Thermodynamics We Can See! Adapted from ATS 541 notes (Dr. Susan van den Heever)

Transcription

1 Thermodynamics We Can See! Adapted from ATS 541 notes (Dr. Susan van den Heever)

2 What we have learned so far ~ Moist Adiabatic Latent Heating ρ ρ Condensation ~ Dry Adiabatic

3 Cloud Types

4 Skew- T log- P

5 Skew- T log- P Techniques LCL - LiIing CondensaJon Level Level at which a liied parcel becomes saturated (an esjmate of the cloud base height). To find the LCL, follow a dry adiabat from the temperature of a parcel, and a saturajon mixing rajo line from the dew point of the parcel. The LCL occurs where these two lines intersect (Fig. 1). LFC - Level of Free ConvecJon Level at which a liied parcel begins a free accelerajon upward to the equilibrium level. The LFC is found where the parcel temperature becomes warmer than the environmental temperature (Fig. 1). For the LFC to be reached, sufficient forcing is required to overcome any CIN.

6 LFC LCL Figure 1: Skew-T Ln-P plot from Dodge City, KS at 0000 UTC on 15 September 2004

7 EL - Equilibrium Level In the absence of water loading and entrainment, the parcel freely ascends to the EL. Level at which a liied parcel becomes cooler than the environmental temperature and is no longer posijvely buoyant. The EL is the point above the LFC where the parcel path crosses the temperature trace SomeJmes referred to as level of neutral buoyancy (Fig. 2). CCL - ConvecJon CondensaJon Level The height to which a parcel of air if heated from below will rise adiabajcally unjl just saturated (height of the base of cumuliform clouds) - produced solely from surface heajng The level at which the saturajon mixing rajo through the dewpoint temperature of the parcel intersects the temperature curve of the sounding (Fig. 2)

8 CT - ConvecJon Temperature For forecasjng thunderstorms it is best to lii parcels from the forecast high temperature for the day to the CCL. The convecjve temperature is the temperature to which the surface must be heated to start the formajon of convecjve clouds through solar heajng It is indicajve of the temperature to which we need to raise the surface to inijate convecjon without dynamical forcing. The CT is found by following the dry adiabat through the CCL to the surface (Fig. 2). AT

9 EL LFC CCL CT Figure 2: Skew-T Ln-P plot from Dodge City, KS at 0000 UTC on 15 September 2004

10 CAPE and CIN ConvecJve Available PotenJal Energy (CAPE) The maximum energy available to an ascending parcel, according to parcel theory (Fig. 3) On a thermodynamic diagram this is called posijve area, and can be seen as the region between the pseudoadiabat and the environmental sounding, from the parcel's level of free convecjon to its level of neutral buoyancy (EL). CAPE is a measure of instability through the depth of the atmosphere and is related to updrai strength in thunderstorms. AT

11 CAPE is given by: EL CAPE =!R d " (T vpar! T venv )dln p (1) or CAPE = EL LFC g T (z)! T (z) v v " dz (2) T v (z) LFC where T v is the virtual temperature of a parcel lifted from the LFC and the overbar refers to the environmental values. AT

12 LFC CIN =!R d " (T vpar! T venv )dln p (3) SFC AT

13 EL CAPE LFC CIN Figure 3: Skew-T Ln-P plot from Dodge City, KS at 0000 UTC on 15 September 2004

14 Thermodynamic Stability

15 Semipermanent high-pressure systems Water world Real world

16 Subsidence Inversion Subsidence associated with high- pressure systems produces stable air aloi, which can trap pollutants near the surface Covers hundreds of thousands of square kms and can persist for days Los Angeles is at the eastern edge of the semipermanent North Pacific antcyclone frequently experiences pollutant trapping from subsidence Short-lived anticyclones can lead to episodic pollution trapping in affected areas

17 Side view Note how species might get mixed; mixing relatively slow between hemispheres

18 There is an isothermal layer just above the tropopause, which reduces the amount of stratification in the lower stratosphere Isentropes as a path of stratosphere troposphere exchange

19 adiabats = isentropes? Starting with the First Law: Now with and taking We find (Write ln θ and differentiate) Combining Or constant Lines of constant are also lines of constant entropy. Transformations in which S remains constant, and therefore is constant, are called isentropic transformation. ISENTROPES are lines of constant, and thus constant entropy.

20 Isentropic back- trajectories h^p:// Isentropic trajectories account for adiabajc verjcal mojons that air parcels may experience en route to their desjnajons. However, in the near- surface layer an air parcel cannot always be traced isentropically because the isentropic surface on which it is travelling may either intersect the ground or be ill- defined in an unstable boundary layer. HYSPLIT: h^p://ready.arl.noaa.gov/hysplit- bin/trajtype.pl?runtype=archive Trace motion back in 3D to estimate the likely source region(s) of pollution, etc. diabatic vs. adiabatic processes?

21 Courtesy Prof. Wayne Schubert

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23 Holton et al., Rev. Geophys., 1995

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