Climate Indices of Precipitation

Monthly maximum of 1-day and consecutive 5-day precipitation

Here, the maximum precipitation is determined on a single day or within a period of 5 days.

For the Monthly maximum 1-day precipitation (Rx1day): Let RR_ij be the daily precipitation amount on day i in period j. The maximum 1-day value for period j are:

Rx1day_j = max (RR_ij)

For the Monthly maximum consecutive 5-day precipitation(Rx5day):

Let RR_kj be the precipitation amount for the 5-day interval ending _k, period j. Then maximum 5-day values for period j are:

Rx5day_j = max (RR_kj)

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Simple precipitation intensity index

The Simple precipitation intensity index (SDII) indicates the average daily rainfall for a period.

Let RR_wj be the daily precipitation amount on wet days, w (RR ≥ 1mm) in period j. If W represents number of wet days in j, then: 

SDII_j = ∑_^Ww=1(RR_wj)/W

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Annual number of days exceeding a certain precipitation value

These indices refer to a threshold X of precipitation that is exceeded per day. It indicates the annual number of days on which at least X millimeters of precipitation has fallen.

Let RR_ij be the daily precipitation amount on day i in period j and X the user defined threshold. Count the number of days where:

RR_ij≥ Xmm

Typical values for X are 10 mm and 20 mm with associated indices R10mm and R20mm.

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Maximum length of dry spell

The maximum length of a dry period (consecutive dry days, CDD) is the longest period of consecutive days in which no or less than 1 mm of precipitation has fallen.

Let RR_ij be the daily precipitation amount on day i in period j. Count the largest number of consecutive days where:

RR_ij < 1mm

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Maximum length of wet spell

The maximum length of a rainy season (consecutive wet days, CWD) indicates the longest period in which there has been at least one millimeter of rainfall per day without interruption.

Let RR_ij be the daily precipitation amount on day i in period j. Count the largest number of consecutive days where:

RR_ij ≥ 1mm

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Annual total precipitation exceeds a threshold (percentile)

For this index, the total annual precipitation sum is determined and compared with the average total rainfall of normal climate period. From this one can read, to what extent a year deviates from the long-term average.

Let RR_wj be the daily precipitation amount on a wet day w (RR ≥ 1.0mm) in period i and let RR_wnXX be the XX^th percentile of precipitation on wet days in the reference period. If W represents the number of wet days in the period, then:

RXXp_j = ∑_^Ww=1(RR_wj) für alle RR_wj > RR_wnXX

Typical percentiles XX for this index are 95 and 99. The corresponding indices are then called R95pTOT and R99pTOT. A typical frame for the climate normal period is 1961 - 1990.

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Annual total rainfall in rainy days

This index indicates the annual total precipitation in wet days (PRCPTOT).

Let RR_ij be the daily precipitation amount on day i in period j. If I represents the number of days in j, then

PRCPTOT_j = ∑_^Ji=1(RR_ij)

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References

This information is taken from:

http://etccdi.pacificclimate.org/list_27_indices.shtml

The following references are given there:

• Karl, T.R., N. Nicholls, and A. Ghazi, 1999: CLIVAR/GCOS/WMO workshop on indices and indicators for climate extremes: Workshop summary. Climatic Change, 42, 3-7.
• Peterson, T.C., et al.: Report on the Activities of the Working Group on Climate Change Detection and Related Rapporteurs 1998-2001 (pdf, not barrier-free). WMO, Rep. WCDMP-47, WMO-TD 1071, Geneve, Switzerland, 143pp.

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