Some atmospheric physics definitions

1
Some atmospheric physics definitions
Zonal wind (west to east)? ve zonal winds are
westerlies
Seasons (looking down on plane of ecliptic from
above)?
Meridional wind (south to north)?
At (northern summer) solstice maximum
North-South bias in heating
S
Titan
N
Sun
E.g., typical northern summer surface wind map is
Axis of rotation also points out of page in both
cases
S
Westerlies
Latitude
Easterlies
N
At equinox no North-South bias in heating
Longitude
2
Planetocentric Solar Longitude (Ls) and its
relationship to seasons
Ls0 Northern spring equinox
Ls90 Northern summer solstice
Perihelion (Ls278)?
Shortest distance
Sun
Empty focus
x
Longest Sun-planet distance
Aphelion
90?
Ls270 Northern winter solstice
Ls180 Northern autumn equinox
3
Tropospheric streamfunctions from TitanWRF (mean
north-south circulation)
Solid contours gt ve streamfunction gt clockwise
circulation Dotted contours gt -ve streamfunction
gt anti-clockwise circulation
Northern spring equinox (Ls0)?
Northern summer solstice (Ls90)?
0 - 20 km
Northern fall equinox (Ls180)?
Northern winter solstice (Ls270)?
4
Coriolis effect and zonal winds
As air parcels move towards equator they move
away from the axis of rotation gt Air parcels
must slow down to conserve their angular
momentum gt Equatorial easterlies
As air parcels move away from the equator they
move towards the axis of rotation gt Air parcels
must speed up to conserve their angular
momentum gt Mid-latitude westerlies
Explains trade winds (easterlies) and jet streams
(westerlies) in Earths atmosphere
5
Connecting zonal wind to meridional circulation
(southern summer)?
(2) As air moves towards equator, Coriolis effect
gt zonal wind decreases (3) Zonal wind increases
as air moves away from the equator in the winter
hemisphere (5) Zonal wind decreases as air
moves towards equator (6) Zonal wind increases
as air moves away from equator into the summer
hemisphere
Hadley cell (overturning cell found in mean
meridional circulation)?
(2)?
(3)?
u lt 0
u ltlt 0
u gt 0
u gtgt 0
Height
(1)?
(4)?
w gt 0 u 0
w lt 0 u gt 0
(6)?
(5)?
u gt 0
u 0
N pole
Latitude
S pole
Eq
6
Example of how wind directions relate to
meridional circulation
Plot shows 3 wind components (not
streamfunction!) u black lines (ve is
solid, -ve dotted)? v colored lines (ve is
red, -ve is blue)? w shaded contours (ve is
pink/yellow, -ve is blue/green)? And location of
wind reversal (westerly to easterly, passing
through zero)
Ls 300 (just after northern winter solstice)?
(3)?
(2)?
Height
u ltlt 0
u gtgt 0
u gt 0
(1)?
(4)?
u lt 0
w gt 0 u 0
w lt 0 u gt 0
(5)?
(6)?
u gt 0
u 0
Latitude
7
What about tides?
Eccentric orbit around Saturn gt time-varying
gravity field (tides)
t1800 hrs
t0 hrs
1 orbit 1 Titan day (16 Earth days) as Titan
is tidally locked
t1200 hrs
t600 hrs
Tides primarily affect tropospheric winds
Tidal accelerations repeat once every orbit.
Start of orbit/day
Halfway through orbit/day
TitanWRF now includes tidal effect, but earlier
runs did not.
8
Simple demo of the effect of tides (this used
TitanWRF winds from a run without tides
rotation of wind vectors to mimic tidal effects)?

Rapidly time varying tidal accelerations (period
1 Titan day)?
time varying background zonal wind field

Each color has a local start time differing by
only two Titan hours. The difference in results
is huge. How much the balloon surfs the flow
depends on (1) the phase of the tide when the
path begins, and (2) the strength of the
background wind.
Paths of passive balloons started at 4km altitude
and at (0E, 45S)?
Latitude (degrees N)?
Longitude (degrees east)?
Would look more complex but similarly diverse
using winds from a run with tides
9
Winds from a Titan general circulation model
How can they be used to help trajectory planning
for a Titan aerobot mission?

• Current knowledge of Titan winds is very poor
• Mean winds have been inferred over most latitudes
  from temperatures, but only for z gt 100km and
  for some seasons
• Lower down, we only have one profile at one
  location and time (from Huygens)?
• gt
• Current Titan GCMs (general circulation models)
  cant be tested against observations (gt cant be
  validated / calibrated)?
• gt
• Winds produced by these models are highly
  unreliable

10
What are we pretty confident we know about the
circulation?

• Basic pattern of Hadley cells (zonal mean
  circulation in the meridional / vertical plane)
  in different seasons
• General pattern of zonal winds (easterlies and
  westerlies) associated with these cells
• General pattern of wind rotation due to tidal
  forcing

What dont we know about the circulation?

• Specific pattern and strength of zonal winds
  (easterlies and westerlies) associated with
  Hadley cells
• Details of wind due to combination of tides and
  other forces (Coriolis, pressure gradient)?
• Weather - i.e., transient effects such as large
  scale waves or small scale waves / turbulence

11
E.g., zonal wind (latitude vs height) for
different seasons
Northern fall equinox (Ls180)?
Northern summer solstice (Ls90)?
Northern winter (Ls300) time of Huygens entry

• Equinox easterlies only right at surface
• Northern summer easterlies up to 7km in summer
  low latitudes (westerlies below 1km)?
• Late northern winter easterlies up to 13km in
  summer low latitudes (westerlies below 2km)?

But the details of these results will change for
different TitanWRF simulations, and will change
even more if taken from other models
12
An extreme case shown for the Huygens season (Ls
300)?
Standard TitanWRF run
New TitanWRF run
Zonal mean temperature
Zonal mean temperature
Zonal mean zonal wind
Zonal mean zonal wind
The observed wind reversal during Huygens entry
(at 10 degrees south) was at 7km. We predict a
wind reversal (in the zonal mean flow) at 11km
in our standard run, but at 25km in our new
run which matches Titan far better in the
stratosphere (not shown).
13
TitanWRF near-surface winds (with tides) for 4
times of day at Ls 0
Front linked to planetary wave
Front linked to planetary wave
Moving feature due to tides (period of 1 Titan
day)?
Moving feature due to tides
Front linked to planetary wave
Front linked to planetary wave
Moving feature due to tides
Moving feature due to tides
14
TitanWRF near-surface winds (with tides) for 4
times of day at Ls 90
15
A major concern related to near-surface winds
Map of inferred dune directions (Lorenz,
Radebaugh and the Cassini radar team)?
Latitude (deg N)?
-60 -30 0 30
60
-
Longitude (deg W)?
Dunes stop abruptly at the west side of features
then curve around and restart to the east of
them, suggesting they formed in westerly winds

• Observations of equatorial dunes and other
  features suggest that they formed in westerly
  winds
• But all Titan models (and basic dynamics)
  predict easterlies at the equatorial surface
  almost all the time!
• Still trying to understand if this can be
  explained by e.g. time of day effects, but so far
  no success
• Seems to require a whole new dynamical mechanism

Cassini radar image
16
Key features of the general wind pattern

• For some latitudes and seasons there are no
  expected large-scale wind reversals westerlies
  easterlies from the surface to 20km
• The location of any predicted wind reversal
  changes with latitude for a given season, and
  with season for a given latitude
• Tidal forces produce wind rotation, or at least
  a periodic strengthening and weakening of the
  wind speed in the zonal / meridional direction
• Large scale (planetary) waves pass through
  unpredictably
• The circulation is complex (even at low
  resolution where small scale flows - turbulence,
  flow around topography, etc. - are ignored)?