"""
plot H's and L's on a sea-level pressure map
(uses scipy.ndimage.filters)
"""
import numpy as np
import matplotlib.pyplot as plt
import sys
from mpl_toolkits.basemap import Basemap, NetCDFFile, addcyclic
from scipy.ndimage.filters import minimum_filter, maximum_filter
def extrema(mat,mode='wrap',window=10):
"""find the indices of local extrema (min and max)
in the input array."""
mn = minimum_filter(mat, size=window, mode=mode)
mx = maximum_filter(mat, size=window, mode=mode)
# (mat == mx) true if pixel is equal to the local max
# (mat == mn) true if pixel is equal to the local in
# Return the indices of the maxima, minima
return np.nonzero(mat == mn), np.nonzero(mat == mx)
if len(sys.argv) < 2:
print 'enter date to plot (YYYYMMDDHH) on command line'
raise SystemExit
# get date from command line.
YYYYMMDDHH = sys.argv[1]
# open OpenDAP dataset.
try:
data=NetCDFFile("http://nomad1.ncep.noaa.gov:9090/dods/gdas/rotating/gdas"+YYYYMMDDHH+".grib2")
except:
data=NetCDFFile("http://nomad2.ncep.noaa.gov:9090/dods/gdas/rotating/gdas"+YYYYMMDDHH+".grib2")
# read lats,lons.
lats = data.variables['lat'][:]
lons1 = data.variables['lon'][:]
nlats = len(lats)
nlons = len(lons1)
# read prmsl, convert to hPa (mb).
prmsl = 0.01*data.variables['prmslmsl'][0]
# the window parameter controls the number of highs and lows detected.
# (higher value, fewer highs and lows)
local_min, local_max = extrema(prmsl, mode='wrap', window=25)
# create Basemap instance.
m =\
Basemap(llcrnrlon=0,llcrnrlat=-80,urcrnrlon=360,urcrnrlat=80,projection='mill')
# add wrap-around point in longitude.
prmsl, lons = addcyclic(prmsl, lons1)
# contour levels
clevs = np.arange(900,1100.,5.)
# find x,y of map projection grid.
lons, lats = np.meshgrid(lons, lats)
x, y = m(lons, lats)
# create figure.
fig=plt.figure(figsize=(12,6))
cs = m.contour(x,y,prmsl,clevs,colors='k',linewidths=1.)
m.drawcoastlines(linewidth=1.25)
m.fillcontinents(color='0.8')
m.drawparallels(np.arange(-80,81,20),labels=[1,1,0,0])
m.drawmeridians(np.arange(0,360,60),labels=[0,0,0,1])
xlows = x[local_min]; xhighs = x[local_max]
ylows = y[local_min]; yhighs = y[local_max]
lowvals = prmsl[local_min]; highvals = prmsl[local_max]
# plot lows as blue L's, with min pressure value underneath.
xyplotted = []
# don't plot if there is already a L or H within dmin meters.
yoffset = 0.022*(m.ymax-m.ymin)
dmin = yoffset
for x,y,p in zip(xlows, ylows, lowvals):
if x < m.xmax and x > m.xmin and y < m.ymax and y > m.ymin:
dist = [np.sqrt((x-x0)**2+(y-y0)**2) for x0,y0 in xyplotted]
if not dist or min(dist) > dmin:
plt.text(x,y,'L',fontsize=14,fontweight='bold',
ha='center',va='center',color='b')
plt.text(x,y-yoffset,repr(int(p)),fontsize=9,
ha='center',va='top',color='b',
bbox = dict(boxstyle="square",ec='None',fc=(1,1,1,0.5)))
xyplotted.append((x,y))
# plot highs as red H's, with max pressure value underneath.
xyplotted = []
for x,y,p in zip(xhighs, yhighs, highvals):
if x < m.xmax and x > m.xmin and y < m.ymax and y > m.ymin:
dist = [np.sqrt((x-x0)**2+(y-y0)**2) for x0,y0 in xyplotted]
if not dist or min(dist) > dmin:
plt.text(x,y,'H',fontsize=14,fontweight='bold',
ha='center',va='center',color='r')
plt.text(x,y-yoffset,repr(int(p)),fontsize=9,
ha='center',va='top',color='r',
bbox = dict(boxstyle="square",ec='None',fc=(1,1,1,0.5)))
xyplotted.append((x,y))
plt.title('Mean Sea-Level Pressure (with Highs and Lows) %s' % YYYYMMDDHH)
plt.show()
