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Transcript
```ATS/ESS 452: Synoptic Meteorology
Friday 08 January 2016
• Review Material
• Overview of Maps
• Equations of Motion
• Continuity
What is Synoptic Meteorology?
Definition: Synoptic meteorology is the study of the structure
and evolution of weather systems on spatial scales of order
1000+ km
Synoptic means “view together” or “view at a common point.”
In this course, you’ll learn to view different parameters of the
atmosphere together in order to examine/analyze the weather.
What do YOU expect to get out of this course?
BUT…this is not a weather forecasting course…instead you’ll
begin to learn how the weather “works.”
Quick Map Introduction & What’s
What type of map is this? What important information is shown?
Notice how the winds flow with respect to
the isoheights.
What is this?
What happened
here?
Heavy rain is forecasted for the TN Valley
Relate the location of the upper-level low and it’s associated
vorticity to our forecast.
Much more goes into making a local forecast, but what’s
happening at 500 mb can tell you a lot and is very important
on the synoptic scale!
Corresponding 60-hour 300mb forecast chart. What does this chart show?
The information on this chart (jet level dynamics) will later be important for the
ageostrophic wind equation. In general, lift/vertical motion is enhanced in the
right-rear and left-front quadrants of jets
Notice in this case, the upper-level low is vertically stacked. This isn’t always
the case, and in fact, is not good for a developing cyclone. Want to see a
westward tilt with height for a developing cyclone – more on this later
Later forecast… the previous shortwave that was affecting us is forecasted to move into
New England
Highly amplified trough pattern has developed over the mid-US
Check out the air pouring out of Canada  POLAR air mass
What do would you expect after the trough passes the TN Valley?
Now… notice how there is a westward tilt with height of the cyclone
Also, very strong cold air advection.
Scales of Motion
Scale
Approximate
Length Scale
Approximate Time
Scale
Planetary
> ~ 6000 km
> 1 week
Synoptic
Scale
1000 – 6000 km
1 day – 1 week
Mesoscale
1 – 1000 km
1 hour – 1 day
Microscale
< 1 km
< 1 hour
Types of Phenomena
Winds, Longwave
pattern
Shortwaves, Fronts,
Jet Streaks
Thunderstorms, Sea
Breezes
Turbulence, Boundary
Layer Phenomena
This information is important for simplifying the equations that describe our atmosphere through a
process known as scale analysis
**Determine which physical processes are most important and which can you
neglect.
Variables, Coordinates, & Units
• Cartesian coordinate system (x, y, z) is the most typical coordinate system used in
meteorology
- x: zonal (East-West) direction; positive towards the east
- y: meridional (North-South) direction; positive to the north
- z: vertical (up-dpwn) direction; positive upward
• What other coordinate system do you think we will use?
- Pressure coordinates
Quantity
Unit
SI units and notation
Length
Meter
M
Time
Second
S
Mass
Kilogram
Kg
Temperature
Kelvin
K
Velocity
Meter/Second
m s-1
Force
Newton
N (kg m s-2)
Pressure (force/area)
Pascal
Pa (N m-2 or kg m-1 s-2)
Energy or work
Joule
J (N m or kg m2 s-2)
Variables, Coordinates, & Units
Wind velocity are based on the time rate of change in the distance along its respective
coordinate axes following the airflow
zonal wind:
𝑢=
meridional wind:
𝑣=
∆𝑥
∆𝑡
∆𝑦
∆𝑡
∆𝑧
vertical wind:
𝑤 = ∆𝑡
**start to refer to directions using these names
The Coriolis parameter (f) is related to the spin of the Earth and is given by:
𝑓 = 2Ω𝑠𝑖𝑛𝜑
where φ is the latitude and Ω is the rate of Earth’s rotation (7.292 x 10-5 rad s-1)
Synoptic Scale Governing Equations
Frames of reference:
• Eulerian – object is fixed in space; it doesn’t move; changes due to air moving
through the object; Referred to as the local derivative in equations
• Lagrangian – object is allowed to move with the air; Referred to as the total
derivative in equations
• What is cold air advection (CAA)? What is warm air advection (WAA)?
• Cold air advection is the process in which the wind blows from a region of cold air to a
region of warmer air
• Warm air advection is the process in which the wind blows from a region of warm air
to a region of cooler air
CAA
WAA
Geostrophic Wind
**A scale analysis of the *horizontal* governing equations reveals that only the pressure
gradient force and Coriolis are important on the synoptic scale
The resulting balance between these terms is called geostrophic balance, which yields
the geostrophic wind
**At the synoptic scale, in the Northern hemisphere, the geostrophic wind blows parallel
to isobars (isoheights) with low pressure (low heights) to the left.
Geostrophic balance allows the governing equations to be simplified considerably and
allows the development of quasi-geostrophic theory (chapter 2)
Notice how wind in this 500mb map is flowing west to east, generally
paralleling the isoheights  geostrophic balance
Geostrophic balance does not always apply  surface
Vertical Momentum Equation
dw
1 Dp
=+ fucosj - g + FZ
dt
r Dz
Vertical
acceleration
Vertical
PGF
Coriolis
gravity
friction
Which terms are left if you perform a scale analysis on this equation?
All that remain are the vertical PGF and gravity.
The balance between these two terms is called hydrostatic balance.
**Synoptic scale motions are largely in hydrostatic balance in the vertical direction
If we are to assume hydrostatic balance, then are we assuming no vertical air
motion (i.e. no vertical acceleration)?
NO… they’re just very small compared to the vertical PGF and gravity
Continuity Equation
• This relation is based on the conservation of mass (in a closed system, mass will not
change).
• The kinematic method uses the continuity equation to estimate vertical motion
- Not very accurate
- Errors in actual wind observations will lead to very large errors in the vertical
motion estimate
• Convergence of winds at the surface leads to vertical motion
• What do you think convergence aloft leads to? Divergence of winds aloft?
(sinking air)
(rising air)
• Being able to correctly diagnose and forecast divergence and convergence
aloft is an important component of this course
Most 700mb charts show negative values of “omega”…. Realize the
differences between w and ω
```