Re: Lambert Conformal Grid

From: David Brown <dbrown_at_nyahnyahspammersnyahnyah>
Date: Wed Aug 28 2013 - 15:53:00 MDT

Agnes (and Rick):
Well the model this GRIB is associated with is the "North Pacific Hurricane Wave Model" so it seems natural that it is centered over the Pacific. I am attaching a plot of the area covered by the data as
specified in the GRIB lat/lon coordinate attributes. As the message indicates, the error stems from attempting to mask the conformal projection. The masking routine expects the plot to lie solely in the Northern or Southern
hemisphere. That is because its purpose is to remove the "hole" in the projection that occurs wrapping around the pole. See the two frames of the first plot in the masked LC example page if this is not clear (http://www.ncl.ucar.edu/Applications/lcmask.shtml).

For a regional LC projection that does not include the polar area, there is no need to mask the projection.
So the first suggestion is to remove:
 res@gsnMaskLambertConformal = True

Second if you set up the projection based on the lat/lon variable attributes in the GRIB file you can do a "native" projection, which is faster and presents the data as the GRIB producer output it.
The projection resources you would use are:

res@mpProjection = "lambertconforma
res@mpLambertParallel1F = lat2d@Latin1 ; (36.228)
res@mpLambertParallel2F = lat2d@Latin2 ; (36.228)
res@mpLambertMeridianF" : lat2d@Lov ; (-122.246)
res@mpLimitMode = "corners"
res@mpLeftCornerLonF = lon2d@corners(0) ; (-168.719)
res@mpLeftCornerLatF = lat2d@corners(0) ; (-8.035)
res@mpRightCornerLonF = lon2d@corners(2) : (-45.67911)
res@mpRightCornerLatF = lat2d@corners(2) ; (42.22018)

Additionally set

res@tfDoNDCOverlay = True

and don't use the lat2d, lon2d coordinate attributes.

However, you can use any projection for this data if you don't set tfDoNDCOverlay and do use the lat2d, lon2d coordinates.

Hope this helps.
 -dave

On Aug 28, 2013, at 1:31 PM, Rick Brownrigg <brownrig@ucar.edu> wrote:

> Hi,
>
> I don't know about the source of the error message, but what strikes me is the value for mpLambertMeridianF is somewhat unusual for a map of CONUS: a value of -125 puts the center of projection off the coast of California. A value of -98 would be more typical.
>
> Just FWIW
> Rick
>
> On Aug 28, 2013, at 1:21 PM, Agnes Lim Huei Ni <alim@ssec.wisc.edu> wrote:
>
>> Hi,
>>
>> I have a grb file that contains temperature covering CONUS and part
>> south america.
>> I had set the following resources to plot it on a lambert conformal grid
>> res@mpProjection = "LambertConformal"
>> res@mpLambertParallel1F = 20.000
>> res@mpLambertParallel2F = 40.000
>> res@mpLambertMeridianF = -125.000
>> res@gsnMaskLambertConformal = True
>>
>> The following errors appear
>> (0) mask_lambert_conformal: warning: you are not authorized to specify.
>> (0) a maxlat that is above the equator and a minlat that is below
>> (0) the equator. No masking will take place.
>>
>> The information in the grb file for T, latitude and longitude in the
>> data file are listed belowe.
>>
>> Please advice.
>>
>> Thanks
>> Agnes
>>
>> Variable: anal_T
>> Type: float
>> Total Size: 479364732 bytes
>> 119841183 values
>> Number of Dimensions: 3
>> Dimensions and sizes: [lv_HYBL0 | 99] x [g3_x_1 | 869] x [g3_y_2 | 1393]
>> Coordinates:
>> lv_HYBL0: [1..99]
>> Number Of Attributes: 13
>> center : US National Weather Service - NCEP (WMC)
>> long_name : Temperature
>> units : K
>> _FillValue : 1e+20
>> coordinates : g3_lat_1 g3_lon_2
>> level_indicator : 109
>> gds_grid_type : 3
>> parameter_table_version : 2
>> parameter_number : 11
>> model : North Pacific Hurricane Wave Model
>> forecast_time : 0
>> forecast_time_units : hours
>> initial_time : 05/24/2008 (00:00)
>>
>> Variable: lat2d
>> Type: float
>> Total Size: 4842068 bytes
>> 1210517 values
>> Number of Dimensions: 2
>> Dimensions and sizes: [g3_x_1 | 869] x [g3_y_2 | 1393]
>> Coordinates:
>> Number Of Attributes: 11
>> corners : ( -8.035, -5.586509, 42.22018, 36.89176 )
>> long_name : latitude
>> GridType : Lambert Conformal Secant or Tangent, Conical or bipolar
>> units : degrees_north
>> Latin2 : 36.228
>> Latin1 : 36.228
>> Dy : 7920
>> Dx : 8774
>> Lov : -122.246
>> Lo1 : -168.719
>> La1 : -8.035
>>
>> Variable: lon2d
>> Type: float
>> Total Size: 4842068 bytes
>> 1210517 values
>> Number of Dimensions: 2
>> Dimensions and sizes: [g3_x_1 | 869] x [g3_y_2 | 1393]
>> Coordinates:
>> Number Of Attributes: 11
>> corners : ( -168.719, -80.83456, -45.67911, 154.7749 )
>> long_name : longitude
>> GridType : Lambert Conformal Secant or Tangent, Conical or bipolar
>> units : degrees_east
>> Latin2 : 36.228
>> Latin1 : 36.228
>> Dy : 7920
>> Dx : 8774
>> Lov : -122.246
>> Lo1 : -168.719
>> La1 : -8.035
>>
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Received on Wed Aug 28 15:53:09 2013

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