Transcript KL_Extremes_gr

Kuala Lumpur, Malaysia, 8th-11th November 2012
Climate Extremes
© Crown copyright Met Office
Contents
• What is ‘Extreme’ and why use indices?
• Calculating Extremes using CDO
• Cautionary Note: Comparing Extremes in
Model and Observed data
© Crown copyright 2007
What is ‘Extreme’?
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)
CCl/CLIVAR/JCOMM Expert Team on Climate
Change Detection and Indices (ETCCDI)
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
ETCCDI indices
Internationally coordinated core set of
28 descriptive indices describe frequency,
amplitude, and persistence of moderate
extremes
Extremes Indices – temperature
based
Indicator name
ID
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
Growing 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
GSL
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
Warm spell duration indicator
Annual count of days with at least 6 consecutive days when TX>90th
percentile
Days
Cold spell duration indicator
Annual count of days with at least 6 consecutive days when TN<10th
percentile
Days
Diurnal temperature range
Monthly mean difference between TX and TN
ºC
WSDI
CSDI
DTR
APPENDIX A: List of ETCCDMI core Climate Indices
Extremes Indices – precip based
Indicator name
ID
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
Annual total precipitation divided by the number of wet days (defined as
PRCP>=1.0mm) in the year
Mm/day
SDII
Simple daily intensity index
R10
Number of heavy precipitation
days
Annual count of days when PRCP>=10mm
Days
R20
Number of very heavy
precipitation days
Annual 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
PRCPTOT
Annual total wet-day
precipitation
Annual 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)
10% days exceed
23.2º (av. 36 days
per year)
23.2º
1961
1990
• On average, in the baseline period, 10% of days (36/37 days will exceed this
threshold)
Example: Calculating TX90p
(warm days)
• Calculate the average number of times that same threshold is exceeded in a
future period
58% days exceed
23.2º (av. 212 days
per year)
23.2º
2070
2100
(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
12.7
mm
• These are‘ heavy rainfall days’
• Calculate the average amount of rain per year that occurs in ‘heavy’
events.
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.
12.7
mm
2070
2100
Calculating Indices with CDO
To calculate some of these indices with CDO for whole model fields
we 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.
A word of warning on Validating Extremes
… from a GCM grid to the point of interest.
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
Questions
Acknowledgements: John Caesar (Met Office), ETCCDI
© Crown copyright Met Office