NWS-COMET Hydrometeorology Course 9 –24 May 2000

1. NWS-COMETHydrometeorology Course9 –24 May 2000 Meteorology Primer Presented by: Pete Stamus Tues-Wed, 9-10 May 2000 Hydromet 00-3

2. Peter A. Stamus Research Associate - Senior Meteorologist CSU/Cooperative Institute for Research in the Atmosphere (CIRA) and Colorado Research Associates (CORA) 303-415-9701 x224 303-415-9702 (fax) stamus@co-ra.com

3. Purpose of the primer • Basic understanding of meteorological processes. • Starting point for the rest of Hydromet • To give you a semester-long Introduction to Meteorology course in 8 hours.

4. Atmosphere StructureFun facts • Standard atmosphere • Very long term average for mid-latitudes • Average surface pressure 1013 mb • Average surface temperature 59 oF • 1/2 of the mass of the atmosphere (500 mb) below 6 km (3.7 miles)

6. Atmosphere StructureFun facts • Lapse rate (decrease in temperature in the vertical) Troposphere: +15 oC (at sfc) to ~ -50 oC (at 10 km) -6.5 oC / km

8. Water vapor in the atmosphereThe most important parameter we attempt to measure and forecast. • Clouds • Precipitation • Energy Transfer

9. Evaporation and Condensation

10. Evaporation and Condensation • Evaporation • Fast molecules escape, slower remain cooling process • Condensation • Slower molecules collide, form droplets, droplets fall, faster molecules remain warming process

11. Evaporation and Condensation (cont.) • The Evaporation/Condensation process transfers heat energy to the atmosphere • Latent Heat of Condensation

13. Evaporation and Condensation (cont.)Fun facts • Wind enhances evaporation • Warm water evaporates faster than cool water • Air temperature affects evaporation rate • Cool air, slower molecules, condensation more likely, slows evaporation • Warm air can hold more water vapor before saturation than cold air

14. Saturation Vapor Pressure

15. Relative Humidity and Dew Point Parcel B Parcel A Pressure at 1000 mb T = 10 oC (50 oF) e = 12.3 mb es = 12.3 mb T = 20 oC (68 oF) e = 12.3 mb es = 23.7 mb RH = (e / es) x 100 = 100% RH = (e / es) x 100 = 52% Therefore: Td = 10 oC for Parcel B Dew point = Temperature to which air must be cooled at constant pressure to reach saturation. It is a measure of the air’s actual water vapor content. Relative Humidity is a measure of the degree of saturation of the air.

16. Energy Budget • Incoming solar • Emitted long-wave • Transfer with latitude • Long-term balance

19. Energy Transfer with latitude

20. Daily and Seasonal Energy Balance

21. Lab 1 Basic Surface Features/Moisture

22. Atmospheric Pressure • Pressure = total weight of air above • Air is compressible, so gravity concentrates most air molecules near the surface • Atm pressure decreases with height rising air cools, sinking air warms • Greatest pressure variation in vertical, butsmallerhorizontal variations produce winds and weather systems

23. Pressure and terrain

24. Pressure and volume

25. Pressure and volume (cont.)

26. Typical 500 mb map

27. Lab 2 3-D Atmospheric Structure

28. Wind • Differential heating of land/ocean leads to pressure differences in the atmosphere • Pressure differences are forces that lead to atmospheric motions

29. Wind (cont.) • Newton’s Laws of Motion • First Law: Objects at rest remain at rest and objects in motion remain in motion, provided no force acts on the object • Second Law: Force equals mass times the acceleration produced F = ma • To determine wind direction and speed, need to know the forces that affect horizontal movement of the air

30. Wind (cont.) • Forces that lead to the wind • pressure gradient force (PGF) • Coriolis force (C) • centripetal force (c) • gravity (g) -- doesn’t effect horizontal motions • friction (F) Net Force = PGF + C + c + g + F • If these forces add to zero, then (1) The air remains at rest; or, (2) The air remains in motion along a straight path at a constant speed

31. Wind (cont.) • pressure gradient force (PGF) • Moves air from higher pressure to lower pressure • Coriolis force (C) • Apparent force due to the Earth’s rotation • Acts to turn wind to the right in the Northern Hemisphere • centripetal force (c) • Inward directed, keeps parcels rotating around pressure centers • gravity (g) • Always acts downward; vertical motions only • friction (F) • Acts opposite to the direction of motion; retards motion

35. Typical Flow

36. Idealized surface flow

37. Lab 3 Wind

38. Atmospheric Stability • Stable vs. Unstable • Dry and Moist Adiabatic Processes • Skew-T diagrams

39. Atmospheric Stability (cont.) • Stable vs. Unstable Stable equilibrium Unstable equilibrium

40. Atmospheric Stability (cont.) • Adiabatic Processes • Parcel of air expands and cools, or compresses and warms, with no interchange of heat with the surrounding environment • An adiabatic process is reversible • If the parcel doesn’t saturate, cooling or warming occurs at the dry adiabatic lapse rate • Constant in our atmosphere10 oC / km

41. Atmospheric Stability (cont.) • If the parcel does saturate… • Condensation (RH = 100%), Latent Heat released • Latent Heating offsets some of the cooling • Cooling at slower rate: moist adiabatic lapse rate • Not constant, varies with temperature and moisture Average value ~ 6 oC / km • Not reversible (heat added, moisture probably removed) • Pseudo-adiabatic process

42. Absolutely Stable

43. Absolutely Unstable

44. Conditionally Unstable

45. Growth of a thunderstorm

46. Orographic effects

47. Skew-T diagram • Convenient way to look at the vertical structure of the atmosphere • Determine unreported meteorological quantities • Parcel stability • Observations or model output

48. Skew-T diagram (cont.) • Basic Definitions • mixing ratio (w) • mass of vapor to mass of dry air • saturation mixing ratio (ws) • maximum for a given T and P • wet-bulb temperature (Tw) • equilibrium T when water evaporates from a wetted-bulb thermometer at a rate where latent heat lost is balanced by flow of heat from surrounding warmer air • potential temperature (2) • temperature of air if brought dry-adiabatically to 1000 mb • vapor pressure (e) • partial pressure of water vapor

49. Skew-T diagram (cont.) • Basic Definitions (cont.) • virtual temperature (Tv) • temperature dry air at pressure P would have so its density equals that of a moist parcel at T and P • dew point temperature (Td) • temperature of a parcel cooled to saturation at constant P • relative humidity • 100 x (mixing ratio / saturation mixing ratio) • specific humidity (q) • mass of vapor to mass of moist air (nearly the same as mixing ratio) • equivalent temperature (Te) • temperature air would have if all its latent heat were released

50. Skew-T diagram (cont.) • Basic Definitions (cont.) • equivalent potential temperature (2e) • temperature of a parcel if all moisture condensed out (latent heat released) then the parcel brought dry-adiabatically to 1000 mb • Convective condensation level (CCL) • Height where rising parcel just becomes saturated (condensation starts) • Convective temperature (Tc) • T that must be reached for a surface parcel to rise to CCL • Lifting condensation level (LCL) • Height where parcel becomes saturated by lifting dry-adiabatically • Level of free convection (LFC) • Height where parcel lifted dry-adiabatically until saturated, then moist-adiabaticaly, first becomes warmer than the surrounding air