Loop or function for sequence of matrices of sst data - r

Question

I have been looking for a way to select the frequency of values within a range in R from a sequence of matrices of sea surface temperature (sst) by month. These data are arranged as (sst, 80*40*172) i.e., (sst, longitude,latitude, month) for the north Atlantic. I'am interested in those frequencies because I use them to calculate the surface area of a range in sst for every month say between 4°C and 12°C isotherms. I have used R frequently for statistical analyses of time-series and spatial data, but I'am not used to programming so my approach is probably not the most efficient. I have succeeded in extracting frequencies for sst values of say >12 °C for a all latitudes in one month:
My data from IRI/LDEO climate research web site
dat <- read.table("c:/Temp/[Y+X]datatable.tsv", header=FALSE, sep="\t")
A 5x5 sample matrix of the first month
Nov 1981 30.5000 31.5000 32.5000 33.5000
9.50000 21.7906 21.9431 22.1324 22.1662
8.50000 21.7267 21.8573 21.9981 21.8757
7.50000 21.6644 21.7781 21.8960 21.7393
6.50000 21.5989 21.7025 21.8044 21.6304
I basically skip the longitude, row labels I also intend to add the date latter. First I extract the first month as a matrix (t) and apply two custom functions surface area of
lat strip SA and surface area of all lat strips for the month MSA.
ro=2:81
co=2:41
t <- as.matrix(dat[ro,co])
Then
SA <- function (lat,tmu){
l <- c(t[,lat]>=tmu,0)
la <- as.data.frame(l)
x <- la[,1]
n <- length(x)
sau <- array(0,n)
x. <- lat
for (i in 1:n) sau[i] <- (x[i]*111.320)*(cos(x.*(3.1415/180))*111.320)
s <- as.matrix(sum(sau))
}
MSA <- function(tmu){
m <-1:40
su <- array(0,0)
for (i in 1:40) su[i] <-SA(m[i],tmu)
ms <- as.data.frame(su)
sa <- as.data.frame(colSums (ms))
return(sa)
}
Functions SA and MSA or surface area an surface of one latitude (lat) strip SA and area of all strips for the month SAM, for temperature upper limit (tmu).
Matrix (s) from function SA has the surface area of sst> tmu(say 12 °C) for latitude(lat)(say at 30°N) and Matrix (sa) from function SAM has the sums of all the latitudes (from 30°N to 70°N).This does the job for one month and I can repeat the functions 12 times to obtain the year or 172 times for the 16 years this way:
I define a starting latitude (lat) then a step of 81 longitude cells (st) to extract next and the desired temperature;
lat= 30.5
st=81
t=12
Then calculate the total surface area for each month;
SA1 <- {
i=0*st
t <- as.matrix(dat[ro+i,co])
SA(lat,t)
MSA(12)
}
SA2 <- {
i=1*st
t <- as.matrix(dat[ro+i,co])
SA(30.5,t)
MSA(12)
}
My question is if it is possible to create a loop or a function that would iterate through all months that is 172 times and so skip repeating SA1,SA2...SA172. Thanks in advance.

Answer 1

If I understand your question, your code reduces to this
#create some dummy data
lat <- 30:33
lon <- 6:9
sst <- array(rnorm(length(lat) * length(lon) * 12, mean = 21, sd = 4),
dim = c(length(lat), length(lon), 12), dimnames = list(lat, lon, 1:12))
#the actual code
SA <- function (test, tmu){
colSums(test >= tmu) * 111.320 ^ 2 *
cos(as.numeric(rownames(test)) * (pi/180))
}
apply(sst, 3, SA, tmu = 21)
If this is not correct, please make your question more clear and provide a reproducible input and output dataset.