Weather analysis

1
Weather analysis

• From the planetary scale to the mesoscale

2
Objectives of weather analysis

• Key objectives
• To understand the vertical structure of the
  troposphere in terms of atmospheric processes in
  the BL and aloft
• To learn to use gempak to display operational
  atmospheric measurements and NWP model diagnostic
  fields
• To gain familiarity with operational atmospheric
  measurements, and their limitations, mainly from
  remote sensing probes satellite, radar
• In the process of looking at soundings, weather
  charts, UL charts, etc, we
• gain a sense of the large range of scales in the
  atmosphere, and begin to see weather data as a
  dynamically consistent depiction of the
  atmosphere in 3-D and time, at scales ranging
  from the planetary to the mesoscale
• grasp the concept of advection of a scalar
  quantity, and estimate the sign and magnitude of
  geostrophic advection on charts contouring
  geopotential height and the scalar quantity
• understand the relationship between vertical
  shear of the geostrophic wind and temperature
  advection
• gain a qualitative understanding of the 3-D
  structure and evolution of baroclinic systems -
  frontal disturbances hw 3, the Oct 1996 frontal
  evolution and the main project, E Coast
  cyclogenesis case

3
(1) Scales of atmospheric motion

• (try this site)

Note two spectral peaks (a) A maximum at about
2000 km (b) A minimum at about 500 km

shifted x10 to right
inertial subrange
1
100
10
1000
Gage and Nastrom (1985)
wavelength km
4
Scales of weather

• Air motions at all scales from planetary-scale to
  microscale explain weather
• planetary scale low-frequency (10 days
  intraseasonal) e.g. blocking highs (ca 10,000 km)
  supports anomalous wx events
• Size controlled by planetary rotation f (plan
  vort adv gt rel vort adv)
• synoptic scale cyclonic storms and
  planetary-wave features baroclinic instability
  (ca 3000 km) deep stratiform clouds
• Size controlled by df/dy
• mesoscale tropical cyclones, organized
  convection (MCSs), frontal rainbands various
  instabilities synergies (100-500 km)
  stratiform convective clouds
• Size controlled by f, depth of troposphere,
  terrain size
• microscale individual thunderstorm cells static
  instability (1-5 km) convective clouds
• Size controlled by entrainment and perturbation
  pressures

5
energy
FAfree atmos. BLbound. layer L long waves WC
wave cyclones TCtropical cyclones cbcumulonimb
us cucumulus CATclear air turbulence From
Ludlam (prior to Gage/Nastrom)
6
1 year
waves vortices eddies
1 month
tropical wave hurricane squall
line cumulonimbus tornado waves/billows
long wave anticyclone/ cyclone jet
stream front KH waves/billows
1000 km
1 day
1 hour
lee waves (wake)
Cumulus cell
Boundary layer eddies
1 km
1 min
1 m
1 sec
Molecular dissipation
7
From Dutton
8
(2) advection
f
fDf
v
Advection of a scalar quantity f
l
Dn
Advection of a scalar quantity f by the
geostrophic wind
Z
since
f
where A is the area bounded by two height
contours and two scalar contours
fDf
ZDZ
vg
Dn
Dm
this area is not generally a parallelogram the
area is referred to as a solenoid
l
9
(2) advection examples
black lines are height contours arrows show
geostrophic wind direction

• thermal advection pattern in a cyclone/anticyclone
  couplet in the northern hemisphere
• the magnitude of the advection can be estimated
  from the area of the solenoids

10
thickness advection
dashed 1000-500 mb thickness solid SLP
note this is a very qualitative method since the
sea-level geostrophic wind rarely is
representative of the 1000-500 mb mean wind
11
(3) geostrophic wind shear and thermal advection
thickness lines
layer-mean temperature
thickness
12
vu
warm
vT
vl
cold
thickness lines
thickness lines
veering winds
backing winds
geostrophic thickness advection
? thermal advection is larger when the mean
geostrophic wind is larger, and when the wind
shear (VT) is larger
13
(3) geostrophic wind shear and thermal advection
veeringWAA (by the geostrophic
wind) backingCAA (opposite in the SH)
14
(4) Baroclinic systems

• Facts
• Water vapor concentration is highly variable in
  time space
• Precipitation occurrence is concentrated in time
  space
• We saw that increases in sfc T, and in low-level
  humidity, destabilizes the atmosphere
• Departures from static stability (ie buoyancy)
  cannot explain all updrafts, clouds, and precip
  why does it rain in winter ??
• There must be another key mechanism that produces
  updraft, cloud and precip
• baroclinic instability
• Derives its energy from the mean wind
  (large-scale)
• Results in wave disturbances aloft
• Produces surface cyclones and fronts

15
synoptic-scale atmospheric circulations ? leave
the details for ATSC 5160

• Baroclinic vs barotropic
• Example of barotropic circulation??
• Focus on frontal disturbances (baroclinic
  instability)
• Size
• Balance
• Geostrophic
• Quasi-geostrophic (UL divergence, vertical motion
  )
• Vertical structure
• Westward tilt of GP height, wind and temperature
  fields
• Quadrature phase difference between 1000-500 mb
• Tilt is consistent with thermal advection,
  thickness, thermal wind balance
• Life cycle
• Rapid growth
• Mature and decaying phases
• Mesoscale aspects
• Fronts, frontogenetic circulation
• Rainbands