Announcements

1
Announcements

• Writing Assignment 1 and Quiz 1 are available to
  pick up.
• Writing Assignment 2 is due next Tuesday (Oct
  4). Check webpage for detailed instructions.
• Homework 2 will be posted and handed out in
  class on Thursday and is due the following
  Thursday (Oct 6) at the beginning of class - no
  exceptions.
• Extra credit Homework 1 question is due with
  Homework 2 next Thursday. If you did not get a
  copy, pick one up from the TA or myself, or get
  it off website homework page.
• We still have several Homework 1 and Quiz 0
  papers that need to be picked up

2

Surface parcel in its environment stable case
Stable Atmosphere Example
Environmental lapse rate at 4C per 1000 m.
3

Surface parcel in its environment unstable case
Unstable Atmosphere Example
Environmental lapse rate now at 11C per 1000 m.
4

Surface parcel in its environment conditionally
unstable case
Conditionally unstable Atmosphere Example a two
step process
Environmental lapse rate now at 9C per 1000 m.
5

Surface parcel in its environment skew-T
Skew - T
lapse rate - how temperature varies with
height environmental lapse rate - variation of
temperature with height at a certain time and
place (T on graph) dry adiabatic lapse rate -
decrease in temperature of dry air upon ascent,
10C/1km (green lines on skew-T) moist adiabatic
lapse rate - decrease in temperature of saturated
air upon ascent, on average 6C/1km (cyan dashed
lines - the further left on the diagram, the less
water in the air, the closer the curve comes to
the dry adiabatic slope)
5AM
T
Td
dry
moist
6

Skew-T Review - now include moisture!
Reading the temperatures on a skew-T
Td
T
7

What happens to rising air parcels in Tucson?
Example 1 Tucson, 28 September 12 UTC (5 AM LT)
Parcel starts at 25C near surface with an 8C
dew point (see last slide).
5AM
T
Td
25C
8

Stability of the Tucson Environment
Is this a stable or unstable environment?
T
5AM
Td
T25C
This is consistent with a lack of clouds with
vertical extent hanging over Tucson yesterday.
Note that some mid-level clouds were observed,
consistent with the environments saturation
level (dew point equals air temperature) at
around 5600 m.
9

What happens to rising air parcels in Tropics?
Example 2 Tropics, 13 August 1997 03 UTC (6 PM
LT)
Parcel starts at 28C near surface with 16 g/kg
of water vapor.
Is this sounding stable or unstable?
28C
16 g/kg
10

Cloud Photo of Tropical Sounding
Cumulus congestus
13 August 1997 1800 LT
Cloud base at 750 mb
11

Cloud Formation Free Convection
Cloud Forms
As the saturated air rises above the LCL, water
vapor from the parcels condense onto CCN forming
the liquid water droplets that make up a cloud.
Thermal rising to form a cloud
Air Sinking
Air Sinking
Mature Column
Lifting Condensation Level
Starting column
If energetic enough, the thermals will reach the
lifting condensation level (LCL) for the surface
air - the level at which surface air saturates.
Above this level, parcels rise with moist
adiabatic lapse rate. Will rise above LCL freely
if a conditionally unstable environment exists.
Warm air near surface expands, becomes less dense
than its surroundings and rises with dry
adiabatic lapse rate (thermals)
warm air
Surface warming
Adopted from http//www.piercecollege.com/offices/
weather/stability.html
12

Cloud Formation Orographic Uplift
T-2C, Td-2C (stable)
Windward
Leeward
-4C
T8C, Td0C (unstable)
T4C, Td4C (neutral)
4C
Altitude (m)
T18C, Td2C (unstable)
T10C, Td10C (stable)
Rain shadow
12C
T20C, Td12C
Fig. 5.12
Lifted air is warmer and drier when it returns to
the surface. What happened to the water?
Water condensed and remained suspended in the air
as cloud droplets or fell out as rain.
13

Other ways to form a cloud?
Free Convection
Topography
Low-level Convergence
Fronts
Fig. 5.8