Source code for pyart.retrieve.advection

Advection calculations.


import copy

import numpy as np
from netCDF4 import num2date
from scipy.ndimage import shift

from ..config import get_fillvalue

# Based off work by Christoph Gohlke <>

[docs]def grid_displacement_pc(grid1, grid2, field, level, return_value="pixels"): """ Calculate the grid displacement using phase correlation. See: Implementation inspired by Christoph Gohlke: Note that the grid must have the same dimensions in x and y and assumed to have constant spacing in these dimensions. Parameters ---------- grid1, grid2 : Grid Py-ART Grid objects separated in time and square in x/y. field : string Field to calculate advection from. Field must be in both grid1 and grid2. level : integer The vertical (z) level of the grid to use in the calculation. return_value : str, optional 'pixels', 'distance' or 'velocity'. Distance in pixels (default) or meters or velocity vector in m/s. Returns ------- displacement : two-tuple Calculated displacement in units of y and x. Value returned in integers if pixels, otherwise floats. """ # create copies of the data field_data1 = grid1.fields[field]["data"][level].copy() field_data2 = grid2.fields[field]["data"][level].copy() # replace fill values with valid_min or minimum value in array if "valid_min" in grid1.fields[field]: min_value1 = grid1.fields[field]["valid_min"] else: min_value1 = field_data1.min() field_data1 =, min_value1) if "valid_min" in grid2.fields[field]: min_value2 = grid2.fields[field]["valid_min"] else: min_value2 = field_data2.min() field_data2 =, min_value2) # discrete fast fourier transformation and complex conjugation of field 2 image1fft = np.fft.fft2(field_data1) image2fft = np.conjugate(np.fft.fft2(field_data2)) # inverse fourier transformation of product -> equal to cross correlation imageccor = np.real(np.fft.ifft2(image1fft * image2fft)) # shift the zero-frequency component to the center of the spectrum imageccorshift = np.fft.fftshift(imageccor) # determine the distance of the maximum from the center # find the peak in the correlation row, col = field_data1.shape yshift, xshift = np.unravel_index(np.argmax(imageccorshift), (row, col)) yshift -= int(row / 2) xshift -= int(col / 2) dx = grid1.x["data"][1] - grid1.x["data"][0] dy = grid1.y["data"][1] - grid1.y["data"][0] x_movement = xshift * dx y_movement = yshift * dy if return_value == "pixels": displacement = (yshift, xshift) elif return_value == "distance": displacement = (y_movement, x_movement) elif return_value == "velocity": t1 = num2date(grid1.time["data"][0], grid1.time["units"]) t2 = num2date(grid2.time["data"][0], grid2.time["units"]) dt = (t2 - t1).total_seconds() u = x_movement / dt v = y_movement / dt displacement = (v, u) else: displacement = (yshift, xshift) return displacement
[docs]def grid_shift(grid, advection, trim_edges=0, field_list=None): """ Shift a grid by a certain number of pixels. Parameters ---------- grid: Grid Py-ART Grid object. advection : two-tuple of floats Number of Pixels to shift the image by. trim_edges: integer, optional Edges to cut off the grid and axes, both x and y. Defaults to zero. field_list : list, optional List of fields to include in new grid. None, the default, includes all fields from the input grid. Returns ------- shifted_grid : Grid Grid with fields shifted and, if requested, subset. """ if trim_edges == 0: trim_slice = slice(None, None) else: trim_slice = slice(int(trim_edges), -int(trim_edges)) shifted_grid = copy.deepcopy(grid) # grab the x and y axis and trim shifted_grid.x["data"] = grid.x["data"][trim_slice].copy() shifted_grid.y["data"] = grid.y["data"][trim_slice].copy() # shift each field. if field_list is None: field_list = grid.fields.keys() for field in field_list: # copy data and fill with nans data = grid.fields[field]["data"].copy() data =, np.nan) # shift the data shifted_data = shift(data, [0, advection[0], advection[1]], prefilter=False) # mask invalid, trim and place into grid shifted_data = shifted_data, copy=False, fill_value=get_fillvalue() ) shifted_data = shifted_data[:, trim_slice, trim_slice] shifted_grid.fields[field]["data"] = shifted_data return shifted_grid