Symmetric instability Sources: • Texts: – Martin p. 224-228 – Holton p. 277-281 – Lackmann does not cover this topic • Meted modules: – an operational approach to slantwise convection : highly recommended (by Kent Johnson), 28 min • Try the case exercise (location: BC) – listen to in class: heavy banded snow (by J. Moore), 34 min • This is a survey of conveyor belts, trowal, and (in section 3) symmetric instability – CSI pitfalls: the use and misuse of CSI : more advanced (by David Schultz), 33 min • Required reading material • Real-time charts (PV and SI): – Canadian maps (chart description ) he’s dead Jim – SPC meso -analysis , click on winter weather
21
Embed
Symmetric instability Sources: Texts: – Martin p. 224-228 – Holton p. 277-281 – Lackmann does not cover this topic Meted modules: – an operational approach.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Symmetric instability
Sources:•Texts:
– Martin p. 224-228– Holton p. 277-281– Lackmann does not cover this topic
•Meted modules:– an operational approach to slantwise convection : highly recommended (by Kent Johnson), 28 min
• Try the case exercise (location: BC) – listen to in class: heavy banded snow (by J. Moore), 34 min
• This is a survey of conveyor belts, trowal, and (in section 3) symmetric instability – CSI pitfalls: the use and misuse of CSI : more advanced (by David Schultz), 33 min
• Required reading material
•Real-time charts (PV and SI):– Canadian maps (chart description) he’s dead Jim– SPC meso-analysis, click on winter weather
1. examples2. static and inertial instability3. SI as inertial instability on isentropic surfaces4. basic state energy release in an SI exchange5. a computational method to determine SI (PV)
Example 1
What causes this precip?
N. DakotaS. Dakota
Example 2
E. Nebraska
Jet
PSI?
M
rela
tive h
um
idit
y (
%)
PV min
Jet
0
0
rela
tive h
um
idit
y (
%)
pvor (thte,wnd)
Example 3 (Schumacher et al. 2015)
700 mb, 21 UTC on 16 Feb
Simulated composite radar reflectivity (dBZ, color shading), 700-hPa potential temperature (red contours every 2 K), 700-hPa frontogenesis [thick black contours in K (100 km)−1 h−1, 700-hPa PV (negative values shaded in gray where the environment is statically stable), 700-hPa wind barbs
21 UTC on 16 Feb
SW NE
potential temperature (thin solid contours every 4 K), potential vorticity (negative values shaded in PVU), frontogenesis [thick dashed lines every 5 K (100 km)−1 h−1 above 5], and simulated reflectivity (the 0- and 5-dBZ contours are shown in blue)
Example 3 (Schumacher et al. 2015)
Banded precipitation
• Single- and multiple-banded clouds and precipitation are common, esp. in frontal systems
• They are often aligned with the thickness contours (thermal wind) and occur where they are tightly packed.
• possible cause: symmetric instability with moisture (PSI/MSI/CSI)
• PSI and frontogenesis commonly co-exit– PSI requires EPV<0– frontogenetic circulation requires (geostrophic) PV>0 (ellipticity condition for Sawyer-Eliassen eqn)
• SI is often ‘blamed’ a posteriori, it is not prognosed well (b/o inadequate model-resolution)
Condition for moist or potential SI (MSI):q
e lines steeper than M linesor: dqe/dz < 0 along M linesor: dM/dx <0 along qe linesor: equivalent PV (EPV) < 0
But: MSI only occurs if the atmosphere is potentially and inertially stable
Example:
Rimoist Emanuel (1983)
zf
y
u
x
v
yz
u
xz
v
zf
y
u
x
v
yx
w
z
u
xz
v
y
w
kfP
eee
eee
ee
)ˆ(
PV (EPV)
note: the condition for conditional instability: dqe
*/dz <0
MSI: an intuitive explanation
M = fy-ug
70
60
4030
dM/dy>0
M = absolute zonal momentum
see also: Jim Moore’s meted module on frontogenetic circulations & stability)
Also important for the effective release of the instability: moisture Overlay RH
And frontogenesis … why?
Mapping PI, PSI, frontogenesis, and RH
900-700 mb
Dt
DF
p
EPVg
EPVg
Characteristics of bands due to the release of MSI
1. Two-dimensional, aligned nearly along the thermal wind.
2. Condition for MSI is met in the region of the bands.
3. This region should be close to saturation. MSI by itself is not a sufficient condition for banded precip. MSI is ubiquitous, as is upright PI. We need qe to be close to qe
*, or RH close to 100%.
4. Bands should move at the speed of the flow at the level of MSI, in the cross-band direction.
5. Spacing of bands is proportional to the depth of unstable layer/slope of moist isentropes.
6. Ascent should be nearly along the moist adiabats.
Bandedness vs MSI
• Byrd 1989: 27 events in OK-KS• 80% of banded cases had
EPV<0 and high RH
• Xu 1992: numerical study• Initial EPV anomaly small:• Single band develops
• grid spacings of at most 10 km are required to capture the most unstable MSI mode (Knight and Hobbs 1988, Persson and Warner 1993)
• mesoscale models, incl the current ETA (12 km), should be able to capture most MSI-induced circulations (as well as frontogenetical flow)
• Even high-resolution models tend to underpredict the rainfall variability, and also the integrated amount of rainfall
• The existence of SI alone is not sufficient to initiate convection (need moisture)
• SI is not a forcing mechanism for slantwise ascent over a front (frontogenesis is … SI leads to slantwise convection within the frontogenetic circulation)
• The terms slantwise convection and SI are not interchangeable
• Upright convection always prevails over slantwise convection