19.4. Scaling Variables

This notebook demonstrates the use of the scale parameter to plot the Absolute Vorticity.

Import Packages

from datetime import datetime, timedelta

import metpy.calc as mpcalc
from metpy.plots import declarative
from metpy.units import units
import numpy as np
import xarray as xr

Get Data

# Set the date/time of the model run
date = datetime(2017, 2, 24, 12)

# Remote access to the dataset from the NCEI Archive
ds = xr.open_dataset('https://www.ncei.noaa.gov/thredds/dodsC/model-gfs-g3-anl-files-old'
                     f'/{date:%Y%m}/{date:%Y%m%d}/gfsanl_3_{date:%Y%m%d_%H}00_000.grb')

# Subset data to be just over the U.S. for plotting purposes
ds = ds.sel(lat=slice(70,10), lon=slice(360-140, 360-60))

Scaling Data

Since vorticity values are on the order of \(10^{-5}\) we want to scale them to fit common contouring values of whole numbers, so we want to set the scale attribute to be 1e5.

# Set the plot time with forecast hours
plot_time = date + timedelta(hours=0)

# Set attributes for plotting contours
cfill = declarative.FilledContourPlot()
cfill.data = ds
cfill.field = 'Absolute_vorticity_isobaric'
cfill.level = 500 * units.hPa
cfill.time = date
cfill.contours = list(range(-46, 47, 2))
cfill.colormap = 'PuOr_r'
cfill.colorbar = 'horizontal'
cfill.scale = 1e5
cfill.smooth_field = 3

cntr2 = declarative.ContourPlot()
cntr2.data = ds
cntr2.field = 'Geopotential_height_isobaric'
cntr2.level = 500 * units.hPa
cntr2.time = date
cntr2.contours = list(range(0, 10000, 60))
cntr2.linecolor = 'black'
cntr2.linestyle = 'solid'
cntr2.clabels = True
cntr2.smooth_field = 3

# Set the attributes for the map
# and put the contours on the map
panel = declarative.MapPanel()
panel.area = [-125, -74, 22, 52]
panel.projection = 'lcc'
panel.layers = ['states', 'coastline', 'borders']
panel.title = f'{cfill.level.m}-hPa Abs. Vorticity Valid at {date}'
panel.plots = [cfill, cntr2]

# Set the attributes for the panel
# and put the panel in the figure
pc = declarative.PanelContainer()
pc.size = (15, 15)
pc.panels = [panel]

# Show the figure
pc.show()