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Wind and Rain

Wind and Rain. MCC Synoptic Week 11-15 March 2013, UCD. Surface Weather Systems. Weather systems in the northern hemisphere generally move from west to east due to the earth’s rotation. Movement of tropical systems such as hurricanes are more variable.

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Wind and Rain

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  1. Wind and Rain MCC Synoptic Week 11-15 March 2013, UCD

  2. Surface Weather Systems • Weather systems in the northern hemisphere generally move from west to east due to the earth’s rotation. Movement of tropical systems such as hurricanes are more variable. • In the northern hemisphere, winds blow anti-clockwise around lows such as depressions, and clockwise around highs. • When the isobars (lines of equal pressure) become more closely spaced, then winds increase. That is, the closer the isobars over a particular area, the higher the wind speed.

  3. Geostrophic wind • Typically the wind speed at 2000 feet / 600m • Assume air parcel moves from rest • P is pressure gradient force • Co is Coriolis = 2 Ω SinΦ • Co acts at right angles Low P 1000 1004 Co 1008 High

  4. Balanced Geostrophic flow • Balance when P=Co, ie equal and opposite • Vg is the Geostrophic wind • Blows parallel to isobars in free atmosphere • Forecasters measure Vg from scale Low P 1000 Vg 1004 1008 Co High

  5. A typical weather chart This is the chart for Monday

  6. Cyclonic curved flow • Ce is centrifugal force due to circular motion • Ce reduces P • Co must reduce to maintain balance • Vg must reduce to Vgr which is the gradient wind • Forecasters make correction for curvature to get Vgr • Example eye of a storm Low P Vg Vgr Ce Co High

  7. Anticyclonic curved flow • Ce acts in unison with P • Co must increase to maintain balance • Vg must increase to Vgr • Forecaster makes correction for radius of curvature to get Vgr • Example periphery of a winter High Low P Ce Vg Co Vgr High

  8. Another complication ! • A difference between curvature of isobars and trajectories occurs when systems in motion • Strongest winds on south flank of eastwards moving depression • Strongest winds on north flank of westwards moving depression • Similar for mobile anticyclones L L

  9. Surface wind flow Low P  V Vg • Near ground friction(F) reduces wind speed • Co must reduce • Balance upset • Vectors realign so that P+Co=F • V-the real wind is reduced and blows towards low pressure  F Co  High

  10. Surface wind flow • Over the Sea V=2/3 Vgr, and is backed approximately 15 degrees to the isobars(depending on stability) • Over the Land V =1/2 Vgr and is backed as much as 40 degrees to the isobars(depending on roughness of ground and stability) L 15 0 H L 40 0 H

  11. Measuring Wind • A confusion of units! • Beaufort Forces • Knots (Nautical Miles per hour) • Miles per hour • Kilometres per hour • Metres per second

  12. Windy or Calm?Admiral Francis Beaufort • Born in Navan • Hydrographer to the Royal Navy • Devised one of the first wind scales in 1805, from Force 0 to Force 12 • http://www.mii.connect.ie/history/beaufort/beaufort.html

  13. Beaufort Scale on Land and Sea

  14. Thomas Romney Robinson • Born Dublin, 1792 • longtime director of the Armagh Astronomical Observatory • 4-cup anemometer design, 1846 http://star.arm.ac.uk/history/instruments/Robinson-cup-anemometer.html

  15. William Henry Dines • Born London, 1855 • Dines Pressure Tube Anemometer • Pressure difference between tube mouth and sides moves float in sealed chamber • Allows instrument to be remote to recorder. onlinelibrary.wiley.com/doi/10.1256/wea.38.05/pdf

  16. Fundamentals of Wind • Measured at 10m above the ground • (Always be aware that Malin Head is much higher. Treat wind readings from oil platforms, ships etc with caution). • Mean Speed – average over a ten-minute period • Gust Speed – highest instantaneous wind speed • Gusts normally do the damage!!

  17. Wind Speed and Gusts • Wind speed mentioned in marine observations, forecasts, and warnings is the average speed over a 10 minute interval. • Wind gusts may be up to 70% higher than the average wind speed. • For example, if the average wind speed is 25 knots, occasional gusts up to 40 knots can be expected, depending on stability of the air-mass.

  18. Surface wind • Speed: 1knot = 0. 514 m/s = 1. 15 mph • Direction: Direction fromwhich wind blows measured clockwise from true North • A veer is a clockwise change • A back is an anticlockwise change • Mean speed is average over 10 minute period • Gusts and lulls are rapid fluctuations due to obstacles and instability which are called turbulence Speed Time

  19. Pressure and drawing of Isobars • Plotted values are reduced to MSL • Isobars join areas of equal pressure • Back to wind, low pressure to the left (Buy’s Ballots Law) • On large Atlantic charts-4hPa intervals • On hourly charts –1 hPa intervals • A pascal =1 Pa = 1N/m2 • A hecto Pascal = 100 Pa = 10 N /m2 • 100Pa = 1mb = 1 hPa High X 1005 X 1002 X 997 Low X 999 X 998 x1002 X 1008 X 1008 X 1013

  20. The Sea Breeze • An onshore breeze which develops in coastal areas on a warm day. • Differential heating between the land and sea.

  21. Sea breeze formation Two columns of air At dawn:

  22. Sea breeze formation As land heats up a circulation develops

  23. 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.

  24. Landbreeze • Another thermally driven circulation. • Sea warmer than land at night. • Usually weaker than the sea breeze. • Very rarely exceeds 10 kt.

  25. 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

  26. Bureau of Meteorology

  27. Seabreeze front • Offshore wind opposes sea breeze • Enhanced convergence • Tightening temperature and humidity gradients

  28. Sea Breeze Summary • Nice cooling breeze on the coast. • Can bring in offshore stratus to spoil a sunny day right on the coast • Useful for yachtsmen and inshore fishermen • Enhanced convection can lead to some severe weather.

  29. A good sea breeze day

  30. Mountain Airflow • Modification of broadscale winds Deflection, channelling and shelter Effect on depressions and fronts • Lee waves • Locally induced winds Katabatic and anabatic winds Valley wind circulations • Downslope winds Föhn and Chinook winds Bora wind

  31. Deflection • Factors favouring deflectionover mountain barrier: Long barrier Perpendicular wind flow Concave barrierUnstable air • Factors favouring deflection around mountain barrier: Short barrierOblique wind flow Convex barrier Stable air

  32. Channelling Gaps in barrier strengthen wind flowe.g. Mistral (between Alps & Massif Centrale)

  33. Katabaticwind • Down-slope wind, usually nocturnal • Speed: a few knots • Depth: typically ~100 m • Best on even, gentle slopes Cooling

  34. Anabatic wind • Day-time up-slope wind • Speed: 5–10 knots • Depth: up to 200 m • Best on smooth, hot slopes Heating

  35. Föhn/ Chinook winds Condensation & release of latent heat Warm Cool

  36. Wind Flow over Mountains The Irish Meteorological Service www.met.ie

  37. Mountain Waves from Above The Irish Meteorological Service www.met.ie

  38. Bureau of Meteorology

  39. Bureau of Meteorology

  40. Bureau of Meteorology

  41. Lenticular Altocumulus The Irish Meteorological Service www.met.ie

  42. Fog, Rain, Drizzle and Showers • Fog, Drizzle and Rain distinguished by DROP SIZE • If droplets are suspended in the air (not falling) then we have FOG or MIST (drop size up to 0.2mm diameter) • Falling droplets from 0.2mm to 0.5mm are termed DRIZZLE • Drops of greater size constitute RAIN

  43. Rain and Drizzle Rates

  44. Rain and Showers • Rain • Primarily large geographical scale • Origin in dynamical processes • Showers • Small spatial scale (500m – 20Km) • Convective in origin • Much higher rates of rainfall • Can be embedded in larger scale rain bands

  45. Fronts Versus Showers Fronts -give widespread rain • Warm • Cold • Occlusion Showers - small Scale • 20km • last 10-20mins • Convective - • develop over warm sea in winter

  46. Forced Ascent • Air forced to rise • Stratus cloud forms on higher ground • Drizzle or rain likely

  47. Convection - creates instability Warm Cooler Cooler Warm air rising Warm air rising Air in contact with high ground is warmer than free air at the same height. Hot Hot

  48. Convection Showersand thunderstorms Warm Cooler Cooler Hot Hot

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