University:
Glendale Community College
Course:
GLG 101 | Physical Geology
Academic year:

2022
Views:

193

Pages:

10
Author:

jazzily607hm

1 8. A TMOSPHERIC S TABILITY Reading Assignment: • A&B: Ch. 5 (p. 118-132) • LM: Lab# 8 1. Lapse Rates Lapse rate: - rate at which temperature decreases with height. - in [K / km] or [°C / km] - a positive value indicates decrease of T with height •Troposphere - general decrease in T with height •Three types of lapse rates: a) ELR - Environmental Lapse Rate b) DALR - Dry Adiabatic Lapse Rate c) SALR - Saturated (wet) Adiabatic Lapse Rate a) Environmental Lapse Rate •Actual air T we measure: i.e. Observed air T at any height •Varies in space and with time •Upper air T sounding 2 b) Dry Adiabatic Lapse Rate • Rate which a non-saturated air parcel cools as it rises • Rate: 2500 Height (m) 2000 1500 1000 500 0 0 5 10 15 Temperature (C) − 10 K − 1 K − 10 K = = 1000 m 100 m 1 km 20 25 3 Why does this decrease in T occur? Recall: •Adiabatic Process - a physical change of the state of the air parcel that does not involve exchange of energy with the air surrounding the air parcel. •Pressure and density decreases with height ⇒ As air parcel rises - expands & cools c) Saturated Adiabatic Lapse Rate •Air is saturated - condensation occurs ⇒ Release of latent heat of vaporization (LV) ⇒ Keeps the parcel warmer than it would otherwise ⇒ Decrease in T with height is not as great •Dependent on the amount of moisture in the atmosphere •More moisture in the atmosphere the greater will be the release of LV ⇒ warmer the air will remain •Range: 4 to More moisture 9 K km-1 Less moisture 4 1000 Height (m) 800 600 400 200 DALR SALR = 9 K/km SALR = 4 K/km 0 10 12 14 16 18 20 Temperature (C) • When air cools to the Tdew (saturation!), then convert from DALR to SALR • At base of clouds Tair = Tdew =Lifting condensation level 1200 Height (m) 1000 DALR SALR 800 600 400 200 0 8 10 12 14 16 Temperature (C) 18 20 5 2. Atmospheric Stability •Talked about air parcels moving up & down in the atmosphere but have not talked WHY Stability: Tendency of an air parcel to move vertically following an initial dislocation (up or down) Atmospheric stability is dependent on T and ρ •The thermal stratification of the air: vertical temperature profile, environmental lapse rate, ELR •To determine stability, vertical motions of air parcels are assumed to be adiabatic processes Principle: •Temperature differences between an air parcel and its surrounding lead to density differences and thus to buoyancy forces in upward (positive) or downward (negative) direction. è Air warmer than its surroundings will tend to rise (because of its lower density) è Air cooler than its surroundings will tend to sink (because of its greater density) è Air at the same temperature as its surroundings will tend to remain at the same height (because there is no density difference) 6 In Practice: •Can use the lapse rates to determine: o Which direction the air will move? o How far the air will move? Method •Compare the environmental air T (given by the ELR) with the DALR or SALR •Move along the dry (or saturated adiabat) •If T on the adiabat is less than ELR - STABLE 2500 Height (m) 2000 1500 1000 500 0 0 5 10 15 20 25 Temperature (C) •If T on the adiabat is greater than ELR - UNSTABLE 2500 Height (m) 2000 1500 1000 500 0 0 5 10 15 Temperature (C) 20 25 7 •If T on the adiabat is same than ELR - Neutral 2500 Height (m) 2000 1500 1000 500 0 0 5 10 15 20 25 Temperature (C) •Conditionally unstable or conditional instability: stable or unstable depends on whether the air is saturated or dry 2500 Height (m) 2000 1500 1000 500 0 0 5 10 15 Temperature (C) 20 25 8 ELR is not normally just a simple line (see Lab 8) • Air pollution will get trapped under an inversion o Inversion - T increases with height o Lapse - T decrease with height lapsestability.doc 2/27/01 9 If there are unstable conditions and the air rises •Cools as it rises •May cool down to the dew point T •Condensation - clouds Where air sinks - warms up •T moves away from Tdew - so no clouds will form •Clear air lapsestability.doc 2/27/01 10 Problem: An air parcel at the surface is not saturated and has a Temperature of 30°C and Tdew=25°C. Assuming it moves adiabatically and the SALR is -8.5 °C km-1 what will the air T be at 3000 m? 3500 3000 Height (m) 2500 2000 1500 1000 500 0 0 5 10 15 20 Temperature (C) 25 30 35