EOF: Empirical Orthogonal Function

In metR: Tools for Easier Analysis of Meteorological Fields

Empirical Orthogonal Function

Description

Computes Singular Value Decomposition (also known as Principal Components Analysis or Empirical Orthogonal Functions).

Usage

EOF(
  formula,
  n = 1,
  data = NULL,
  B = 0,
  probs = c(lower = 0.025, mid = 0.5, upper = 0.975),
  rotate = NULL,
  suffix = "PC",
  fill = NULL,
  engine = NULL
)

Arguments

formula	a formula to build the matrix that will be used in the SVD decomposition (see Details)
n	which singular values to return (if NULL, returns all)
data	a data.frame
B	number of bootstrap samples used to estimate confidence intervals. Ignored if <= 1.
probs	the probabilities of the lower and upper values of estimated confidence intervals. If named, it's names will be used as column names.
rotate	a function to apply to the loadings to rotate them. E.g. for varimax rotation use stats::varimax.
suffix	character to name the principal components
fill	value to infill implicit missing values or NULL if the data is dense.
engine	function to use to compute SVD. If NULL it uses irlba::irlba (if installed) if the largest singular value to compute is lower than half the maximum possible value, otherwise it uses base::svd. If the user provides a function, it needs to be a drop-in replacement for base::svd (the same arguments and output format).

Details

Singular values can be computed over matrices so formula denotes how to build a matrix from the data. It is a formula of the form VAR ~ LEFT | RIGHT (see Formula::Formula) in which VAR is the variable whose values will populate the matrix, and LEFT represent the variables used to make the rows and RIGHT, the columns of the matrix. Think it like "VAR as a function of LEFT and RIGHT". The variable combination used in this formula must identify an unique value in a cell.

So, for example, v ~ x + y | t would mean that there is one value of v for each combination of x, y and t, and that there will be one row for each combination of x and y and one row for each t.

In the result, the left and right vectors have dimensions of the LEFT and RIGHT part of the formula, respectively.

It is much faster to compute only some singular vectors, so is advisable not to set n to NULL. If the irlba package is installed, EOF uses irlba::irlba instead of base::svd since it's much faster.

The bootstrapping procedure follows Fisher et.al. (2016) and returns the standard deviation of each singular value.

Value

An eof object which is just a named list of data.tables

left

data.table with left singular vectors

right

data.table with right singular vectors

sdev

data.table with singular values, their explained variance, and, optionally, quantiles estimated via bootstrap

There are some methods implemented

• summary

• screeplot and the equivalent ggplot2::autoplot

• cut.eof

• predict

References

Fisher, A., Caffo, B., Schwartz, B., & Zipunnikov, V. (2016). Fast, Exact Bootstrap Principal Component Analysis for p > 1 million. Journal of the American Statistical Association, 111(514), 846–860. \Sexpr[results=rd]{tools:::Rd_expr_doi("10.1080/01621459.2015.1062383")}

See Also

Other meteorology functions: Derivate(), GeostrophicWind(), WaveFlux(), thermodynamics, waves

Examples

# The Antarctic Oscillation is computed from the
# monthly geopotential height anomalies weighted by latitude.
library(data.table)
data(geopotential)
geopotential <- copy(geopotential)
geopotential[, gh.t.w := Anomaly(gh)*sqrt(cos(lat*pi/180)),
      by = .(lon, lat, month(date))]

eof <- EOF(gh.t.w ~ lat + lon | date, 1:5, data = geopotential,
           B = 100, probs = c(low = 0.1, hig = 0.9))

# Inspect the explained variance of each component
summary(eof)
screeplot(eof)

# Keep only the 1st.
aao <- cut(eof, 1)

# AAO field
library(ggplot2)
ggplot(aao$left, aes(lon, lat, z = gh.t.w)) +
    geom_contour(aes(color = after_stat(level))) +
    coord_polar()

# AAO signal
ggplot(aao$right, aes(date, gh.t.w)) +
    geom_line()

# standard deviation, % of explained variance and
# confidence intervals.
aao$sdev

# Reconstructed fields based only on the two first
# principal components
field <- predict(eof, 1:2)

# Compare it to the real field.
ggplot(field[date == date[1]], aes(lon, lat)) +
    geom_contour_fill(aes(z = gh.t.w), data = geopotential[date == date[1]]) +
    geom_contour2(aes(z = gh.t.w, linetype = factor(-sign(stat(level))))) +
    scale_fill_divergent()

metR documentation built on Oct. 14, 2024, 5:09 p.m.