[MITgcm-support] A query about EXF

[Abhisek Chakraborty]

Dear Martin Losch,

Many thanks for your detailed explanation.

As per your prescription, accurate computation of net Qsw and Qlw
definitely needs Qswdown and Qlwdown, which I have to obtain again from NWP
model. In such a scenario, it is better to use my present configuration of
EXF (becasue all other parameters are available with me except
precipitation and runoff which I can use from long term climatology).

Thanks for your suggestions once again.

regards,
Abhisek


On Thu, Oct 10, 2013 at 4:22 PM, Martin Losch <Martin.Losch at awi.de> wrote:

> Hi,
>
> your forcing data set (as you describe it) is imcomplete.
>
> The model requires a net heat flux at the ocean surface. The net heat flux
> (Qnet) consistes of different components, but the ocean doesn't care about
> them (except for the short wave heat Qsw, but that's not so important
> here). Typically you have
>
> Qnet = Qsw(shortwave) + Qlw(longwave) + Qsh(sensible) + Qlh(latent)
> Qsw = QswDown (from the sun refracted by the atmosphere) + QswUp
> (reflected from the surface) = Qswdown*(1-albedo)
> Qlw = QlwDown (also called counter radiation by the atmosphere) + QlwUp
> (=BoltzmannConstant * emissivity * surfaceTemp^4)
>
> The turbulent fluxes Qsh and Qlh are usually computed from bulk formulae.
> To first order Qsh ~ windSpeed*(Tair-SST), and Qlh ~ windSpeed*(qair -
> qSurface), where qSurface is the saturation  specific humidity at the
> surface temperature SST (a nonlinear function, either a polynomial or an
> exponential function or what the meteorologist come up with). One example
> of these bulk formulae for the ocean is implemented in
> pkg/exf/exf_bulkformulae.F, a somewhat simpler set is in
> pkg/seaice/seaice_ocean_budget.F. As input they need the wind speed
> (derived from uwind/vwind or read in separately), the surface temperature
> provided by the ocean (or sea ice) model, the air temperature Tair and
> specific humidity qair (typically at 2m height above the surface, but
> that's a detail). A data set that provides both Tair/qair and Qsh/Qlh has
> somehow computed Qsh/Qlh based on surface temperatures from an ice-ocean
> different from what you have (and unknown to you).
>
> The extremely important contribution QswDown can be computed from
> astronomical parameters (see e.g. Parkinson and Washington, 1979), but the
> refraction by the atmosphere needs to be estimated, which is difficult if
> you don't have an estimate of a clould cover.
>
> A very large contribution to the net heat flux is the long wave
> contribution Qlw. QlwUp can be computed from the surface temperature, but
> QlwDown needs to be specified. You can approximate it based on the surface
> temperature (someCorrectionFactor*BoltzmannConstant*Tair^4) but that's
> really not very good, because you actually need the temperature at the
> bottom of the clouds, and there is no way to reconstruct that reliably from
> surface Tair.
>
> Currently the exf does not have the option of reading in turbulent heat
> fluxes. Even if you modify the code to make it do so, your plan of running
> the model just with wind, Tair, qair, and turbulent heat fluxes will fail
> (the model will cool down very quickly and explode), if you don't provide
> incoming radiation. (Just setting them to zero will not do it!!)
> I repeat Dimitris suggestion that, if you really want to use these
> precomputed fluxes, you construct Qsw(net!!!) and Qlw(net!!!) somehow
> yourself, then add up the four components and read in the total heat and
> fresh water flux (so uwind, vwind, hflux, swflux, sflux, stress will be
> computed). This will only work if you don't have an ice-model, which
> requires the "default" uwind, vwind, tair, qair, swdonw, lwdown.
>
> Martin
>
>
> On Oct 10, 2013, at 10:03 AM, Abhisek Chakraborty <abhisek.sac at gmail.com>
> wrote:
>
> > Dear Dimitris Menemenlis,
> >
> > Actually I am presently running a global ocean (excluding poles)
> configuration using forcings from NCEP reanalysis. However, I am planning
> to use forcings from some other data sets (which have 10 m winds, 2m air
> temperature, 2m sp. humidity, sensible and latent heat fluxes only). For
> such a data set, I wish to use the EXF package. The present configuration
> of EXF needs other parameters ,.e.g., shortwave, longwave radiations,
> precipitations etc. So my query is how to use EXF for the mentioned data
> sets as far as the forcing of the global ocean model set up is considered.
> >
> > regards,
> > Abhisek
> >
> >
> > On Thu, Oct 10, 2013 at 11:28 AM, Menemenlis, Dimitris (3248) <
> Dimitris.Menemenlis at jpl.nasa.gov> wrote:
> > what's so special about this hs/hl? what are you trying to study/achieve?
> >
> > Dimitris Menemenlis
> >
> > On Oct 9, 2013, at 10:35 PM, Abhisek Chakraborty wrote:
> >
> >> Dear Dimitris Menemenlis,
> >>
> >> Thanks for your prompt reply.
> >>
> >> I understand that for the default configuration of EXF I need to use
> ustress, vstress (from 10m winds) and hflux, swflux and sflux.  However, I
> don't have any fluxes (e.g. shortwave or long wave, precipitaion etc.)
> other than HS and HL.  In such a scenario, I can compute hflux as the sum
> of HS and HL. But what I should do for the other fluxes (swflux, sflux
> etc.). Should I use some climatology or simply zero?
> >>
> >> regards,
> >> Abhisek
> >>
> >
> >
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