applying the formula into the data

Hi. Can someone please help me to understand this formula:

where;

SSPI is SPI value below the threshold

K is drought threshold

T is duration of drought processes

and if I can apply it to my matrx data in Matlab?

[Star Strider]

What is the calculation doing?

What are , , and and the rest? Are they vectors or matrices, and how does n affect them?

[Nurul Ain Basirah Zakaria]

it is an index,

i have a for instance,

420x32x20 as time(precipitation value)xlonxlat

so i need to:

find all the SSPI is SPI value below the threshold in each grid (threshold is k=-1)

and then T is how many there is of drought month (which is month with value -1 till -3