Estimate cloud cover from shortwave radiation

Inverts the ratio of measured to clear-sky potential shortwave radiation to a cloud fraction, following Martin & McCutcheon (1999). Works at any timestep: daily input is evaluated on an internal hourly grid (built in tz) and aggregated back to daily.

Usage

calc_cc(
  datetime,
  airt,
  swr,
  relh = NULL,
  dewt = NULL,
  lat,
  lon,
  elev = 0,
  tz = NULL
)

Arguments

datetime

Date or POSIXct timestamps matching swr.

airt

air temperature, degC.

swr

measured downwelling shortwave radiation, W m-2.

relh

relative humidity, percent; used when dewt is absent.

dewt

dew point temperature, degC.

lat, lon

decimal degrees.

elev

elevation, m above sea level.

tz

timezone that datetime refers to; used to expand daily dates into hours. Default NULL, which uses the tzone attribute of datetime when it carries one, otherwise "UTC".

Value

numeric vector of cloud cover fractions (0-1), same length as datetime. Values that cannot be inferred (measured shortwave at or above clear sky, or night) are linearly interpolated from neighbours.

Examples

d <- data.frame(Date = seq(as.Date("2024-01-01"), by = "day", length.out = 10),
                airt = 18, swr = c(300, 120, 280, 90, 310, 150, 200, 260, 80, 290))
calc_cc(d$Date, airt = d$airt, swr = d$swr, relh = 70,
        lat = -38.08, lon = 176.27, elev = 280)
#>  [1] 0.6223841 1.0000000 0.6795007 1.0000000 0.5858550 0.9794251 0.8746414
#>  [8] 0.7288839 1.0000000 0.6412350