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Re: [ferret_users] dynamic height calculations [CORRECTION]



Thanks a lot William, i followed your suggestion and it works.
 
1) The temperature is the insitu temperature rather than potential temperature and the density is also from the insitu temperature and salinity. its not the potential density.

2) units are also fine in the go dynamic_height script units are in dyn-cm so divide by 100 to get in dyn-m.

i am also attaching my script to calculate the dynamic height from the formula. i think both the plots are in agreement. please comment !

regards, saurabh

On Sat, Dec 17, 2016 at 2:31 AM, William Kessler <william.s.kessler@xxxxxxxx> wrote:
Sorry for confusion. My original discussion of the unit conversion below was wrong. The corrected discussion is here:

The 1E5 in the second command contains the unit conversions:
- multiplying by 1e4 converts depth in m to pressure in Pa via the hydrostatic eqn: P=rho*g*z
   Here rho*g ~ 1025*9.8=10045 is approximated as 10000
- dividing the result by 10 defines the quantity  “dynamic meter” (units J/kg = m2/s2)
- multiplying by 100 converts dyn-m to dyn-cm

BK

> On Dec 16, 2016, at 5:09 AM, William Kessler <william.s.kessler@xxxxxxxx> wrote:
>
> A few things I notice:
>
> * The chapter you attach is from a very good book (Tomczak and Godfrey) that has one of the clearest explanations of this stuff. Find this on p23-26, which is essential stuff for an oceanographer to know.
>
> Godfrey likes “steric height” in meters, while most others use “dynamic height” in J/kg=m2/s2=dyn-m. The difference is only a factor of g. Unfortunately Godfrey does not explain the units question completely, especially not the meaning of “dynamic meter”.
>
> His definition eq(2.5) is an integration in pressure, which is not how it is done in practice. In the ferret script:
> let SVanom = 1/rho_UN(dyn_s,dyn_t,dyn_p) - 1/rho_UN(35,0,dyn_p)
> let/title="Dynamic Height(dyn-cm)" DYN_HT  = 1E5 * dyn_mask * SVanom[z=@din]
>
> The 1E5 in the second command contains the unit conversions:
> - multiplying by 1e4 converts pressure in dbar to the SI unit Pa
> - dividing by 10 (really should be g) makes the result in geopotential units J/kg, which is defined as “dynamic meter"
> - multiplying by 100 converts to dyn-cm
> This seems sloppy but is the way oceanographers have always done it. Note that using 10 instead of g and a hidden 1000 instead of 1025 for density are compensating errors! (9.8*1025=10045, not 10000). Few if any aspects of the ocean are measured to a precision of less than half a percent, and geostrophic calculations are also a large approximation. There would be no point in fixing the sloppiness. Dynamic meter is simply a defined quantity that approximates distance to acceptable accuracy. As long as we all do it the same way, it works.
>
>
> Two mistakes in your calculation:
>
> * It is incorrect to use potential temperature for the dynamic ht calculation. The point of the differencing is to compare densities of water at the same depth. Use in situ T (See T&G)
>
> * The main mistake in your calculation is the subtraction of 1000 in defining ro0 and ro. You’re thinking of sigma-theta, but here it is actual density. I think that accounts for most of the difference in the two plots.
>
> let ro0=rho_un(35,0,z[g=tt])-1000                   !! potential density at T=0, S=35, P
> let al0=1/ro0                                                    !! specific volume i.e. alpha0
>
> let ro=rho_un(sa,pot,z[g=tt])-1000                  !! potential density
> let al=1/ro                                                        !! specific volume i.e. alpha
>
> let dif=al-al0                                                    !! alpha - alpha0
>
> => Try fixing these two things and see if the plots agree better.
>
> Billy K
>
>> On Dec 16, 2016, at 12:11 AM, saurabh rathore <rohitsrb2020@xxxxxxxxx> wrote:
>>
>>
>> Hi Ferret users,
>>
>> might be my query is not posted in ferret mail group. hence i am forwarding it again.
>>
>> regards, saurabh
>>
>>
>>
>>
>> ---------- Forwarded message ----------
>> From: saurabh rathore <rohitsrb2020@xxxxxxxxx>
>> Date: Fri, Dec 16, 2016 at 12:50 PM
>> Subject: dynamic height calculations
>> To: ferret users <ferret_users@xxxxxxxx>
>>
>>
>> hi ferreters,
>>
>> i am trying to compute dynamic height by using potential temperature (deg. Celsius) and salinity (psu).
>>
>> 1) i used the go dynamic_height.jnl. after using this go script i got the plot but i am confused about the units mentioned by the plot. For dynamic height it is showing dyn-cm instead for dyn-m. i checked the script and found that there is no unit conversion. it is straight forward taking the depth mentioned by the data which is specified for depth i.e. Z[G=Temp] (argo temperature data) in meters. please tell me about the units used in this go script.
>>
>> 2) Moreover in the go dynamic_hight script it is defined as SVanom = SV - SV(35,0,0) [ in SV(35,0,0) the depth is zero instead of SV(35,0,z[g=temp]), why the depth is mentioned as zero instead fo variable according to the depth in data ?]
>>
>> you the see the script by the command go/help dynamic_height.jnl
>>
>> these are the commands i issued in the calculation.
>>
>> use ptemp_argo.nc                  ! potential temp
>> use "/media/salt_argo.nc"        ! salinity
>>
>> let dyn_S=salt[d=2]
>> let dyn_T=ptemp[d=1]
>> let dyn_P=z[gz=ptemp[d=1]]
>>
>> set region/z=0:2000                ! for dynamic height at z=0 and reference depth is 2000 m
>>
>> go dynamic_height
>>
>> fill dyn_ht[l=@ave]
>>
>> go fland
>>
>> frame/file=1.png
>>
>> 3) but if i do my own calculation for this computation but using the following steps
>>
>> use ptemp_argo.nc
>> use "/media/salt_argo.nc"
>>
>> let tm=ptemp[d=1]
>> let sa=salt[d=2]
>>
>> let ro0=rho_un(35,0,z[g=tt])-1000                   !! potential density at T=0, S=35, P
>> let al0=1/ro0                                                    !! specific volume i.e. alpha0
>>
>> let ro=rho_un(sa,pot,z[g=tt])-1000                  !! potential density
>> let al=1/ro                                                        !! specific volume i.e. alpha
>>
>> let dif=al-al0                                                    !! alpha - alpha0
>> let ro_ref=ro[z=2000]                                      !! reference potential density i.e. density at reference level of no motion
>>
>> let h1=dif*ro_ref
>> let h=h1[z=0:2000@din]                                 !! dynamic height at surface z=0 with reference to z=2000 m
>>
>> fill h[l=@ave]                                                  !! units in m
>> go fland
>>
>> frame/file=2.png
>>
>> but the plot is different from the go dynamic_height script. the dynamic height is more from the script as compare to my script.  i am attaching both the plot and the reference document for the computation.
>>
>> regards, saurabh
>>
>> --
>>
>>
>> REGARDS
>>
>> Saurabh Rathore
>> Research Scholar (PhD.)
>> Centre For Oceans, Rivers, Atmosphere & Land Science Technology
>> Indian Institute Of Technology, Kharagpur
>> contact :- 91- 8345984434
>>
>>
>>
>> --
>>
>>
>> REGARDS
>>
>> Saurabh Rathore
>> Research Scholar (PhD.)
>> Centre For Oceans, Rivers, Atmosphere & Land Science Technology
>> Indian Institute Of Technology, Kharagpur
>> contact :- 91- 8345984434
>> <1.png><2.png><02P-Basics.pdf>
>




--


REGARDS

Saurabh Rathore
Research Scholar (PhD.)
Centre For Oceans, Rivers, Atmosphere & Land Science Technology
Indian Institute Of Technology, Kharagpur
contact :- 91- 8345984434

Attachment: mdt.jnl
Description: Binary data

Attachment: dydep.jnl
Description: Binary data

Attachment: by_formula.png
Description: PNG image

Attachment: by_go_script.png
Description: PNG image


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