© Crown copyright Met Office Climate Extremes Kuala Lumpur, Malaysia, 8 th -11 th November 2012.
-
Upload
raymond-dickerson -
Category
Documents
-
view
214 -
download
2
Transcript of © Crown copyright Met Office Climate Extremes Kuala Lumpur, Malaysia, 8 th -11 th November 2012.
© 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