© Crown copyright Met Office

Climate Extremes

Kuala Lumpur, Malaysia, 8th-11th November 2012

© Crown copyright 2007

Contents

• What is ‘Extreme’ and why use indices?

• Calculating Extremes using CDO

• Cautionary Note: Comparing Extremes in Model and Observed data

Wide range of space and time scales• From very small scale (precip) to large scale (droughts)

Definitions?• High impact events• Unprecedented events (in the available record)• Rare events (long return periods)• Exceedance of a relatively low threshold

(indices, such as 10th percentile of daily temperature or 95th percentile of daily precipitation amounts)

• Persistence of weather conditions (droughts)• Climatic extremes (e.g. extreme seasons)

What is ‘Extreme’?

from ETCCDI:• Definition of 28 core extreme

indices• Organization of regional

workshop• WMO-guide on extremes, 2009,

targeted at NMHSs around the world

http://cccma.seos.uvic.ca/ETCCDI

CCl/CLIVAR/JCOMM Expert Team on Climate

Change Detection and Indices (ETCCDI)

Internationally coordinated core set of 28 descriptive indices describe frequency, amplitude, and persistence of moderate extremes

ETCCDI indices

Extremes Indices – temperature based

APPENDIX A: List of ETCCDMI core Climate Indices

IDIndicator name

Definitions UNITS

FD0 Frost days Annual count when TN(daily minimum)<0ºC Days

SU25 Summer days Annual count when TX(daily maximum)>25ºC Days

ID0 Ice days Annual count when TX(daily maximum)<0ºC Days

TR20 Tropical nights Annual count when TN(daily minimum)>20ºC Days

GSLGrowing season Length Annual (1st Jan to 31st Dec in NH, 1st July to 30th June in SH) count between first

span of at least 6 days with TG>5ºC and first span after July 1 (January 1 in SH) of 6 days with TG<5ºC

Days

TXx Max Tmax Monthly maximum value of daily maximum temp ºC

TNx Max Tmin Monthly maximum value of daily minimum temp ºC

TXn Min Tmax Monthly minimum value of daily maximum temp ºC

TNn Min Tmin Monthly minimum value of daily minimum temp ºC

TN10p Cool nights Percentage of days when TN<10th percentile Days

TX10p Cool days Percentage of days when TX<10th percentile Days

TN90p Warm nights Percentage of days when TN>90th percentile Days

TX90p Warm days Percentage of days when TX>90th percentile Days

WSDI Warm spell duration indicator Annual count of days with at least 6 consecutive days when TX>90th percentile Days

CSDI Cold spell duration indicator Annual count of days with at least 6 consecutive days when TN<10th percentile Days

DTR Diurnal temperature range Monthly mean difference between TX and TN ºC

Extremes Indices – precip based

IDIndicator name

Definitions UNITS

RX1day Max 1-day precipitation amount Monthly maximum 1-day precipitation Mm

Rx5day Max 5-day precipitation amount Monthly maximum consecutive 5-day precipitation Mm

SDIISimple daily intensity index Annual total precipitation divided by the number of wet days (defined as

PRCP>=1.0mm) in the yearMm/day

R10Number of heavy precipitation

daysAnnual count of days when PRCP>=10mm Days

R20Number of very heavy

precipitation daysAnnual count of days when PRCP>=20mm Days

Rnn Number of days above nn mm Annual count of days when PRCP>=nn mm, nn is user defined threshold Days

CDD Consecutive dry days Maximum number of consecutive days with RR<1mm Days

CWD Consecutive wet days Maximum number of consecutive days with RR>=1mm Days

R95p Very wet days Annual total PRCP when RR>95th percentile Mm

R99p Extremely wet days Annual total PRCP when RR>99th percentile mm

PRCPTOTAnnual total wet-day

precipitationAnnual total PRCP in wet days (RR>=1mm) mm

Alexander et al., JGR, 2006; also in IPCC, 2007

Example: Calculating TX90p (warm days)

• Calculate threshold exceeded by the 10% hottest days (Tmax) in baseline period (i.e. 1961-90)

• On average, in the baseline period, 10% of days (36/37 days will exceed this threshold)

10% days exceed 23.2º (av. 36 days per year)

23.2º

1961 1990

Example: Calculating TX90p (warm days)

• Calculate the average number of times that same threshold is exceeded in a future period

23.2º

2070 2100

58% days exceed 23.2º (av. 212 days per year)

(these are synthetic data, not from real projections!)

R95PTOT- Total annual rainfall on heavy rain days

• Similarly, calculate the 95th percentile of wet days only (5% wettest ‘wet days’, i.e. days >1mm) in baseline

• These are‘ heavy rainfall days’• Calculate the average amount of rain per year that occurs in ‘heavy’ events.

12.7mm

R95PTOT- Total annual rainfall on heavy rain days

• Identify the ‘heavy’ rainfall days in the future• Sum the rainfall that falls on those days to give average per year.

2070 2100

12.7mm

Calculating Indices with CDO

To calculate some of these indices with CDO for whole model fieldswe can either use CDO defined extremes operators, or our own.

For the percentage of warm days:

cdo eca_tg90p ifile1 ifile2 ofile

cdo timsum –gt ifile1 ifile2 ofile

cdo mulc,100 –divc,[days] ofile ofile.percent

Some of the CDO extremes operators are not always robust with PRECIS data, but we can calculate them by using other CDO operators together.

… from a GCM grid to the point of interest.

A word of warning on Validating Extremes

Individual station vs. area averages

26 stations in a 25km×25km area (black bars) and their area averages, (red bars).

The area average (c.f. model grid box output) is considerably and inconsistently different to most individual stations

Model grid box vs. point observations

Average(Extreme) ≠ Extreme(Average)

Rules of thumb:

Usually model output has reduced range of values and reduced variability, but it depends on the physiography of the grid box

Trends should be the same if dependent on large scale phenomena (e.g. major mode of variability, climate change) included in the model and observational world

© Crown copyright Met Office

Questions

Acknowledgements: John Caesar (Met Office), ETCCDI