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 atmosphere 10 o C / 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 o C / 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 (w s ) maximum for a given T and P –wet-bulb temperature (T w ) 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 (T v ) temperature dry air at pressure P would have so its density equals that of a moist parcel at T and P –dew point temperature (T d ) 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 (T e ) temperature air would have if all its latent heat were released

50 Skew-T diagram (cont.) Basic Definitions (cont.) –equivalent potential temperature ( 2 e ) 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 (T c ) 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

51 Skew-T diagram (cont.) Basic Definitions (cont.) –Positive area (or CAPE) Area between the sounding and the moist adiabat that intersects the CCL, above the CCL. Proportional to the amount of energy the parcel gains from the environment. –Negative area (or CIN) Area between the sounding and the dry adiabat that intersects the CCL, below the CCL. Proportional to the energy needed to move the parcel. –Equilibrium level (EL) Height where the temperature of a buoyant parcel again becomes equal to the environment. –Wet bulb zero Height above ground where the wet bulb first reaches zero degrees Celsius. This is the level where hail will begin to melt.

52 Skew-T diagram

53 Skew-T diagram Isobars

54 Skew-T diagram Isotherms

55 Skew-T diagram Dry adiabats

56 Skew-T diagram Moist adiabats

57 Skew-T diagram Saturation Mixing Ratio

58

59

60 LFC EL + LI CIN CAPE

61 LFC EL + LI CIN CAPE

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63

64

65

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67

68 Lab 4 Stability Analysis and Soundings

69 Fronts and Air masses Air mass types and sources Frontal types Frontal structure in 3-D

70 Fronts and Air masses (cont.) Air mass types –Four general categories: PPolar source (also A for Arctic source) TTropical source ccontinental (land regions) mmaritime (ocean regions) Source regions –Generally flat and uniform composition –Light surface winds –Places dominated by high pressure Arctic plains (ice/snow covered) Subtropical oceans Desert regions

71 Fronts and Air masses (cont.) Air masses of North America: (1) cP and cAContinental Polar and Continental Arctic Cold (very cold in winter) and dry, stable conditions. (2) mPMaritime Polar Cool, moist and somewhat unstable. Forms in polar regions, then moves over oceans. (3) mTMaritime Tropical Very warm and moist. Forms over the eastern Pacific and the Caribbean sea and Gulf of Mexico. (4) cTContinental Tropical Hot, dry, and unstable conditions. Forms over northern Mexico and the southwestern U.S. during the summer.

72 Air mass source regions

73 Fronts and Air masses (cont.) A front is a transition zone between air masses of different densities. Frontal types: –Cold front Zone where colder air is replacing warmer air –Warm front Zone where warmer air is replacing a retreating colder air mass. –Stationary front Zone that has little or no movement. –Occlusions 2 types: Warm and Cold Occur during mature phase of storm development

74 Cold front structure

75 Warm front structure

76 3-D Frontal Structure

77 Cold occlusion

78 Cold occlusion structure

79 Warm occlusion

80 Warm occlusion structure

81 Flow associated with developing Low

82 Lab 5 Stability Analysis and Soundings (cont.)