Transcript Isentropic Analysis
Isentropic Analysis
Advanced Synoptic M. D. Eastin
Isentropic Analysis
Outline:
• Basic Idea • Construction of Isentropic Maps • Interpretation of Vertical Motion • Example Case • Can we neglect Diabatic Processes?
• Advantages / Disadvantages
Advanced Synoptic M. D. Eastin
Isentropic Analysis: Basic Idea
Limitations of QG Analysis:
•
Quantitative
results require inverting a Laplacian operator (not easy to do…) •
Qualitative
results require evaluation of vertical derivatives (noisy…) • QG forcing terms could offset each other (Q-vectors help…) • Several restrictive assumptions → small Rossby number (
R o
) → horizontally-uniform static stability (
σ
)
Isentropic Analysis:
Offers a
practical alternative
for diagnosing
vertical motion
• Results are generally consist with those obtained from QG analysis Can diagnose horizontal and vertical moisture transport Can visualize vertical motion near fronts • Conceptually simple and insightful Advanced Synoptic M. D. Eastin
Isentropic Analysis: Basic Idea
Underlying Assumption:
•
Adiabatic flow
→ air parcels flow along potential temperature (
θ
) surfaces → air parcels are “
thermodynamically constrained
” • This constraint is a major advantage over other coordinate systems since there is no dynamical constraint “holding” air parcels along isobaric or geopotential surfaces.
Advanced Synoptic M. D. Eastin
Isentropic Analysis: Constructing Maps
Overview:
• Can use observations or numerical model output • At each location (or for each sounding), find the pressure level that corresponds to the isentropic surface
chosen
for analysis (Example:
θ = 296-K
) Plotting
pressure values
on an isentropic surface provides “
system topography
” (much like plotting geopotential height on pressure surfaces) Plotting
winds
on an isentropic surface provides “
3-D flow
”
θ = 296-K Greensboro Sounding H L 705 mb H 296 K
Advanced Synoptic M. D. Eastin
Isentropic Analysis: Interpretation
Parallel Flow:
Any flow oriented
exactly parallel
to the isobars is horizontal motion (analogous to geostrophic flow on isobaric surfaces)
Non-Parallel Flow:
Any
cross-isobar flow
implies the presence of vertical motion Winds pointing toward lower pressure → ascent Winds pointing toward higher pressure → descent Greater crossing angles → stronger vertical motions
θ = 296-K H θ = 296-K H L L H
Advanced Synoptic
H
M. D. Eastin
Isentropic Analysis: Vertical Motion
Three Mechanisms:
• Using the definition of omega (
ω
) and evaluating horizontal derivatives on an isentropic surface, we find three mechanisms which can cause vertical motion:
p
t
V
c
p
p
t
Term A Term B Term C Term A
:
Local pressure tendency
• Accounts for local changes in the pressure surfaces at a fixed location • Often a small contribution to total
ω
• Can be eliminated by assuming steady-state (“frozen wave approximation”)
Term B
:
Pressure advection
• Analogous to temperature advection • Evaluated via the cross-isobar wind component (see previous slide
**
) • Often the
dominant term
in total
ω
• Can be evaluated with (or without) removing the system motion (
c
) Advanced Synoptic M. D. Eastin
Isentropic Analysis: Vertical Motion
Three Mechanisms:
• Using the definition of omega (
ω
) and evaluating horizontal derivatives on an isentropic surface, we find three mechanisms which can cause vertical motion:
p
t
V
c
p
p
t
Term A Term B Term C Term C
:
Diabatic forcing
• Heating / cooling due to condensation, evaporation, radiation, etc.
• Can make significant contributions to total
ω
, but often much smaller than Term B • Can also be neglected [more on this later…] Advanced Synoptic M. D. Eastin
Isentropic Analysis: Example Case
QG-Omega Interpretation: 500mb heights and vorticity 500mb heights and SLP Strong PVA Weaker PVA H Strong CAA Moderate WAA L
• Basic QG forcing terms
cancel
over TX and LA → no vertical motion? → Q-vectors… • Basic QG forcing clearly implies ascent across NC and SC Advanced Synoptic M. D. Eastin
Isentropic Analysis: Example Case
QG-Omega Interpretation: 500mb heights and 700mb ω 500mb Q-vectors / Convergence
• Q-vector forcing implies ascent across
both
TX/LA and NC/SC • Analyzed total vertical motion (ω) → Strong ascent over NC/SC → Weak ascent over TX/LA Advanced Synoptic M. D. Eastin
Isentropic Analysis: Example Case
Isentropic Interpretation: …with Mixing Ratio Pressure/ Winds on 296-K surface
• Isentropic forcing (via cross-isobar flow) implies strong ascent across NC / SC / GA / FL and only weak ascent across TX / LA • Accounting for “moisture supply” suggests the SE should experience heavy precipitation and the TX/ LA region should not Advanced Synoptic M. D. Eastin
Isentropic Analysis: Example Case
Isentropic Interpretation: …with Composite Radar Reflectivity Pressure / winds / mixing ratio 296-K surface
• Isentropic forcing (via cross-isobar flow) implies strong ascent across NC / SC / GA / FL • and only weak ascent across TX / LA • Accounting for “moisture supply” suggests the SE should experience heavy precipitation and the TX/ LA region should not Radar confirms the isentropic analysis!!!
Advanced Synoptic M. D. Eastin
Isentropic Analysis: Neglect Diabatic?
Can We Neglect Diabatic Processes?
Unsaturated parcels → conserve potential temperature (θ) → motion (upward) along isentropic (θ) surfaces Saturated parcels → conserve equivalent potential temperature (θ e ) → motion is still upward, but ascent is stronger Thus, neglecting diabatic processes only results in an
underestimation
but the qualitative results remains the same of isentropic lift Advanced Synoptic M. D. Eastin
Isentropic: Advantages / Disadvantages
Advantages:
Clear (visual) depiction of air parcel motion and three-dimensional airflow including vertical motion and moisture transport Conceptual simplicity Adiabatic assumption is valid most of the time → when it’s violated the qualitative answer remains unchanged and
ω is underestimated
QG assumptions of small
R o
and uniform
σ
are
not needed
Disadvantages:
Computations must be performed to interpolate pressure, wind, and moisture data onto isentropic surfaces Isentropic analysis
fails to provide an insightful dynamic interpretation
regarding cause and effect (as QG theory does…) Occasionally, potential temperature does
not
increase with height (complicating practical application) User must select the
appropriate isentropic surface
wisely • Relevant surfaces vary with season, latitude, and phenomenon • Surfaces that fall between 850mb and 700mb are used most often • Look at multiple isentropic surfaces!!!
Advanced Synoptic M. D. Eastin
Isentropic Analysis: Websites
Real-time Analyses:
WxCaster College of DuPage: University of Oklahoma http://www.wxcaster.com/isentropic.htm
http://weather.cod.edu/analysis/ http://hoot.metr.ou.edu/upperair/isen/ Advanced Synoptic M. D. Eastin
References
Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume I: Principles of Kinematics and Dynamics.
Oxford University Press, New York, 431 pp.
Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume II: Observations and Theory of Weather Systems. Oxford University Press, New York, 594 pp.
Byers, H., 1938: On the thermodynamic interpretation of isentropic charts.
Mon. Wea. Rev
.,
66
, 63-68.
Carlson, T. N., 1998:
Mid-latitude Weather Systems
, AMS, 343 pp.
Hoskins, B. J., 1991: Towards a PV-theta view of the general circulation.
Tellus
,
43
, 27-35.
Lackmann, G., 2011:
Mid-latitude Synoptic Meteorology – Dynamics, Analysis and Forecasting
, AMS, 343 pp.
Montgomery, R. B., 1937: A suggested method for representing gradient flow on isentropic surfaces.
Bull. Amer. Meteor. Soc
.,
18
, 210-212.
Moore, J. T., 1993: Isentropic analysis and interpretation: Operational application to synoptic and mesoscale forecast problems. NWS Training Center Manual, 99 pp.
Advanced Synoptic M. D. Eastin