Fog forecasting at FMI - forecaster

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Fog forecasting at FMI- forecasters view

• Vesa Nietosvaara

Photo Jenni Teittinen
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Contents

• Introduction
• Climatology
• Rules of thumb
• Localizing fog
• Satellite information
• Model support
• Observations
• Real life
• Case studies

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Introduction

• At FMI fog is forecasted for
• General public in the media (overview)
• Aviation forecasts (as precisely as possible)
• Marine forecasts (overview -precise)
• The central service at 6th floor takes care of
  most of the general and marine weather
  forecasting
• Regional services especially aviation
  forecasting

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Climatology

• Stratus and fog very common in Finland. The
  probability of St/fog is at its maximum during
  late spring and winter.
• For example, in southern and central Finland fog
  is observed roughly 15-25 of the days in winter.
  
• In summer the fog probability is only 5-10.

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More climatology

• Fog axis at upslopes of Salpauselkä (100 km
  north of south coast)
• Northern Finland hills often foggy (Rovaniemi
  airport)
• Duration of fog
• advection fog Oct-Dec even days
• Radiation fog early summer vs. late summer in
  early summer not so frequent, but more persistent!

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Rules of thumb

• The basic rules known to each forecaster
• For example, the requirements for the formation
  of radiation fog
• Advection fog rules
• Plus a lot of silent knowledge, especially at the
  aviation forecasting !
• Local knowledge and experience (based on
  climatology)
• - Air streams, wind directions favourable for fog
  formation

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Wind direction vs. fog probability

• A climatological study within COST 722 is being
  done currently at FMI (Jukka Julkunen, Rovaniemi)
  
• Fog climatology for 10 selected airports in
  Finland.

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Localizing the fog

• Less and less manual SYNOPs and METARs.
• More automatization.
• ? fog mapping purely based on observations is
  very coarse and unreliable.
• Satellite observations are crucial
• MSG used, but not yet as effectively as we wish
• AVHRR traditionally the most used instrument
• Our experience is that even few observations
  combined with satellite images allow a
  satisfactory start for fog analysis
• A good mesoanalysis of the fog is needed!!

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Satellite information

• Some examples of available satellite products at
  FMI
• AVHRR
• daytime 0.6 0.9 10.8 µm combinations
• daytime 0.6 1.6 10.8 µm combinations,
• night-time 3.7 10.8 12.0 µm combinations,
• night-time 3.7 10.8 µm difference images.
• Meteosat-8
• As NOAA, but difference images not yet
  implemented
• Individual images very little use
• MTP still the most used source of information !?

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General experiences

• AVHRR superb in northern latitudes even in Seviri
  era
• Gaps in passes during the afternoon and night are
  problematic
• The 15-minute-interval for Meteosat-8 is
  extremely valuable for nowcasting purposes

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Some other examples of the use of satellite
information

• Fog sheets and their relation to daytime
  convection..
• Dissipation of fog

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Fog sheets and their relation to daytime
convection..
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Dissipation of fog
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Model support

• The general forecasters work mostly with this
  kind of model output when forecasting fog

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Model support

• Or with this kind of products

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Model support

• or with anything they find from the internet ...

http//meteo.icm.edu.pl
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Model support

• But no real fog model is currently available.

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Surface observations

• While classical observations have decreased, new
  observational data has become available
• Ceilometers
• Mast obs
• Sounding data not adequate for fog analysis
  purposes
• Even weather radar network can be used in some
  cases

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Real life

• Forecasters are aware of especially difficult fog
  forecasting issues
• Spring/early summer fog banks at sea
• Fog forming or not forming just prior to sunrise?
• How to actually forecast the dissipation of the
  fog?
• Evaluating visibility is very very difficult.

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Case studies

• Irene Suomi local fog case 8.9.2002 at Gulf of
  Finland
• Leena Upola fog case in northern Lapland 4.9.2003

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Photo Jenni Teittinen 8.9.02, Kruunuvuorenselkä
Case Study Marine Fog on the Coast of Helsinki
8.9.2002

• Yhtä sakeaa sumua olen kohdannut 35 vuoden
  aikana vain kolmasti.' PORKKALANNIEMI 8.9.02
• Kun laiva oli miekassa, lähestyi tutkalla
  tehdyn havainnon mukaan etelästä purjevene, joka
  väisti kohti rantaa (kadotti paikan
  todennäköisesti). Meiltä ei sitä optisesti sumun
  takia nähty, vain tutkalta. Jälkeenpäin kuultiin,
  että kaksi purjevenettä oli harhautunut rantaan.
  Molemmissa oli ollut pakolaisia!!!! Auttamaan
  tullut polisiisvene ajoi sekin kivikkoon! ''
  KUSTAANMIEKKA 8.9.02
• Koko Kruunuvuorenselkä oli paksussa sumussa,
  näkyvyys 50 m. Itse etenin kohti Hevossalmea
  tutkaa hyväksi käyttäen. Koko Kruunuvuorenselkä
  oli täynnä veneitä, noin 20. Osa oli neljän,
  viiden veneen ryhmissä paikallaan''
  KRUUNIVUORENSELKÄ 8.9.02

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• Dramatic and unexpected marine fog
• the inlet of Kustaanmiekka was closed by Helsinki
  Vessel Traffic Service for about an hour in
  8.9.2002 afternoon because several boats had got
  lost in the ship lane
• tens of boaters were caught by the fog during the
  day several alarm notices to the Maritime Rescue
  Co-ordination Centre in Helsinki (MRCC Helsinki)
• weather conditions inland warm and sunny late
  summer day
• fogs fairly rare on Finnish coastal seas in
  autumn because surface waters are typically warm
  after the summer

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• Methods and Goals of the Study
• Where and when? mapping of the evolution of fog
• satellite data
• in situ observations (synoptic weather
  stations, boats, ships, the Coast Guard, etc.)
• Why? determination of physical conditions
• sea surface temperature (from satellite images
  processed by SYKE)
• meteorological factors (synoptic weather
  stations, soundings, mast observations)
• origin of the foggy air reversed model run with
  SILAM dispersion model
• Predictability? consideration of the
  forecasting aspects
• as a meteorologist
• from the viewpoint of HIRLAM (HIgh Resolution
  Limited Area Model)

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• Fog evolution 7.-9.9.2002
• initial formation at night 7.-8.9.2002
• partial dissipation around noon
• movement and extent of fog patches in the
  afternoon determined by the local changes in wind
  field
• gradual dissipation at Finnish coast during the
  following night as the wind turned to north
• Typical features of marine fog formation
• sea surface temperature pattern sharp change in
  horizontal because of upwelling in the northern
  Gulf of Finland
• foggy air travelling a long distance over warm
  water before arriving in the area of cold sea
  surface
• strengthening of high pressure and related
  weakening of wind

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• Forecasting meteorologist
• sea surface temperature in major role in this
  case but also in any marine fog case
• meteorologists need more tools to observe the fog
  gt more cooperation with other authorities?
• Forecasting HIRLAM
• progress from ENO to the new version
• results fairly good with climatological sea
  surface temperature gt how about the effects of
  upwelling?

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Occasionally fog in early morning 4.9.2003
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Satellitepicture (NOAA 345), (the first picture
in this morning)

• Western Lapland is cleared up.
• In north-western part some upper clouds,
  developing showers? (near the upper-through)
• In eastern Lapland the cloudcover is thinning,
  but low stratus can be distinguished in black
  colour.
• Green dots are EFRO and EFPU

4.9. 02.18z
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Wind forecast

• almost the same wind speed at EFRO and at EFPU
• weak wind helps the fog formation

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Military Metars in Pudasjärvi

• 040250z 24004kt 4000 br few005 sct058 08/08
  Q1007
• 040320z 24004kt 4000 br sct006 bkn062 08/08
  Q1007
• 040350z 29005kt 5000 br bkn004 sct060 08/08
  Q1008
• 040400z 29006kt 2000 dz ovc003 08/08 Q1008
• 040410z 29006kt 0800 fg ovc003 08/08 Q1008
• 040420z 28004kt 0500 45fg ovc002 09/08 Q1008
• 040450Z 27005kt 0300 fg ovc002 09/08 Q1008
• 040520z 28004kt 0400 fg vv002 09/08 Q1008
• 040550z 28005kt 0400 fg vv002 09/08 Q1009
• 040602Z 26004kt 3000 br OVC002 08/08 Q1009
• 040620Z 26004kt 8000 VV001 08/08 Q 1009
• 040650Z 23003kt 9999 sct003 bkn010 09/07 Q1009
• 040720z 21004kt 9999 sct010 10/9 Q 1009

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Conclusions

• The model and radar products are displayed in a
  way which does not help in forecasting stratus.
• Infrared or interpreted satellitepictures and
  surface observations are useful in large scale
  cloud edge is approaching, a rough estimate of
  cloudbase.
• Metar-observations twice in hour almost realtime
  data, but air temperature and dewpoint
  temperatures are rounded off and information is
  lost, which is troublesome when we are near
  saturation.
• In this particular case we were able to say, that
  fog/stratus is coming and dispersing in the
  accuracy of (/- )3 hours
• It isnt nearly enough, what we are promised to
  do
• Limits for amending are very near each other in
  poor weather!

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Summary

• better realtime observations from ground to
  1500ft (significant cloud base) masts and
  profilers, low level soudings?
• Serviceable, practical tool especially for
  radiation fog situation, where fog is developing
  at the same time in the whole district, not
  advecting.
• Practical visual, graphic, easy to outline. It
  also tries to warn in situations learned with
  experience (rule of thumb)
• Rapid temperature change, change in radiation
  balance, (scattering/coming upper cloud),
  changing separation between air temperature and
  dew-point temperature.