Station lists and bias corrections Jemma Davie, Colin Parrett, Richard Renshaw, Peter Jermey © Crown Copyright 2012 Source: Met Office© Crown copyright Met Office

Outline Use of the model background Station lists −Surface −Aircraft −Upper air Background and buddy checks © Crown copyright Met Office

Use of the background Comparing observations with the background The background is a 6 hour forecast It has useful information because it contains observational information from previous analyses Left plot shows the rate at which 12- hour 500 hPa height forecast error degrades in ECMWF’s system on removal of satellite data. (Fisher 2004) It takes ~7 days before the influence of the satellite data is “forgotten”.

© Crown copyright Met Office Station Lists Contain information about which observations should or should not be rejected, bias corrections, probabilities of gross error, thinning and observation errors Static and updated sections Station lists for different observation types (surface, aircraft, sonde etc.) are updated in different ways Reject TEMP temperature reports from station ID between 700hPa and 400hPa Correct SYNOP pressure reports from station ID by 20hPa Reject marine reports over land Do not reject PILOT wind reports Reject aircraft reports above 90hPa

© Crown copyright Met Office Surface Report types: Land SYNOP, MOBSYN, METAR, ships and buoys Variables: Pmsl, Pstn, wind, 2m temperature, 2m relative humidity, visibility For each station: Calculate O-B statistics Make a decision about whether to reject reports from the station

© Crown copyright Met Office Surface – Non-pressure variables FEW OBSERVATIONS  Check previous month ENOUGH OBSERVATIONS  Check if statistics are good enough for accepting  Check if statistics are bad enough to reject station  If neither, do further checks involving whether the station was rejected for the previous month to make the decision

© Crown copyright Met Office Surface – Pressure Create a histogram of the O-B values Smooth the histogram Find peak and construct a 15 bin window centred at the peak Calculate mean and standard deviation of O-B in the window Compare to limits to make decision Negative of bias used as correction

© Crown copyright Met Office Aircraft Observation types: Airlines, AIREP/ADS, AMDAR Variables: temperature and wind Rejections can be made for individual aircraft or for an airline AMDAR levels – High ( hPa), Medium ( hPa) and Low (>700hPa) Corrections for AMDAR temperature

© Crown copyright Met Office Upper Air Observation types: TEMP, PILOT, drop sondes, wind profilers Variables: wind, temperature, relative humidity Can be rejected for different cycles The whole profile can be rejected, or just some layers For each station: Statistics Check wind direction bias Initial rejections and corrections Check whether to increase PGE Smoothing Final decision

© Crown copyright Met Office Upper Air – Initial rejections Compare statistics with limits Correct for radiation errors in temperature – Hawson correction

© Crown copyright Met Office Map model level decisions to decisions of pressure layers Upper Air – Initial rejections

© Crown copyright Met Office Take into account previous month’s rejections Combine for winds SuspectRejectPrevious Reject Updated Reject Number of Reports Updated Reject Upper Air – Updated rejections

© Crown copyright Met Office Upper Air – Smoothing Check stations for increasing PGE Layer rejections are smoothed Final rejection decisions made All layer rejection Low layer rejection Upper layer rejection Mid layer rejection

© Crown copyright Met Office Background and buddy checks Probability of gross error updated by consistency checks, the background check and the buddy check. Background check - compares with model background Probability density σ 2 = σ O 2 + σ B 2 Buddy check – checks against nearby observations If resulting PGE > 0.5, final flag is set and the observation is not assimilated.

© Crown copyright Met Office Summary Station lists − Static part: default settings for observation types and variables overall, as well as a few checks − Updated part: rejections and corrections for individual stations Quality control in Observation Processing System for individual reports − Consistency checks, background check and buddy check update PGE − If PGE exceeds 0.5, report not assimilated Observation feedbacks − Could be used to identify systematic errors and improve the observation record − Archiving ODBs for conventional observations

Thank you for listening. Any questions? © Crown Copyright 2012 Source: Met Office© Crown copyright Met Office