[ecco-support] ECCO Mixed Layer Depths criteria
Martha Buckley
marthabuckley at gmail.com
Tue May 2 11:11:20 EDT 2017
Thanks Matt! ECCO v4 does output DRHODR. I'd be curious about getting
your code at some point, but for my present analysis, which involves
comparing to other products (without DRHODR available) and observations, I
can't really take that route. Unfortunately, I think my only choices are
(1) use whatever arbitrary MLD definition is output by the various ocean
data products I am using or (2) recompute MLD from some standard definition
for all products using monthly mean T/S, neither of which is preferable.
With that said, the focus of my work is not on mixed layer depths, it is on
predictability of upper ocean heat content, so the accuracy of the MLD
definition may not be key.
Regarding the different criteria in different regions, I thought that using
a density equivalent of a temperature threshold took care of this to some
degree since the thermal expansion coefficient is smaller at low
temperatures.
Thanks!
Martha
On Mon, May 1, 2017 at 11:41 PM, Matthew Mazloff <mmazloff at ucsd.edu> wrote:
> Hello
>
> To get MLD from a model solution one should really look at first and
> second vertical derivatives of density. Does ECCO v4 output DRHODR
> diganostic? If so I can share code to get MLD.
>
> Observations are too noisy to look at vertical derivatives of density so
> people typically use thresholds. But an appropriate threshold is regionally
> variable so there are somewhat complicated algorithms for the global
> problem (e.g. http://mixedlayer.ucsd.edu). So to compare to data it is
> appropriate to use a threshold criteria.
>
> I have looked at MLD from a density threshold criteria, from the Holte
> algorithm, and from DRHODR in the MITgcm. Really they aren’t qualitatively
> different, but there are quantitative differences with the threshold
> criteria tending to give deeper values by missing some restratification
> events. I like the DRHODR method, but since it can’t be compared to obs
> the threshold is nice
>
> I would suggest using a potential density cutoff is fine for most
> applications, just tune it to your location. E.g. 0.3 may be fine in
> subtropics but may want 0.2 in higher lats. MLD is very nonlinear, but
> isn’t as bad as mixing *layer* depth, which is the instantaneous depth of
> mixing and is very non-linear and should be calculated on the fly from
> vertical diffusivity or KPP_HBL or some other method. MLD is sort of the
> integrated mixing layer depth….so maybe even taking it from pot dense
> calculated from monthly mean T and S is OK for an approximate average
> value, though obviously not preferable!
>
> Hope this quick brain dump helps!
>
> Matt
>
>
>
>
>
>
>
> On May 1, 2017, at 7:44 PM, Fenty, Ian G (329C) <Ian.Fenty at jpl.nasa.gov>
> wrote:
>
> Martha,
>
> I wouldn't say that there is a single MLD definition for ECCO v4 state
> estimates. Yes, there is a MLD output field that uses the 0.8 deg C
> criteria but I believe that MLD has no prognostic effect on the solution.
> Model output can be used to calculate alternative MLDs. For example, Gael
> provides three alternative MLD criteria (Kara, Suga, Boyer) to show the
> wintertime and summertime MLD plots as part of the standard analysis
> package.
>
> See pp 140-142 of
> ftp://snowwhite.jpl.nasa.gov/data4/Release3//doc/standardplots.pdf
>
> In the Labrador Sea I used the following criteria to identify the base of
> mixed layer: the shallowest depth level with a potential density exceeding
> the profile minimum by 13 g m^−3.
>
> Note that ECCOv4 uses the GGL90 package not KPP so there is no KPPhbl
> field.
>
> If you want exact monthly-mean MLDs from the state estimate using an
> alternative criteria (e.g., potential density cutoff) then the forward
> model would have to be rerun since the MLD diagnosed from monthly-mean T
> and S are almost certainly not equal to the monthly mean of MLD because of
> nonlinearities.
>
> -Ian
>
>
>
> On 5/1/17, 7:18 PM, "ecco-support-bounces at mit.edu on behalf of
> Menemenlis, Dimitris (329C)" <ecco-support-bounces at mit.edu on behalf of
> Dimitris.Menemenlis at jpl.nasa.gov> wrote:
>
> Hi Martha, I don't have answer for your question. I return discussion
> to ECCO support, and also include Daria in cc, in case someone else has
> advice for you.
>
> On May 1, 2017, at 6:39 PM, Martha Buckley <marthabuckley at gmail.com>
> wrote:
>
>
>
> Thanks for the replies. It seems that all the ECCO products use the
> 0.8C density equivalent.
>
>
> The reason I am asking is that I am calculating predictability
> timescales for temperature integrated over the maximum climatological
> (e.g., wintertime) MLD for a variety of different ocean data products. I
> am trying to be as comparable as possible between
> products in the definition of MLD, but the MLD definitions for the
> products varies wildly (e.g. 0.4 kg/m^3 for ECDA and 0.8 C for ECCO v4).
> The question is whether the most comparable approach is to apply the same
> criteria to all models/data (using for example
> monthly output) or whether there is a better approach.
> Thanks!
> Martha
>
>
>
> ---------- Forwarded message ----------
> From: Wang, Ou (329B) <Ou.Wang at jpl.nasa.gov>
> Date: Mon, May 1, 2017 at 1:43 PM
> Subject: Re: [ecco-support] ECCO Mixed Layer Depths criteria
> To: "ECCO support list, wider membership" <ecco-support at mit.edu>
>
>
> The same default parameters were used for ECCOv4:
> hMixCriteria = -.8 _d 0
> dRhoSmall = 1. _d -6
> hMixSmooth = 0. _d 0
>
>
> The criteria of the density equivalent of 0.8C is applied to the model
> profiles at the model time step.
>
>
> Ou Wang
>
>
> From: <ecco-support-bounces at mit.edu> on behalf of "Menemenlis,
> Dimitris (329C)" <Dimitris.Menemenlis at jpl.nasa.gov>
> Reply-To: "ECCO support list, wider membership" <ecco-support at mit.edu>
> Date: Monday, May 1, 2017 at 10:19 AM
> To: "ECCO support list, wider membership" <ecco-support at mit.edu>
> Subject: Re: [ecco-support] ECCO Mixed Layer Depths criteria
>
>
>
> Don’t know what criterion was used for ECCOv4
> For cube92, the default parameters were used:
>
>
> C hMixCriteria:: criteria for mixed-layer diagnostic
> C dRhoSmall :: parameter for mixed-layer diagnostic
> C hMixSmooth :: Smoothing parameter for mixed-layer diag
> (default=0=no smoothing)
>
> hMixCriteria = -.8 _d 0
> dRhoSmall = 1. _d -6
> hMixSmooth = 0. _d 0
>
> C-- First method :
> C where the potential density (ref.lev=surface) is larger than
> C surface density plus Delta_Rho = hMixCriteria * Alpha(surf)
> C = density of water which is -hMixCriteria colder than surface
> water
> C (see Kara, Rochford, and Hurlburt JGR 2000 for default criterion)
>
>
>
> See model/src/calc_oce_mxlayer for other options.
>
>
> Another available definition (when using KPP scheme)
> is KPPhbl, which is the “mixing” layer depth.
> The daily maximum of KPPhbl is a very good proxy for
> mixed layer depth most places, except seasonally-sea-ice-covered and
> polar-winter regions.
> (Kara’s criterion is also problematic for polar regions, which tend to
> be salt-stratified.)
>
>
>
> On May 1, 2017, at 9:50 AM, Martha Buckley <marthabuckley at gmail.com>
> wrote:
>
> One follow up question regarding mixed layer depths in ECCO.
>
>
> What is the criteria for the variable "MXLDEPTH" in the ecco v4
> product. I think I recall being told it is the density equivalent of
> 0.8C. Two questions:
> (1) Is this the criteria that was used? It the criteria applied to
> model profiles at the model timestep?
> (2) If so, isn't this criteria a bit large? 0.125 kg/m^3 or 0.5C seem
> common MLD criteria when applying criteria to monthly-averaged data.
> However, criteria that are smaller (~0.03 kg/m^3 or ~0.2 C) are typically
> applied when using raw observational
> profiles.
>
>
> I know that the MLD criteria is somewhat arbitrary, but I want an idea
> of the rational for the chosen criteria.
> Thanks!
> Martha
>
>
> On Fri, Apr 28, 2017 at 6:04 PM, HONG ZHANG
> <hong.zhang at ucla.edu> wrote:
>
>
>
>
>
> -------- Forwarded Message --------
> Subject: Re: [ecco-support] ECCO2 Mixed Layer Depths ConcernsDate: Fri,
> 28 Apr 2017 12:48:59 -0700From: HONG ZHANG
> <hong.zhang at ucla.edu> <mailto:hong.zhang at ucla.edu>To: Dimitris
> Menemenlis
> <dmenemenlis at gmail.com> <mailto:dmenemenlis at gmail.com>, Moisan, John
> R. (WFF-610W)
> <john.r.moisan at nasa.gov> <mailto:john.r.moisan at nasa.gov>CC: Daria
> Halkides
> <halkides at esr.org> <mailto:halkides at esr.org>
>
> On 4/28/17 9:37 AM, Dimitris Menemenlis wrote:
>
>
> Daria or Hong, would you have looked at MXLDEPTH depth in the “cube92”
> solution?
> ftp://ecco2.jpl.nasa.gov/data1/cube/cube92/lat_lon/
> quart_90S_90N/MXLDEPTH.nc/
>
>
> John, why not use the ECCO-V4, which is data-constrained?
> http://www.ecco-group.org/products.htm
> ECCO v4 is on a 1-degree grid, which is closer to your desired 2x2
> degree grid.
>
> Dimitris Menemenlis
>
>
> On Apr 26, 2017, at 9:41 AM, Moisan, John R. (WFF-610W) <
> john.r.moisan at nasa.gov> wrote:
>
> Hello ECCO2’ers,
>
> I would like to make use of the Mixed-Layer Depth model outputs from
> the global model runs 1992-present (Nov. 2016) to drive some simple 1D
> biogeochemical models for CDOM. I have binned to model outputs from the
> jpg site into monthly 2x2 deg. grids. One thing
> of concern that I have is the trends in the global MLDs (see attached
> figure), which are more extreme in the SO. Is there an issue with
> maintaining correct ocean boundary layers, esp. in the SO for this model?
> Has this been observed yet?
>
>
>
>
>
>
>
>
>
>
>
>
> Hi John,
> You're right, MLD from cube92 run has bias in late stage
> because it's a free run for the later period.
> As Dimitris said, you can look at other ECCO product provided at the
> website.
> Please see attached fig showing MLD from ECCOv4 and ECCO llc270 for an
> example,
> where red line is for ECCOv4 (1992-2012) and blue line for ECCO llc270
> (2001-2015) in upper plot
> also MLD for cube92 is also shown in lower plot for reference.
>
> cheers
> Hong
>
>
>
>
>
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> --
> Martha W. Buckley
> marthabuckley at gmail.com
> http://sites.google.com/site/marthabuckley/home
>
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> Martha W. Buckley
> marthabuckley at gmail.com
> http://sites.google.com/site/marthabuckley/home
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--
Martha W. Buckley
marthabuckley at gmail.com
http://sites.google.com/site/marthabuckley/home
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