Forecasting Local Weather

Transcription

1 Forecasting Local Weather Sea/Land Breeze Temperature Dew Fog Frost Snow Thunderstorms Tropical Cyclones

2 Temperatures: Radiation Balance

3 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

4 Temperature Forecasting Techniques Maximum 850 ϴw Empirical relationship between 850 ϴw and maximum temperature Minimum hPa 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 Uses model output of temperatures, combined with regression techniques containing local information

5 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 in the air is heavily dependent upon temperature. Measure Water Vapour content in different ways: Relative Humidity Dew Point Temperature Wet Bulb Temperature Mixing Ratio

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7 NOT ALLOWED IN THE FREE ATMOSPHERE

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9 Td

10 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 The first case can occur if air passes over a body of water The second case can have many causes...

11 Dew 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.

12 Diurnal Variation of Temperature

13 What is Fog? Very low stratus cloud Visibility less than 1Km Visibility 1Km to 18Km Relative Humidity greater than 75% Fog Mist Relative Humidity less than 75% Haze Haze

14 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

15 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

16 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.

17 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.

18 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, slow moving 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.

19 Fog formation Very heavily influenced by the topography 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 Fogs typically formed by combination of processes Makes forecasting fog/dispersal more difficult

20 Radiation Fog Persistent anticyclone to the South West late autumn/winter or early spring can bring fog for many days. Radiation fog.

21 Sea Fog Airmass/sea fog moist SW airflow in a broad warm sector fog on S and W coast. Can persist for days. Sea fog if Td > Tsea

22 Post Cold Frontal Fog Ridge post cold front. Clearing skies. Radiation fog which then advects onto (populated) east coast. Short lived.

23 Sea Fog on the Norwegian Coast The most typical wintertime fog is advection fog when a S wind brings up warm, humid air over the Norwegian Sea and weak SW component towards land > fog in the central areas. The winter with great variations: during a mild winer with many open waters also radiation fog observed during calm, cold nights.

24 Valley Fog in Norway

25 Fog in Switzerland

26 North Italy / Northern Balkans

27 Frost Occurrence Occurs on radiation nights Clear skies+slack winds Anticyclone Ridge or Slack airflow Long night

28 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 fog reduces radiation cooling

29 Frost Definitions Air frost Air temperatures below zero >1m above ground. Ground frost Ground/object/tree temperatures below zero. Can occur without air frost Slight 2o < Td < 0o Sharp 5o < Td < 2o Severe 10o< Td < 5o Very Severe Td < 10o Hoar Frost deposits ice (through sublimation) onto surfaces.

30 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

31 Snow Snow Scenario: Warm front approaching from South, cold surface (Easterly) Widespread Snow Rare airflow Most likely in showers Cold front turns to snow More frequent on higher before clearing ground Showers in cold west to north airflow, or Easterly airflow

32 Snow Forecasting Percentage Probability of Snow 90% 70% 50% 30% 10% T Air hPa Isotherm hPa

33 Warm and Cold Clouds

34 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 Non inductive Charging (NIC) Theory

35 Lightning!

36 Forecasting Thunderstorms Stability Indices Boyden Index I=( hPa) T700hPa Thunder Probable if I > 4/95 Radcliff Index T=ϴw900 T500 Thunder Probable if T 29/30 Potential Instability P=ϴw500 ϴw850 Thunder possible if P 2 (summer) K Index K = (T850 T500) + Td850 (T700 Td700) Thunder possible for K 20

37 Tropical Cyclones

38 Tropical Cyclones 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.

39 Hurricane Isabel (2003)

40 Saffir Simpson Scale Wind Speed (km/h) TS 63 TD Pressure (hpa) <920 > Storm Surge (m) 5 >249

41 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).