# file1 by liwenting

VIEWS: 15 PAGES: 27

• pg 1
```									Meteorology 2603

Lecture 10
3 February 1999

1
Today

 Exam 1: Wednesday 10 February
 Review Session:
– Monday 8 February 1999
– 7:00 PM
– EC 1410
 Thermodynamic Diagram
 Stability

2
 In a moist adiabatic process, assume that
the relative humidity of the air is equal to
or greater than 100 %.
 Recall: The dry adiabatic lapse rate is
– -10oC km-1
– For every 1 km you lift a parcel, the
temperature of that parcel will fall 10oC.

Example: An unsaturated parcel with a temperature of
5oC is lifted 4.5 km. What is the new temperature of the
parcel?
5oC + (-10oC km-1)(4.5 km) = 5oC - 45oC = -40oC
3
 With a moist parcel, the RH100% so
we assume condensation.
 With condensation there is a release of
latent heat.
 This release of latent heat, warms the
parcel.
 The parcel does not cool with height as
fast as does the dry parcel.

4
   The moist (or wet) adiabatic lapse rate
is
– -6oC km-1
– Note: This is an average value!!!
 Since the concentration of water vapor
decreases with height, the release of
latent heat decreases with hight.
 The moist adiabatic lapse rate is not
constant with height!!! It is curved.

5
Thermodynamic Diagram
p Moist adiabats tell you what happens to the
temperature of moist air (RH=100%) as it
rises. If there is no condensation, the
process is reversible.

T
Any time you are going up a moist adiabat, you are making a cloud.
If you condense out all the moisture, you go down dry adiabat.

Problem:        (a) Moist air rising from the surface (T=12oC) will have a
temperature of _________ at 1 km. (b) If dry, the temperature will be? Why?
(a)      T = 12oC + (-6oC km-1) x (1 km) = 6oC
(b)       T = 12oC + (-10oC km-1) x (1 km) = 2oC
6
Stability
   Stable - Lift an unsaturated parcel and the
parcel cools dry adiabatically. In a stable
environment, the parcel will be cooler
than the environment so the parcel will
Parcel
Environment

13C         10C                     1 km

20C         20C
7
Stability
   Neutral - Lift an unsaturated parcel and
the parcel cools dry adiabatically. In a
neutral environment, the parcel will be the
same temperature as the environment so
the parcel will stay at the new level.
Parcel
Environment

10C          10C                    1 km

20C          20C
8
Stability
   Unstable - Lift an unsaturated parcel and
the parcel cools dry adiabatically. In an
unstable environment, the parcel will be
warmer than the environment so the parcel
will move away from the original level.
Parcel
Environment

8C         10C                   1 km

20C        20C
9
Thermodynamic Diagram
   Stability: To determine the stability you
must plot a sounding. A sounding is the
temperature at various heights as

p
The sounding is also
COLD             WARM                 called the environmental
lapse rate (ELR).
T

Note: We also plot dew point on the chart -- we’ll get to that later.   10
Thermodynamic Diagram
   Stability:
– We can evaluate the stability of an
atmospheric layer by comparing the
sounding (blue) to the dry (green) and

CONDITIONALLY
STABLE            UNSTABLE            UNSTABLE      11
Absolute Stability
   Special Kinds of Absolute Stability
– Lapse -- The sounding is absolutely stable.
The lapse rate is just the environmental
sounding.
– Isothermal -- The sounding is absolutely
stable. The temperature is constant with
height.
– Inversion -- The sounding is absolutely
stable. The temperature increases with
height.

12
Absolute Stability
Lapse    Isothermal

Pressure

Inversion
Lapse Rate

Lapse Rate                                          13
Factors to Increase Stability
   Warming Aloft
– Latent heat release
– Sensible heat transport into a region
   Cooling Below
– Latent heat “absorption”
– Sensible heat transfer out of a region
– Air mass moving over a cold surface

14
Factors to Increase Stability

   Compression of a Layer of Air
– Top of the layer will heat (by compression)
more quickly than the bottom of the layer.
– The lapse rate of the layer decreases.

15
Factors to Increase Instability
   Warming Below
– Latent heat release
– Sensible heat transport into a region
– Air mass moving over a warm surface
   Cooling Aloft
– Latent heat “absorption”
– Sensible heat transfer out of a region

16
Factors to Increase Instability

   Lifting of a Layer of Air
– Top of the layer will cool (by expansion)
more quickly than the bottom of the layer.
– The lapse rate of the layer increases.

17
Neutral Stability
   Displace a parcel and it remains where you
put it.
– It doesn’t fly away from it’s original position.
   Dry Neutral
– Environmental lapse rate equals the dry
– Can result from thoroughly mixing air.
   Moist Neutral
– Environmental lapse rate equals the dry
18
Convective Instability
   Lift layer with a dry top and a moist
bottom.
– The top layer cools dry adiabatically.
– The bottom layer cools moist adiabatically.
– Since the top layer cools faster than the
bottom layer, we see a net cooling aloft.
This tends to destabilize the atmosphere.

19
Thermodynamics

   Thermodynamic Diagram:
– Handy little diagram -- Has on it:
» pressure (P), altitude (Z)
» temperature (T)
   (10C/100 m)
   (~0.60C/100 m)
» saturation mixing ratio lines (for RH stuff)

20
Thermodynamics
 Thermodynamic      Diagram (cont.):
– Is used everyday in forecasting
offices
»clouds: type, time of formation, bases,
tops
»thunderstorms: hail, wind gusts,
»precipitation: type, intensity, amount
»turbulence and icing for aircraft
»temperatures: max, min
»fog, freezing rain, rain vs. snow, etc.   21
Norman                   Temperature
Sounding
Sounding

3 February   Dew Point
Sounding
1999

22
23
Thermodynamic Diagram

   Construction:

Altitude in Km or
1,000’s of feet

Pressure levels
in mb.

-400 C                 +300 C
Temperature                   How high is the
500 mb level?
24
Thermodynamic Diagram
   Saturation mixing ratio line (yellow):
p
The saturation mixing ratio line tells
how much H2Ov is in the parcel at
a particular P, T if RH = 100%.
T
What is ws at 1000 mb and -100 C?

What is the RH at 1000 mb when T=240 C and Td=130 C?

If T=200 C and RH = 70%, what is Td at 1000 mb?

25
Thermodynamic Diagram
p

The dry adiabats tell us what happens
to the temperature of a dry air parcel
as it rises or sinks. (RH<100%)
T

What is the temperature of a parcel at 1000 mb and T=200C if lifted to
900 mb? to 600 mb?

What will the temperature of a parcel at 600 mb and T= -200C be if it
sinks to 1000 mb?

26
Temperature of a parcel