Europe and the NH Phil Jones Climatic Research Unit University of East Anglia Norwich, UK

Summary Advantage of pressure over early observed variables Advantage of pressure over early observed variables Earlier MSLP digitization efforts Earlier MSLP digitization efforts CLIWOC – marine data CLIWOC – marine data Pressure data enable robust estimates of wind strength variations to be made over centuries Pressure data enable robust estimates of wind strength variations to be made over centuries

Pressure Advantages Early observers with a barometer almost always knew what they were doing Early observers with a barometer almost always knew what they were doing Temperature, elevation and gravity adjustments can generally be taken care of quite easily Temperature, elevation and gravity adjustments can generally be taken care of quite easily Time of daily observations also generally quite easy to deal with – from detailed observations that have often been made in many locations for 10-year periods Time of daily observations also generally quite easy to deal with – from detailed observations that have often been made in many locations for 10-year periods Pressure is much easier to homogenize than early temperature and precipitation measurements Pressure is much easier to homogenize than early temperature and precipitation measurements

1980s data digitization/analysis efforts Extended monthly grids developed from station data, assuming gridded datasets from the late 19th century for parts of the NH (Europe and North America) Extended monthly grids developed from station data, assuming gridded datasets from the late 19th century for parts of the NH (Europe and North America) Grids based on daily weather maps drawn from 1899 onwards during WW2 (Historical Weather Mapping Project, HWMP), extended in Europe to 1873 using German operational charts Grids based on daily weather maps drawn from 1899 onwards during WW2 (Historical Weather Mapping Project, HWMP), extended in Europe to 1873 using German operational charts Reconstruction technique is Orthogonal Spatial Regression Reconstruction technique is Orthogonal Spatial Regression Extensions of this approach to the Arctic – illustrating problem of the ‘Arctic High’, which resulted from the chart analysts being told there was an Arctic High, so generally in the absence of data they drew one Extensions of this approach to the Arctic – illustrating problem of the ‘Arctic High’, which resulted from the chart analysts being told there was an Arctic High, so generally in the absence of data they drew one Bias introduced is up to 10hPa in northern Canada and the western Arctic before the 1940s Bias introduced is up to 10hPa in northern Canada and the western Arctic before the 1940s Jones, P.D., 1987: The early twentieth century Arctic High - fact or fiction? Climate Dynamics 1, Jones, P.D., 1987: The early twentieth century Arctic High - fact or fiction? Climate Dynamics 1,

1990s data digitization/analysis efforts SH and Antarctic work, also monthly station pressure based SH and Antarctic work, also monthly station pressure based If monthly average MSLP available then daily/sub- daily should be, it just will take some finding If monthly average MSLP available then daily/sub- daily should be, it just will take some finding 77 SH sites from 1991 paper, with many extending back to 1910s

2000s data digitization/analysis efforts HadSLP1 and HadSLP2 HadSLP1 and HadSLP2 EMULATE – daily gridded fields from the 1851 EMULATE – daily gridded fields from the 1851 Daily station data digitized across Europe for , then combined with the maps from HWMP, marine MSLP data from ICOADS and HadSLP2 for monthly continuity Daily station data digitized across Europe for , then combined with the maps from HWMP, marine MSLP data from ICOADS and HadSLP2 for monthly continuity EMULATE has had numerous uses in synoptic climatology – and like Lamb weather types allows for numerous U/G and MSc projects EMULATE has had numerous uses in synoptic climatology – and like Lamb weather types allows for numerous U/G and MSc projects

A summer PC based on A summer MSLP PC based on the period, showing the precipitation, temperature and DTR response

Reconstructions of circulation indices from wind measurements from ships’ logbooks - CLIWOC Reconstructions always better in the winter season/winter half year, which is unfortunately the time when there were least ships

Reduction in observation count a serious issue of early ship-based data – for an atmospheric measure as opposed to SST

Digitization today Emphasis seems to be on Max/Min temperature and precipitation, but it is relatively easy to add in MSLP Emphasis seems to be on Max/Min temperature and precipitation, but it is relatively easy to add in MSLP ECA&D and ENSEMBLES daily digitization (and gridding) has included MSLP, but fewer NMSs have sent long series ECA&D and ENSEMBLES daily digitization (and gridding) has included MSLP, but fewer NMSs have sent long series Many seem unaware of the importance of MSLP data and the fact that it is far easier to homogenize than temperature and precipitation Many seem unaware of the importance of MSLP data and the fact that it is far easier to homogenize than temperature and precipitation Digitization needs to be justified and one important user of weather data is the Insurance Industry Digitization needs to be justified and one important user of weather data is the Insurance Industry Windstorms are the major insurance peril in Europe, and claims far outweigh fluvial or coastal flooding Windstorms are the major insurance peril in Europe, and claims far outweigh fluvial or coastal flooding Daily data are vital to develop long-term trends in circulation features and provide the best means of developing long series that are proxies for windiness Daily data are vital to develop long-term trends in circulation features and provide the best means of developing long series that are proxies for windiness Sub-daily (e.g. 3-hourly) data can be used to provide long series of rapid deepening rates of storms Sub-daily (e.g. 3-hourly) data can be used to provide long series of rapid deepening rates of storms

Pressure Triangle approach developed by Hans Alexandersson

Extension of this back to 1750 for NW Europe Daily MSLP digitized for Paris (back to 1675, with gaps in the 1730s/1740s), London (back to 1693, but with gaps in the 1710s/1720s) and De Bilt (back to 1706) Daily MSLP digitized for Paris (back to 1675, with gaps in the 1730s/1740s), London (back to 1693, but with gaps in the 1710s/1720s) and De Bilt (back to 1706) With these three sites the triangle approach will be able to put the high values seen in the 1880s into a longer-term context With these three sites the triangle approach will be able to put the high values seen in the 1880s into a longer-term context Paris/London MSLP difference is also a very good NAO index for the winter Paris/London MSLP difference is also a very good NAO index for the winter

Conclusions Many long MSLP records exists, but much is monthly Many long MSLP records exists, but much is monthly The daily should exist but finding it is often the problem The daily should exist but finding it is often the problem Many NMSs unaware of the potential usefulness of daily and sub-daily MSLP data Many NMSs unaware of the potential usefulness of daily and sub-daily MSLP data ERA-75 won’t be any better than ERA-40 for the pre-1979 period if more observational data are not added ERA-75 won’t be any better than ERA-40 for the pre-1979 period if more observational data are not added