mitgcm-support</a></div></blockquote></div><br></div></body></html>