Transcript Coastal Meteorology - University College Dublin

The Sea Breeze
• An onshore breeze which develops in
coastal areas on a warm day.
• Differential heating between the land and
sea.
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Forecasting Local Weather
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Sea Breeze (Again!)
Temperature
Dew
Fog
Frost
Snow
Thunderstorms
Tropical Cyclones
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Sea breeze formation
Two columns of air
At dawn:
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Sea breeze formation
As land heats up a
circulation
develops
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How… and When?
• Land temperatures need to be at least 3.5 oC warmer
than sea temperatures …
• They are very common and strong in tropical regions
• In Ireland generally from March to late September.
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It’s not just a coastal thing
• Sea breezes can occasionally penetrate over
50km inland
• Sea breezes can enhance convection due to
convergence, particularly on peninsulas
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Wind Flow over Mountains
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Mountain Waves from Above
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Lenticular Altocumulus
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Temperatures: Radiation Balance
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Typical Diurnal Variation of Temperature
• Min soon after dawn
Temp falls until incoming
shorwave >outgoing longvave
• Max after local noon
Temp rises Until incoming
shorwave <outgoing longvave
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Temperature Forecasting Techniques
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Maximum
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850 qw Empirical relationship between 850 qw and maximum
temperature
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Minimum
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1000-850hPa thickness, using standard tables,
correction for cloud
McKenzie: Uses Maximum Temperature, Td at time of Tmax, and
correction for wind/cloud
Model Output statistics MOS
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Uses model output of temperatures, combined with regression
techniques containing local information
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Moisture in the Atmosphere
• Water can exist in any one of three phases:
• Solid (Ice, Hoar Frost)
• Liquid (Raindrops, Cloud drops, Drizzle, Dew)
• Gas (water vapour)
• The amount of WATER VAPOUR that the air can
hold is heavily dependent upon temperature.
• Measure Water Vapour content in different ways:
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Relative Humidity
Dew Point Temperature
Wet Bulb Temperature
Mixing Ratio
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NOT
ALLOWED IN
THE FREE
ATMOSPHERE
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Td
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Saturation of Air
• For a parcel of air to become SATURATED,
either
1. It must acquire some more moisture, or
2. It must cool down
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The first case can occur if air passes over a body
of water
The second case can have many causes...
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Dew – A Brief Diversion
• DEW forms when water vapour condenses out
onto the earth’s surface.
• Night time bring radiation cooling to the ground
• Grass, exposed metal (cars!) etc cool more rapidly
than roads, footpaths
• On MOST nights the temperature of the ground
falls below the Dew Point
• Exceptions – windy, cloudy weather.
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Diurnal Variation of Temperature
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What is Fog?
Visibility less than
1Km
Visibility 1Km to
18Km
Relative Humidity
greater than 75%
Fog
Mist
Relative Humidity
less than 75%
Haze
Haze
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Formation of Fog
• Ground will cool at night, through loss of heat
through long-wave radiation (clear, calm nights)
• Air at the surface will cool through contact with
the colder ground.
• Fog forms at the surface; initially in a shallow
layer, then it “grows” upwards as the top of the
fog layer loses heat in turn.
• Known as RADIATION FOG
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Fog
Radiation Fog
• On clear nights air near the surface of the earth is
cooled due to outgoing radiation
Conditions Favouring ‘Fog Formation’
Clear skies, long night
Tair and Tdewpoint converging
Little or no wind
Timing – often just after dawn
Freezing Fog is when the temperature is less than zero
and the water droplets in the fog are supercooled. This
is very uncommon in Ireland. NOT just fog with T < 0c
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Formation of Radiation Fog
• Why just after dawn?
• Rising sun heats surface of the ground
•  Evaporation of night-time dew
•  Injection of moisture into the (cold) lowest
layers
•  Condensation into fog droplets
• Usually clears again after a couple of hours.
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Fog
Sea Fog
• Forms when moist air is cooled to
saturation by contact with a cool sea
surface
• Most common in spring and early
summer when the sea is at its coldest
• Temperature of sea relative to
Dewpoint of the Airmass?
• Look for Td greater than 13C or 14C
• Can get Td up to 16C or 17C in Tm
air during the summer.
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Other types of fog
• Advection Fog
• Warm air passing over cold ground (e.g. Warm sector
reaching snow-covered ground).
• Frontal Fog
• Frontal precipitation falls through a dry layer of air,
where it evaporates. The consequent increase in the
water vapour can trigger fog. Typical of weak, slowmoving fronts in the summer months.
• Advected sea or radiation fog
• Fog which has formed in one place but been transported
to another by a (usually gentle) breeze.
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Fog formation
• Very heavily influenced by the topography
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Exposed upslopes (south and southwest of Ireland)
River valleys (e.g. Po valley in Italy)
Flat bogland
Steep valleys which lead to cold-air pooling and
consequent inversions
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Radiation Fog
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Sea Fog
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Post Cold-Frontal Fog
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Sea Fog on the Norwegian Coast
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Valley Fog in Norway
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Continental Anticyclone
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Orientation of Warm Sector
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Fog in Switzerland
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North Italy / Northern Balkans
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Frost Occurrence
Occurs on radiation nights
• Clear skies+Slack winds
•Anticyclone
•Ridge or
•Slack airflow
•Long night
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Frost Conditions
• Often a choice between frost and fog
• Cold, dry air  Favours frost
• More moist air  Fog more likely
• Unusual to have both together
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Frost Definitions
• Air frost -- Air temperatures below zero.
• Ground frost – Ground temperatures below zero
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Slight
Sharp
Severe
Very Severe
-2o < Td < 0o
-5o < Td < -2o
-10o< Td < -5o
Td < -10o
• Hoar Frost -- deposits ice (through sublimation)
onto surfaces.
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Formation of Hoar/Rime Frost
“Radiation night“
with clear skies and
slack winds
We also require a source of moisture
• This is present in the air as a gas Water Vapour
– Cooler air holds less moisture
Dewpoint: Temperature to which air must be cooled
to become saturated with respect to water
Conditions Favouring ‘Wet’ Frost
• Tsurface < 0 °C, Tsurface < Tdewpoint
• Sufficient humidity Tdewpoint -Tair<1.5°C
Frost Criteria
Slight
0º to -2º C
Sharp
-2º to -5º C
Severe
-5 to -10º C
Very Severe Below -10 C
• Tair ,Tsurface and Tdewpoint are converging
On long frosty nights a build up of hoar or
rime frost can lead to a layer of Ice
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Snow
• Widespread Snow Rare
• Most likely in showers
• More frequent on higher
ground
Snow Scenario
•Warm front approaching from South,
cold surface (Easterly) airflow
•Cold front turns to snow before clearing
•Showers in cold west to north airflow,
or Easterly airflow
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Snow Forecasting
Percentage Probability of Snow
90%
70%
50%
30%
10%
T Air
-0.3
1.2
1.6
2.3
3.9
1000-850hPa
1281
1290
1293
1298
1303
0° Isotherm
12
25
35
45
61
1000-500hPa
5180
5238
5258
5292
5334
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Warm and Cold Clouds
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Idealized
Thunderstorm
• Charge separation most
likely occurs during
rebounding collisions
between ice crystals and
large ice hydrometeors
such as graupel and hail
that remain suspended in
the mixed phase zone by
the updraft of a growing
thunderstorm.
-20° C
-10° C
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Non-inductive Charging (NIC) Theory
Forecasting Thunderstorms
Stability Indices
• Boyden Index I=(800-700hPa)-T700hPa
• Thunder Probable if I > 4/95
• Radcliff Index T=qw900 – T500
• Thunder Probable if T  29/30
• Potential Instability P=qw500 - qw850
• Thunder possible if P  -2 (summer)
• K Index K = (T850 – T500) + Td850 – (T700 – Td700)
• Thunder possible for K20
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Lightning!
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Tropical Cyclones
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Tropical Cyclones
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Tropical Cyclones
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Need Sea Temperature above 26.5 C
Low levels of vertical wind shear
“Easterly Wave” in the trade wind flow (Atlantic)
Differences of degree...
• Tropical Depression
• Tropical Storm
• Hurricane
• Bring vast amounts of moisture into the upper
atmosphere.
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Hurricane Isabel (2003)
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Hurricane Isabel (2003)
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Tropical Cyclones / cont
• Most damage / deaths caused by coastal flooding
• Weaken quickly over land
• but.... can bring very heavy rain inland leading to flash
floods
• We watch out for “old” tropical cyclones that get
caught up in mid-latitude weather systems
• Tend to bring very heavy rain (rather than strong
winds).
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