objective digital analog forecasting “is the future in the past?”
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Objective Digital Analog Forecasting
“Is The Future In The Past?”
We’re Going Back …..
Back to the Future
Pattern Recognition Important to recognize the shape and influence of
patterns and teleconnection indices.
Teleconnections: AO, NAO, NAM, PNA, AAO, EA, WP, EP, NP, EAWR,
SCA, POL, PT, SZ, ASU, PDO,
El Nino/La Nina MEI, SOI, Nino1, Nino2, Nino3, Nino4, Nino3.4
Complex interactions in the mid and high latitudes makes forecasting most teleconnection indices difficult beyond a week or two.
55-yr Monthly Temporal Correlation of AO and 1000-500 mb Thickness
55-yr Monthly Temporal Correlation of AO and Precipitation
55-yr Monthly Temporal Correlation of AO and 500 mb Zonal Wind
55-yr Monthly Temporal Correlation of NAO and Surface Temperature
55-yr Monthly Temporal Correlation of PNA and 1000-500 mb Thickness
55-yr Monthly Temporal Correlation of MEI and 1000-500 mb Thickness
55-yr Monthly Temporal Correlation of MEI and Precipitation
Analog Motivation
Monthly/seasonal pattern evolution affected by?
Sea surface temperature anomalies ENSO Snow Cover / Icepack Solar cycle Phytoplankton Vegetation Atmospheric Chemistry Stratospheric Phenomena
Analog forecasting The oldest forecasting method?
Compare historical cases to existing conditions
Previous analog forecasting research yielded limited success
New digital age of analog forecasting
1. Dataset availability 55 Year NCEP Reanalysis 40 Year ECMWF Reanalysis 109 Year Climate Division Data Etc
2. Computational resources – statistical forecasting - ensembling
A sobering perspective…
“…it would take order 1030 years to find analogues that match over the entire Northern Hemisphere 500mb height field to within current observational error.”
From: Searching for analogues, how long must we wait?
Van Den Dool, 1994, Tellus.
Goals
Not seeking exact replication of patterns
Instead, determine sign of the climatological departure using an analog ensemble (on a weekly to monthly time scale)
Analogs require keys keys to matching keys to extracting
Statistically extracting information relevant to current patterns and removing noise.
Analog Components1. Data
Dataset length, frequency, area, variables, filtering
2. Matching Method Parameters, region, search window, threshold method (MAE,
anomaly correlation, RMSE, etc), statically or dynamically
3. Ensemble Configuration Match/date selection, top (1,10,100,1000 matches), ensemble
of single match analysis / ensemble of match analyses / both
4. Forecast Forecasts made from dates acquired from matching Integrate historical dates forward in time to generate
ensemble forecast – mean, probabilistic distributions
Example Analog Forecasts
1. Seasonal tropical thickness forecasts
2. Seasonal San Diego precipitation forecasts
3. 2-4 week mid-latitude forecasts
Seasonal Tropical (20N-20S) Analog Thickness Forecasts
1000-500hPa Thickness as Pattern Descriptor
Fewer degrees of freedom (Radinovic 1975)
Great integrator of: Long wave pattern Global temperature pattern Global lower tropospheric moisture pattern
Large inertia: Not greatly influenced by transient fluctuations (e.g. short-lived convection)
Matching Method?
Instantaneous (unfiltered) thickness analyses?
Filtered thickness analyses?
Choice likely depends on desired forecast length Short term forecast: compare instantaneous analyses Long term forecast: compare filtered analyses
Optimal Filtering F = f(t,L)t = forecast length (lead time)L = verification increment (hour, month, season)
Filtering
30
1( ) ( )
125
i t
SMOOTHi t days
Z t Z i
Seasonal forecasting30-day lagged mean
smoothed thickness
Matching Window for July 1
J D2003
J D2002
J D2001
J D1948
J D1949
J D
J D
J D
J D
J DMatch exact time/date # = 55
Match within 2 wk window # 3000
J D
J D
J D
J D
J DMatch allowed over entire year # 80000
2003
2002
2001
1948
1949
Analog selection for 00 UTC 12 January 2001.
Choose the top 200 (out of 3000 possible or 6%) matches from a 2-week window around the initialization date.
Exclude matching between the year before and after the initialization
Consensus forecast made for each 6-hour initialization time in 1948-1998, approx 80,000 forecasts.
51 years of Analog
Selection:
The DNA of atmospheric recurrence?
P e r c e n t
Skill? Persistence, anomaly persistence?
Convention for seasonal forecasting: Climatology. 54-year mean? 10-year mean? 30-year mean? Previous year?
Tropical (20°S-20°N) monthly mean thickness forecast is evaluated
Skill = MAECLIMO - MAEANALOG
Analog Forecast Skill: 51 year mean
Skill to 8.5 months
Skill to 25 months
Skill to 12 months
Winter/spring 1997 Forecast of 1998 El Nino
Pinatubo hinders analog matching
Spring 1982 prediction of 1983 El Nino
2
Skill (shaded) = MAECLIMO – MAEANALOG: [Red: Skill > 2m ]
Seasonal Precipitation Forecasts
“Dependent” Analog Forecasts
Analogs allow for forecasts of any dependent variable which has a historical record, regardless of what is matched.
Forecasts of dependent variables requires some relationship to the matching parameter
For example – electrical usage – long term record of electrical usage could be determined from dates provided by thickness matching, thanks to the dependence of electricity on temperature, and temperature on thickness.
Precipitation Forecasts
Need an analog ensemble of matching datesAcquired from global thickness matching
Daily historical records of surface parameters with a period as long as that from which the analogs matches were extracted51 years (1948-1998)
MEI and Precipitation CorrelationWith Available GSN Data
Method San Diego precipitation forecasts
Global thickness matching dates Surface precipitation observations Forecast length (1- 365) days
Forecasts averaged over the length of period which is to be forecast e.g., a seasonal (3 month) forecast is composed of an
average of 3 months of 6 hourly forecast initializations (~360 forecasts)
1983 El Nino 1998 El Nino
Seasonal Precipitation Forecast For San Diego Initialized 1982
Seasonal Precipitation Forecast For San Diego Initialized 1997
Mid-Latitude 2-4 Week Thickness Forecasts
Method
Technique similar to seasonal tropical forecasts with the following exceptions:
1-day filtered thickness analysesNH matchingMatching window - 4 weeksForecast length 1-30 days
Observed Analyses 00Z14MAR1993
Analog Ensemble Size
Analog Ensemble Consensus Top (1,10,100,500 analogs)
00Z14MAR1993
Optimal Analog Ensemble Size at Analysis
Analog Skill Length as a Function of Year and Season
Forecast Skill Variability
Distinct periods where analog forecast skill extends to 30 days or beyondENSOBlockingWell represented patterns - good analogs
Forecast confidence?
Example Forecast00Z15JAN1995
Analysis
Week 1
Week 2
Week 3
A flood of unanswered questions…
How does analog forecast skill vary with filtering of thickness in time and space
What is the impact of using another reanalysis dataset (ECMWF, JMS)?
How will mutli-parameter analogs impact skill?
Will temporal sequence matching vs static matching improve analog selection?
Can we blend dynamical prediction systems with analogs to further improve the skill of both?
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