fog forecasting at fmi - forecaster’s view

Fog forecasting at FMI- forecaster’s view

Vesa Nietosvaara

Photo: Jenni Teittinen

Contents

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

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

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%.

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!

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

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.

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!!

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 !?

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

Some other examples of the use of satellite information

• Fog sheets and their relation to daytime convection..

• Dissipation of fog

Fog sheets and their relation to daytime convection..

Dissipation of fog

Model support• The general forecasters work mostly with this

kind of model output when forecasting fog…

Model support• Or with this kind of products…

Model support• …or with anything they find from the

internet ...

http://meteo.icm.edu.pl

Model support

• But no real fog model is currently available.

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

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.

Case studies

• Irene Suomi: local fog case 8.9.2002 at Gulf of Finland

• Leena Upola: fog case in northern Lapland 4.9.2003

Case Study: Case Study: Marine Fog on the Coast of Marine Fog on the Coast of

Helsinki 8.9.2002Helsinki 8.9.2002

``Yhtä sakeaa sumua olen kohdannut 35 vuoden aikana vain kolmasti.'‘``Yhtä sakeaa sumua olen kohdannut 35 vuoden aikana vain kolmasti.'‘ PORKKALANNIEMI 8.9.02PORKKALANNIEMI 8.9.02

``Kun laiva oli miekassa, lähestyi tutkalla tehdyn havainnon mukaan etelästä purjevene, joka väisti kohti ``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 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 kuultiin, että kaksi purjevenettä oli harhautunut rantaan. Molemmissa oli ollut pakolaisia!!!! Auttamaan tullut polisiisvene ajoi sekin kivikkoon! '' KUSTAANMIEKKA 8.9.02polisiisvene ajoi sekin kivikkoon! '' KUSTAANMIEKKA 8.9.02

``Koko Kruunuvuorenselkä oli paksussa sumussa, näkyvyys 50 m. Itse etenin kohti Hevossalmea tutkaa ``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 hyväksi käyttäen. Koko Kruunuvuorenselkä oli täynnä veneitä, noin 20. Osa oli neljän, viiden veneen ryhmissä paikallaan'' KRUUNIVUORENSELKÄ 8.9.02ryhmissä paikallaan'' KRUUNIVUORENSELKÄ 8.9.02

Photo: Jenni Teittinen 8.9.02, Kruunuvuorenselkä

Case Study: Marine Fog on the Coast of Helsinki, 8.9.2002Case Study: Marine Fog on the Coast of Helsinki, 8.9.2002

IntroductionIntroduction

Dramatic and unexpected marine fogDramatic and unexpected marine fog

• the inlet of Kustaanmiekka was closed by Helsinki Vessel Traffic Service for 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 about an hour in 8.9.2002 afternoon because several boats had got lost in the ship lanethe ship lane

• tens of boaters were caught by the fog during the day: several alarm notices tens of boaters were caught by the fog during the day: several alarm notices to the to the Maritime Rescue Co-ordination Centre in Helsinki (MRCC Helsinki)Maritime Rescue Co-ordination Centre in Helsinki (MRCC Helsinki)

• weather conditions inland: warm and sunny late summer dayweather conditions inland: warm and sunny late summer day

• fogs fairly rare on Finnish coastal seas in autumn because surface waters fogs fairly rare on Finnish coastal seas in autumn because surface waters are typically warm after the summerare typically warm after the summer

Methods and Goals of the StudyMethods and Goals of the Study

• Where and when?Where and when? – mapping of the evolution of fog – mapping of the evolution of fog

satellite datasatellite data

““in situ” observations (synoptic weather stations, boats, ships, the Coast Guard, etc.)in situ” observations (synoptic weather stations, boats, ships, the Coast Guard, etc.)

• Why?Why? – determination of physical conditions – determination of physical conditions

sea surface temperature (from satellite images processed by SYKE) sea surface temperature (from satellite images processed by SYKE)

meteorological factors (synoptic weather stations, soundings, mast observations)meteorological factors (synoptic weather stations, soundings, mast observations)

origin of the foggy air: reversed model run with SILAM dispersion modelorigin of the foggy air: reversed model run with SILAM dispersion model

• Predictability?Predictability? – consideration of the forecasting aspects – consideration of the forecasting aspects

as a meteorologistas a meteorologist

from the viewpoint of HIRLAM (HIgh Resolution Limited Area Model)from the viewpoint of HIRLAM (HIgh Resolution Limited Area Model)


IntroductionIntroduction


Synoptic Weather ConditionsSynoptic Weather Conditions


Observed Fog AreasObserved Fog Areas

8.9.2002 at 9 a.m. 8.9.2002 at 9 a.m. ± ± 1 h (local time)1 h (local time) 8.9.2002 at 3 p.m. 8.9.2002 at 3 p.m. ± ± 1 h (local time)1 h (local time)


8.9.2002 at 6 p.m. 8.9.2002 at 6 p.m. ± ± 1 h (local time)1 h (local time) 9.9.2002 at 9.09 a.m. (local time)9.9.2002 at 9.09 a.m. (local time)

Observed Fog AreasObserved Fog Areas

(Source: S

uomen ym

päristökeskus, SY

KE

)(S

ource: Suom

en ympäristökeskus, S

YK

E)


Sea Surface TemperatureSea Surface TemperatureWind roses:Wind roses:

Inkoo BågaskärInkoo Bågaskär Helsinki HarmajaHelsinki Harmaja

28.8.-2.9.0228.8.-2.9.02

2.9.-8.9.022.9.-8.9.02

• at 4 p.m.: fog patches on a at 4 p.m.: fog patches on a narrow zone parallel to the narrow zone parallel to the coast (yellow)coast (yellow)

• later in the evening the fog later in the evening the fog area becomes widerarea becomes wider

• at 6 p.m.: fog is observed also at 6 p.m.: fog is observed also at Harmaja and Isosaari (grey)at Harmaja and Isosaari (grey)

• wind direction changes ca wind direction changes ca 360 degrees within a day: 360 degrees within a day:

land/sea breeze land/sea breeze phenomenonphenomenon

strengthening high pressure strengthening high pressure

• there is correlation between there is correlation between the width of the foggy area the width of the foggy area and the wind directionand the wind direction


The Fog at HelsinkiThe Fog at Helsinki

Difficulties in forecasting the Difficulties in forecasting the afternoon fog:afternoon fog:

• marine fogs fairly rare on autumnmarine fogs fairly rare on autumn

• sunny and warm weather was sunny and warm weather was

expected inland => difficult to estimate expected inland => difficult to estimate

what will happen to the fog during the what will happen to the fog during the

day on a narrow zone of cold sea day on a narrow zone of cold sea

surfacesurface

• no indications of a dramatic fog no indications of a dramatic fog

situation on satellite imagessituation on satellite images

• between 9 a.m. and 9 p.m. (local time) between 9 a.m. and 9 p.m. (local time)

only one synoptic weather station only one synoptic weather station

(Kirkkonummi-Mäkiluoto) reported fog (Kirkkonummi-Mäkiluoto) reported fog

at observation timesat observation times

Significant factors in fog Significant factors in fog

formation:formation:

• sea surface temperature pattern: sea surface temperature pattern:

due to upwelling, the sea surface due to upwelling, the sea surface

temperature was lower at the Finnish temperature was lower at the Finnish

coast compared to the other parts of coast compared to the other parts of

the Gulf of Finland and Baltic Seathe Gulf of Finland and Baltic Sea

• the air had travelled a long distance the air had travelled a long distance

above warm sea before arriving in the above warm sea before arriving in the

Finnish CoastFinnish Coast

• strengthening high pressure and strengthening high pressure and

related weakening of windrelated weakening of wind


Forecasting as a MeteorologistForecasting as a Meteorologist

ConclusionsConclusions

• Fog evolution 7.-9.9.2002Fog evolution 7.-9.9.2002

initial formation at night 7.-8.9.2002 initial formation at night 7.-8.9.2002

partial dissipation around noonpartial dissipation around noon

movement and extent of fog patches in the afternoon determined by the local changes movement and extent of fog patches in the afternoon determined by the local changes

in wind field in wind field

gradual dissipation at Finnish coast during the following night as the wind turned to gradual dissipation at Finnish coast during the following night as the wind turned to

northnorth

• Typical features of marine fog formationTypical features of marine fog formation

sea surface temperature pattern: sharp change in horizontal because of upwelling in sea surface temperature pattern: sharp change in horizontal because of upwelling in

the northern Gulf of Finlandthe northern Gulf of Finland

foggy air travelling a long distance over warm water before arriving in the area of cold foggy air travelling a long distance over warm water before arriving in the area of cold

sea surfacesea surface

strengthening of high pressure and related weakening of windstrengthening of high pressure and related weakening of wind

ConclusionsConclusions

• Forecasting: meteorologistForecasting: meteorologist

sea surface temperature in major role in this case but also in any sea surface temperature in major role in this case but also in any marine fog casemarine fog case

meteorologists need more tools to observe the fog => more meteorologists need more tools to observe the fog => more cooperation with other authorities?cooperation with other authorities?

• ForecastingForecasting: HIRLAM: HIRLAM

progress from ENO to the new versionprogress from ENO to the new version

results fairly good with results fairly good with climatologicalclimatological sea surface temperature => sea surface temperature => how about the effects of upwelling?how about the effects of upwelling?

Occasionally fog in early morning 4.9.2003

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

Wind forecast

• almost the same wind speed at EFRO and at EFPU

• weak wind helps the fog formation

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=

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 isn’t nearly enough, what we are promised to do: Limits for amending are very near each other in poor weather!

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.

fog forecasting at fmi - forecaster’s view

Documents

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daysradiation fog

fog mapping

central finland fog

rovaniemi fog climatology

real fog model

fog formationwind direction

satellite observations