Outline

1. Myneni Lecture 13: Lapse-Rates Feb-20-07 (1 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Further Reading: Chapter 06 of the text book Outline - adiabatic processes - dry and wet adiabatic lapse rates - ascent versus descent

2. Myneni Lecture 13: Lapse-Rates Feb-20-07 (2 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Previously, we discussed various quantities associated with gas phase of water in the atmosphere Specific humidity Saturation humidity Relative humidity Dew Point Important points Air can only hold a finite amount of water vapor; the amount depends on the temperature of the air Any change of phase requires or releases energy Now we want to talk about the liquid phase of water in the atmosphere -> this will introduce the subject of clouds and precipitation Introduction

3. Myneni Lecture 13: Lapse-Rates Feb-20-07 (3 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Clouds form when the capacity of the atmosphere to hold water vapor is exceeded Excess water must go into liquid Usually does not happen because H2O is added but because the air is cooled As the air cools, saturation humidity goes down so the relative humidity goes up Primary method of cooling the air? Lifting Adiabatic Processes • Adiabatic process: Process in which there is no external source or sink of energy • No solar heating • No longwave heating • There can be latent heating • Can allow latent heating because this represent internal energy within the atmosphere • Essentially, we are talking about process where the air parcel does not interact with the surrounding air

4. Myneni Lecture 13: Lapse-Rates Feb-20-07 (4 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Pressure-Temperature Relation • If we raise an air parcel, the pressure of the air around the parcel decreases • The parcel expands and the temperature decreases

5. Myneni Lecture 13: Lapse-Rates Feb-20-07 (5 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Adiabatic Lapse Rate • Adiabatic Lapse Rate: The temperature change of a hypothetical parcel of air as it moves up in the atmosphere • Dry adiabatic lapse rate - the temperature change of a parcel of unsaturated air • Dry adiabatic lapse rate represents the temperature change a parcel will experience if it does not interact with the air around it • It is equal to 10-degrees/km • Constant • This is for a hypothetical parcel (i.e. it is a given)

6. Myneni Lecture 13: Lapse-Rates Feb-20-07 (6 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Example 1 If T(ground) = 295K and we raise the parcel until T=280K, how high have we raised the parcel? Dry Adiabatic Lapse Rate-Examples • Example 2 • If T(2km) = 255K and we move the parcel until T=275K: • Have we moved the parcel up or down? • How far?

7. Myneni Lecture 13: Lapse-Rates Feb-20-07 (7 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Lifting Condensation Level • Level at which clouds will form • Clouds form when T(air) = T(Dew point) • By lifting the air, T(air) decreases • T(dew point) also changes, but not as much • Lapse rate for T(dew point) = 1.8K/1km; • Lapse rate for T(air) = 10K/1km • Therefore, at some point T(air)=T(dew point), i.e. the parcel is saturated • If we keep lifting after clouds form, air will continue to cool because of the adiabatic process • But now water vapor is also being converted to liquid • Releases latent heat of vaporization • This heats the parcel somewhat

8. Myneni Lecture 13: Lapse-Rates Feb-20-07 (8 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Moist Adiabatic Lapse Rate-1 • Moist adiabatic lapse rate: The temperature change of a parcel of saturated air as it rises • “Adiabatic” because part of the temperature change is due to the same processes as a dry parcel (i.e. temperature changes because the pressure around the parcel changes) • “Moist” because part of the temperature change is due to water going from vapor to liquid

9. Myneni Lecture 13: Lapse-Rates Feb-20-07 (9 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University The adiabatic process changes the temperature by 10K/1km Conversion of vapor to liquid releases heat and also leads to a temperature change The moist adiabatic lapse rate is not constant Depends upon surrounding temperature and pressure Usually approximated as 6K/1km Moist Adiabatic Lapse Rate-2 Moist Adiabatic Lapse Rate 2 km Warming due to Latent Heat Release Height 1 km Cooling due to Expansion (dry adiabatic lapse rate 280 290 300 Temperature

10. Myneni Lecture 13: Lapse-Rates Feb-20-07 (10 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Ascent vs Descent Movie • Let’s follow a parcel as it rises above the LCL then comes back down • Initially the parcel’s temperature changes according to the dry adiabatic lapse rate • Once it hits the LCL however, condensation releases latent heat that warms the parcel somewhat as it rises, hence it follows the moist adiabatic lapse rate and it’s temperature doesn’t change as much with height • As the parcel descends, however, there is no condensation so there is no gain or release of latent heat; hence the temperature change is purely due to pressure effects, i.e. it follows the dry adiabatic lapse rate as it warms • Note, when it reaches the bottom, it is warmer than when it started; this is due to warming as latent heat is released • Also, note that the specific humidity will be lower, as will the relative humidity, hence the parcel will be drier Lifting Condensation Level