1 The Thermodynamic Diagram Adapted by K. Droegemeier for METR 1004 from Lectures Developed by Dr. Frank Gallagher III OU School of Meteorology

2 What is it? n The thermodynamic diagram, of which there exist many types, is a chart that allows meteorologists to easily assess, via quantitative graphical analysis, the stability and other properties of the atmosphere given a vertical profile of temperature and moisture (i.e., a sounding).

3 St  ve Diagram

4 to be used in this class

5 Skew-T Log-p Diagram

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8 What Can it Be Used to Estimate? n Cloud base and cloud top height n Expected intensity of updrafts, downdrafts, and outflow winds n Likelihood of hail n Storm and cloud type (supercell, multicell, squall line) n Storm motion n Likelihood of turbulence n Likelihood of storm updraft rotation n 3D location of clouds n Precipitation amount n High temperature n Destabilization via advection, subsidence n And many others….

9 The Stuve Diagram n Construction: C Altitude in Km or 1,000’s of feet Pressure levels in mb. How high is the 500 mb level? C Temperature

10 St  ve Diagram to be used in this class

11 Thermodynamic Diagram n Saturation mixing ratio line (yellow): T p It provides the saturation mixing ratio associated with the dry bulb temperature, or the mixing ratio associated with the dew point. The same line provides both

12 St  ve Diagram to be used in this class

13 Thermodynamic Diagram n Saturation mixing ratio line (yellow): It provides the saturation mixing ratio associated with the dry bulb temperature, or the mixing ratio associated with the dew point. The same line provides both What is w s at p=1000 mb and T=-10 0 C? What is the RH at 1000 mb when T=24 0 C and T d =13 0 C? If T=20 0 C and RH = 70%, what is T d at 1000 mb? T p

14 Thermodynamic Diagram n Dry adiabats (green): Unsaturated air that rises or sinks does so parallel to the dry adiabats. This line simply shows the rate of temperature decrease with height for an unsaturated parcel. T p

15 St  ve Diagram to be used in this class

16 Thermodynamic Diagram n Dry adiabats (green): What is the temperature of an unsaturated air parcel at 1000 mb and T=20 0 C if lifted to 900 mb? to 600 mb? What will be the temperature of an unsaturated air parcel at 600 mb and T= C if it sinks to 1000 mb? T p Unsaturated air that rises or sinks does so parallel to the dry adiabats. This line simply shows the rate of temperature decrease with height for an unsaturated parcel.

17 Temperature of a parcel at 1000 mb T parcel = 20  C

18 Temperature of a parcel at 1000 mb T parcel = 20  C

19 Temperature of a parcel at 1000 mb T parcel = 20  C Parcel is unsaturated, so if lifted to 600 mb, it follows parallel to a dry adiabat (green line) – note that the parcel goes parallel to the NEAREST green line.

20 Temperature of a parcel at 1000 mb T parcel = 20  C Temperature of a parcel lifted dry adiabatically to 600 mb. T parcel = -20  C

21 Thermodynamic Diagram n Moist (pseudo) adiabats (red): T p Saturated air that rises or sinks does so parallel to the moist adiabats. This line simply shows the rate of temperature decrease with height for a saturated parcel.

22 St  ve Diagram to be used in this class

23 Thermodynamic Diagram n Moist (pseudo) adiabats (red): Problem :(a) Moist air rising from the surface (T=12 o C) will have a temperature of _________ at 1 km. (b) If dry, the temperature will be? Why? (a)T = 12 o C + (-6 o C km -1 ) x (1 km) = 6 o C (b) T = 12 o C + (-10 o C km -1 ) x (1 km) = 2 o C T p Saturated air that rises or sinks does so parallel to the moist adiabats. This line simply shows the rate of temperature decrease with height for a saturated parcel.

24 Using the Thermodynamic Diagram to Assess Atmospheric Stability

25 The Thermodynamic Diagram n We’ll use two types of thermodynamic diagrams in this class. –The simpler of the two is the St  ve diagram, and we’ll use this to familiarize you with the use of such diagrams –The more popular (in the U.S.) and more useful is the Skew-T Log-p diagram, which we’ll apply later.

26 St  ve diagram Green Dry Adiabats Red Moist Adiabats Yellow Saturation Mixing Ratio

27 Thermodynamic Diagram n Stability: To determine the stability you must plot a sounding. A sounding is the temperature at various heights as measured by a balloon-borne radiosonde. The sounding is also called the environmental lapse rate (ELR). WARMCOLD T p Note: We also plot dew point on the chart -- we’ll get to that later.

28 Types of Stability Unsat Sat

29 Stability May Vary With Height Stable

30 Example: Dry Neutral Neutral to Dry Processes Unstable to Moist Processes ELR

31 Example: Moist Neutral Stable to Dry Processes Neutral to Moist Processes ELR

32 Example: Absolutely Unstable Unstable to Dry Processes Unstable to Moist Processes ELR

33 Example: Conditionally Unstable Stable to Dry Processes Unstable to Moist Processes ELR

34 Example: Absolutely Stable Stable to Dry Processes Stable to Moist Processes ELR

35 Norman Sounding 3 February 1999 Temperature Sounding Dew Point Sounding

36 Definitions n Lifting Condensation Level (LCL) –The level to which a parcel must be raised dry adiabatically, and at constant mixing ratio, in order to achieve saturation –It is determined by lifting the surface dew point upward along a mixing ratio line, and the surface temperature upward along a dry adiabat, until they intersect.

37 Example: LCL Surface Data T = 10 o C T d = 3 o C Mixing Ratio = 5 g kg -1 T TdTd LCL = 900 mb Data at LCL T LCL = 2 o C Mixing Ratio = 5 g kg -1 Notes: Dry adiabatic ascent from surface Constant mixing ratio RH increases as parcel ascends (T and Td approach one another; RH is 100% at LCL

38 Definitions n Lifting Condensation Level (LCL) –The LCL is CLOUD BASE HEIGHT for a parcel lifted mechanically, e.g., by a front. Remember, it is the LIFTED OR LIFTING condensation level.

39 Example: LCL Surface Data T = 10 o C T d = 3 o C Mixing Ratio = 5 g kg -1 T TdTd LCL = 900 mb Notes: Dry adiabatic ascent from surface Constant mixing ratio RH increases as parcel ascends (T and Td approach one another; RH is 100% at LCL

40 Definitions n Level of Free Convection (LFC) –The level to which a parcel must be lifted in order for its temperature to become equal to that of the environment. –It is found by lifting a parcel vertically until it becomes saturated, and then lifting it further until the temperature of the parcel crosses the ELR

41 Example: LFC Surface Data T = 10 o C T d = 3 o C Mixing Ratio = 5 g kg -1 T TdTd LCL = 900 mb LFC = 840 mb

42 Definitions n Level of Free Convection (LFC) –Any subsequent lifting will result in the parcel being warmer than the environment, i.e., instability. –This is what “free convection” means – the parcel will convect freely after reaching the LFC

43 Example: LFC Surface Data T = 10 o C T d = 3 o C Mixing Ratio = 5 g kg -1 T TdTd LCL = 900 mb LFC = 840 mb

44 Definitions n Equilibrium Level –A level higher than the LFC above which the temperature of a rising parcel becomes equal to that of the environment, i.,e. the parcel has zero buoyancy or is in equilibrium with the environment –It is found by lifting a parcel until its temperature becomes equal to the ELR

45 Example: LFC and EL Surface Data T = 10 o C T d = 3 o C Mixing Ratio = 5 g kg -1 T TdTd LCL = 900 mb LFC = 840 mb EL = 580 mb

46 Definitions n Equilibrium Level –Any subsequent lifting above the EL leads to stability –The EL marks the “top” of thunderstorms, though in reality the upward momentum of updraft air makes thunderstorms overshoot the EL (overshooting top)

47 Example: LFC and EL Surface Data T = 10 o C T d = 3 o C Mixing Ratio = 5 g kg -1 T TdTd LCL = 900 mb LFC = 840 mb EL = 580 mb

48 Definitions n Convective Condensation Level –The level at which convective clouds will form due to surface heating alone. –It is found by taking the surface dew point upward along a mixing ratio line until it intersects the ELR. n Convective Temperature (T c ) –The temperature required at the ground for convective clouds to form. –It is found by taking a parcel at the CCL downward along a dry adiabat to the surface.

49 Example: LCL, CCL, and T c Surface Data T = 10 o C T d = 3 o C Mixing Ratio = 5 g kg -1 T TdTd LCL = 900 mb CCL = 750 mb T c = 23 o C

50 Example: Positive and Negative Areas Surface Data T = 10 o C T d = 3 o C Mixing Ratio = 5 g kg -1 T TdTd LCL = 900 mb LFC = 800 mb EL = 510 mb Positive Area Negative Area Need to push parcel up!!!! Parcel warmer than environment!

51 CAPE n Convective Available Potential Energy –The “positive area” on a thermodynamic diagram, or the area between the MALR and ELR curves in the layer where the parcel is warmer than the environment, is proportional to the energy available in the atmosphere to do the work of lifting/accelerating a parcel vertically. –The theoretical maximum updraft of a thunderstorm is equal to the square root of 2xCAPE

52

53 How Can CAPE Increase?

54 How Can CAPE Increase? n Hotter surface temperature n More low-level moisture n Cool the mid-levels

55 Td T

56 W(surface) = 11 g/kg

57 W(surface) = 14 g/kg

58 W(surface) = 16 g/kg

59 What Changes with Height as a Parcel Rises? T TdTd Below LCL (cloud base) T Td w w s RH

60 What Changes with Height as a Parcel Rises? T TdTd Below LCL (cloud base) T decreases Td decreases w is constant w s decreases RH increases

61 What Changes with Height as a Parcel Rises? T TdTd At LCL T w RH LCL = 900 mb

62 What Changes with Height as a Parcel Rises? T TdTd At LCL T = Td w = w s RH = 100% LCL = 900 mb

63 What Changes with Height as a Parcel Rises? T TdTd Above LCL T Td w w s RH LCL = 900 mb

64 What Changes with Height as a Parcel Rises? T TdTd Above LCL T decreases Td decreases w decreases w s decreases RH = 100% LCL = 900 mb