Re: [ferret_users] Manually interpolate zero degree level

[Russ Fiedler <russell.fiedler@xxxxxxxx>]

Hi,

Ah, I thought that might be a problem. You can use the event mask transformation @EVNT to get the last occurrence and mask out all the preceding
events.

http://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/variables-xpressions/XPRESSIONS#_VPINDEXENTRY_550


It's a bit tricky though.

let event=temp[z=@EVNT:0]  ! Increments every time temp crosses zero starting from the highest
let event_rev = event[z=@max] - event   ! Reverse the order. Zero beneath the lowest crossing and increases thereafter
let temp_mask = if event_rev le 1 then temp  ! We only want the temps surrounding the lowest crossing
let isotherm_0 = temp_mask[z=@WEQ:0]
...

You can also mask like this

let event=temp[z=@EVNT:0] 
let last_cross=event[z=@max]
let next_last=max(last_cross-1,0)
let temp_mask = if event ge next_last then temp
...

c.f.

http://ferret.pmel.noaa.gov/Ferret/faq/finding-the-location-of-last-isotherm



Russ


On 18/05/17 19:32, Jasper Denissen wrote:

Dear Ansley and Russ,

Thank you both for your helpful answers. I was indeed looking for what you mentioned Russ. But there is one remaining problem: In some cases there are multiple zero degree levels; I did some sampling and found out that when I execute:

let/title="geopotential height on 0 degree isotherm" z_g_0 = integrand_0[z=@sum]  ! interpolated Z_g

it gives me the heighest height at which this situation occurs. I'd like for it to give me all interpolated heights at which T=0, so that I can manually select the lowest level (so use this variable and k=@min). Is this possible?

Kind regards,

Jasper Denissen

2017-05-18 2:44 GMT+02:00 Russ Fiedler <russell.fiedler@xxxxxxxx>:

Hi,

I think Jasper actually wants @WEQ rather than @LOC since his independent height variable is pressure not geopotential height.

http://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/variables-xpressions/XPRESSIONS#_VPID_175


let isotherm_0 = temp[z=@WEQ:0]  ! pressure kernel at 0 degrees
let integrand_0 = geopotential*isotherm_0/9.81  !
let/title="geopotential height on 0 degree isotherm" z_g_0 = integrand_0[z=@sum]  ! interpolated Z_g


Cheers,
Russ

On 18/05/17 05:17, Ansley C. Manke wrote:

Hi,

Look at the transformation @LOC which picks out the data at a specified value.

-Ansley


On 5/17/2017 6:14 AM, Jasper Denissen wrote:

Dear ferret users,

I've got two variables, temperature T (K) and geopotential Z (m^2/s^2) on 25 pressure levels and a certain spatial extent. What I want to do is find the exact (read linearly interpolated) height of the zero degree level. All I found in the ferret documentation was interpolate transformations which fill data gaps, but I just need to define one variable which contains the interpolated geopotential height (geopotential / 9.81) at which the interpolated temperature equals zero. If there are several heights where T = 0, please take the lowest height.

I think this should be a fairly simple transformation, but I can't seem to find the answer myself. Someone please help me!

Kind regards,

Jasper Denissen
MSc student Earth & Environment
Wageningen University