1. A new precipitation and drought climatology based on weather patterns
Doug Richardsona, Hayley Fowlera, Chris Kilsbya, Rob Nealb
aNewcastle University
bMet Office
05/09/2017 EMS conference, Dublin

2. Content
Introduction to Lamb Weather Types and the Met Office weather pattern classification.
Comparing the two classifications using daily rainfall characteristics.
UK drought climatology using the Met Office classification.
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3. 1. Lamb Weather Types (LWTs)
LWTs are the most commonly used weather patterns for UK analyses
LWTs are predefined types based on the eight cardinal directions, cyclonic (C) and anticyclonic (A) types and an unclassified type.
Daily pressure maps are assigned to a LWT using rules based on flow characteristics (Jones et al. 1993). CE NW
ASW
e.g. C A
(+)
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4. 1. Met Office classification (MO-30)
Objectively defined by clustering 154 years of 5° x 5° MSLP data (EMULATE).
Larger region than LWTs – useful for capturing large-scale circulation over Atlantic.
There are 30 distinct patterns (WPs).
WPs are ordered from most frequent to least frequent.
Seasonality: Lower-numbered WPs occur more in summer; higher-numbered WPs in winter.
Neal et al. (2016)
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5. 2. Mapping MO-30 to LWTs U SW ASW W S A U C A W C SW NW NW SW
Calculations used for LWT derivation are applied to MO-30 WPs, obtaining a LWT for each WP.
14 WPs have a westerly component in their LWTs, compared to 2 WPs with an easterly component.
This better reflects the predominance of westerly flow over the UK.
E, AE, CE, NE, ANE and CNE LWTs account for ~7% days
C SW NW NW SW
S AS ASW N W
SW S W C A
NW SE CSE C CW
Neal et al. (2016) 1/

6. 2. MO-30 and LWT relationship with daily precipitation
Both classifications associated with daily rainfall for 9 UK regions
A ‘good’ classification would show distinct rainfall distributions for each type.
In general, MO-30 patterns are more distinct in terms of median rainfall than LWTs.
Richardson et al. (2016) 1/
WP (top), LWT (bottom). Ordered driest to wettest L-R.

7. 2. MO-30 and LWT relationship with daily precipitation
The inter-quartile range (IQR) is higher for LWTs than MO-30.
This is especially true for easterly regions and LWTs.
Richardson et al. (2016)
WP (top), LWT (bottom). Ordered driest to wettest L-R. 1/

8. 2. MO-30 and LWT relationship with daily precipitation
MO-30 has distinct WPs that have the same LWT definition.
WP13 and WP14 are both NW LWTs.
Both feature anticyclone to southwest and cyclone to northeast (hence NW flow).
Anticyclone is closer and cyclone further away for WP13, so it is drier than WP14.
NW flow
Richardson et al. (2016)
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9. 2. MO-30 and LWT relationship with daily precipitation
MO-30 has distinct WPs that have the same LWT definition.
WP13 and WP14 are both NW LWTs.
Both feature anticyclone to southwest and cyclone to northeast (hence NW flow).
Anticyclone is closer and cyclone further away for WP13, so it is drier than WP14.
Anticyclone closer to UK.
Cyclone closer to UK.
Richardson et al. (2016)
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10. 2. MO-30 and LWT relationship with daily precipitation
MO-30 has distinct WPs that have the same LWT definition.
WP13 and WP14 are both NW LWTs.
Both feature anticyclone to southwest and cyclone to northeast (hence NW flow).
Anticyclone is closer and cyclone further away for WP13, so it is drier than WP14.
WP13 drier than WP14.
Richardson et al. (2016)
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WP. Ordered driest to wettest L-R.

11. 3. Quantifying drought
Drought quantified using Drought Severity Index (DSI), based on cumulative precipitation deficits.
DSI-3 represents short-term drought, DSI-12 long-term drought.
Calculated for 9 UK regions.
Highest 5% DSI values classed as “drought months”.
Richardson et al. (2016)
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12. 3. MO-30 WP frequencies during drought months (DSI-3)
Decrease in easterly WPs during droughts in eastern regions.
Some increases in westerly WPs during droughts in eastern regions.
Richardson et al. (2016)
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13. 3. MO-30 WP frequencies during drought months (DSI-3)
Mixed results for westerly WPs during droughts in western regions
Easterly WPs tend to increase in frequency during western droughts
Richardson et al. (2016)
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14. 3. Weather patterns associated with drought over entire UK
For droughts spanning the majority of the UK, the following WPs typically increase in frequency: WP6, WP9, WP10, WP12, WP17, WP25.
Additional WPs associated with regional drought.
Neal et al. (2016)
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15. Summary
We can identify which WPs are linked with short/medium/long-term drought for different UK regions
This has potential for a forecast model based on predicting weather patterns and hence drought conditions.
Met Office ‘Decider’ product forecasts probabilistic WP occurrences using seasonal forecast systems e.g. GloSea5.
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16. Questions?
Jones, P. D., Hulme, M. and Briffa, K. R. (1993), A comparison of Lamb circulation types with an objective classification scheme. Int. J. Climatol., 13: 655–663. doi: /joc Neal, R., Fereday, D., Crocker, R. and Comer, R. E. (2016), A flexible approach to defining weather patterns and their application in weather forecasting over Europe. Met. Apps, 23: 389–400. doi: /met.1563 Richardson, D., Fowler, H. J., Kilsby, C. G. and Neal, R. (2017), A new precipitation and drought climatology based on weather patterns. Int. J. Climatol. doi: /joc.5199
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