Dave’s nightmare. Scales of motion Whiteman (2000)
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Transcript of Dave’s nightmare. Scales of motion Whiteman (2000)
Solar Radiation At the Top of the Atmosphere
When sun directly overhead, amount of solar radiation received is:
Q = S (d/d)2
– Where d – mean distance from the sun
– d – distance from sun on particular day
up
Sun
Zenith Angle
When sun is at some other zenith angle, Z (angular distance of sun from local vertical), how much solar radiation is received?
As- Area perpendicular to Sun Ap- Area perpendicular to local vertical Qs As = Qp Ap ; As = Ap cos Z
Qp = S (d/d)2 cos Z Z=0 cos Z = 1; Z=45 cos Z= .71 Z=60 cos Z = .5; Z=90 cos Z= 0 As sun becomes lower on horizon, amount of solar
energy decreases per unit area
up
Z
Sun
As
Ap
Z
Zenith Angle
Zenith angle depends on:– latitude (north +, south -) – time of year. Solar declination angle, - angular distance of the sun
north (+) or south (-) of the equator• = 23.5o on June 21 Summer Solstice• = -23.5o on Dec 22 Winter Solstice• = 0o on Sept 23 (Autumnal Equinox) and March 20 (Vernal Equinox)
– Time of day. Hour angle, h, is 0 at solar noon, when sun is directly north or south of observation point
• h increases by 15 for every hour before or after solar noon• h = -90o at 0600 LT and h = 90o at 1800 LT
Zenith Angle
cos Z = sin sin + cos cos cos h Special cases:
– Solar noon: cos h = 1. Z = – On June 21, sun directly overhead at 23.5oN• On the same day at 40oN, Z = 40 – 23.5o = 17.5o
• At 90oN, Z = 67.5o
Zenith Angle cos Z = sin sin + cos cos cos h Special cases:
At sunrise or sunset, cos Z = 0, h = H = half-day length Cos H = - tan tan At the equator, tan so H = 90o which is 6 hours On the equinoxes, tan = 0 so H = 90o or 6 hours Latitude of polar night (H=0), 90o - |in winter
hemisphere
Azimuth Angle
Azimuth angle, a, angular distance of the sun from due south
At sunrise and sunset, cos a = - sin / cos At the equator, cos = 1, azimuth angle will be most nearly due
east. a = 90 + .On the equinoxes, the sun will rise/set due east/west. At the summer solstice, a = 113.5o
As the latitude increases, the denominator decreases, and the azimuth of sunrise/sunset departs more from due east/west
Maximum Temperatures: Monday, April 15, 2002
Tax Day Storm:
April 15, 2002
Tax-Day Storm (15 April 2002):
• Extensive damage ($4M+) from high winds > 35 m / s
• Record lowest SLP (982mb) at Salt Lake City (SLC)
• Ushered in an extended period of cold/wet weather
• 5-10 year event
• Max temperature change with cold front 16 C / hr
• Prefrontal blowing dust visibility < 1 km, closed roads,
• Rained mud, brownish/orange-colored snow
(J. Shafer)
Todd Foisy. April 15, 2002
Tax Day Storm:April 15, 2002
Bagley. Salt Lake Tribune
Maximum Sustained Wind Speed (mph)
April 14-15, 2002 WBB
2 km away
~1930 UTCJ. Shafer
Limited visibility due to blowing dust
Initial Development:
•Upper-level system moved over a preexisting temperature gradient
1200 UTC, 1500-m pressures = 2mb
1500 UTC
500 kmJ. Shafer
Rapid development during daytime…
1800 UTC 2100 UTC
J. Shafer
Low track and strength
L15z
L18z
L21z
L00z
Hours from 0000 UTC 15 April
J. Shafer
14mb/9hr
Strong cold front over N. UT
WNW, gusting to 48 knots
SSE, gusting to 83 knots
25 km
2130 UTC
Strongest Baroclinity ~20 C / 100 km
J. Shafer
Closer inspection of cyclogenesis/frontogenesis:
1500 UTC
700mb: Temp (2C) and winds
1800 UTC2100 UTC
Dry slot
1800 UTC2100 UTC
Precip mostly postfrontal – N. UT
J. Shafer
Relative humidity gradient with front
Adas RH 2200 UTC
fropa
• Front was stronger where rh gradient was stronger
J. Shafer
• Highest winds correlate well with daytime heating or deep boundary layer
9 PM
11AM
Max gust versus time of day
Characteristics of the damaging winds:
> 25 m/s> 30 m/s
Coverage
•Mainly prefrontal
J. Shafer
Basic frontal dynamics – frontogenesis/lysis
Consider a zonally oriented front with a meridional temperature gradient
Frontogenesis would occur in this case depending on the interaction of:
x
y TT+TT+2T
Differentialdiabatic
heating/cooling
Cross-frontalconfluence Tilting+ +
Basic frontal dynamics – frontogenesis/lysis
Differential heating/cooling: Horizontal gradients in heating/cooling strengthen or weaken the temperature gradient
x
y
T
T+T
T+2T
Basic frontal dynamics – frontogenesis/lysis
Differential heating/cooling: Horizontal gradients in heating/cooling strengthen or weaken the temperature gradient
x
y
T
T+T
T+2T
T+3T
Frontogenesis!
Basic frontal dynamics – frontogenesis/lysis
Cross-frontal confluence: Horizontal winds (deformation) act to increase the horizontal temperature gradient
x
y
T
T+T
T+2T
Basic frontal dynamics – frontogenesis/lysis
Cross-frontal confluence: Horizontal winds (deformation) act to increase the horizontal temperature gradient
x
y
T-T
T
T+T
T+2T
Frontogenesis!
Basic frontal dynamics – frontogenesis/lysis
Tilting: Vertical motion pattern acts to “tilt” a vertically oriented temperature gradient into the horizontal
y
z
T
T-T
T-2T
AdiabaticWarming
AdiabaticCooling
Basic frontal dynamics – frontogenesis/lysis
Adiabatic warming and cooling leads to horizontal temperature contrast
y
z
T
T- T
T-2T
Frontogenesis!
How does orography affect fronts?
Movement • Low-level flow blocking and channeling may retard or accelerate
a front, resulting in a distortion of its “shape”
Frontogenesis/frontolysis• Terrain-induced horizontal flow field may contribute to
frontogenesis or frontolysis • Terrain-induced vertical motion pattern (and associated
adiabatic warming and cooling) may contribute to frontogenesis or frontolysis
Vertical structure• Low-level blocking may act to decouple surface-based and
upper-level portions of front• In some cases, entire lower portion of a front may not be able to
surmount a mountain ridge or range, leaving only upper-level front
Orographic impacts on frontal movement
“The mountain-induced flow advects and distorts a front” (Egger and Hoinka 1992)
Examples– Pre-frontal downslope (Foehn) and low-level blocking of the
post-frontal wind can retard the progression of a front on the windward side of a mountain range
– A mountain-induced anticyclone can act to rotate a front anticyclonically upwind of a mountain range
– Terrain-channeled flow along a valley, plain, or gap can produce acceleration of a front
Early examples of frontal deformation/distortion
Bjerknes and Solberg (1922): Describe how mountains can retard a warm front, resulting in development of “warm-core seclusion” and secondary cyclogenesis
Nor
way
Nor
way
Nor
way
Seclusion
Early examples of frontal deformation/distortion
Bergeron (1928, available in Godske et al. 1957): Orographic distortion of a cold front by topography of Europe
• Frontal retardation windward of Alps• Frontal acceleration in Rhone Gap between Alps/Pyrenees
(Mistral), as well as east of Alps
Terrain channeling
Post-frontal flow becomes oriented along valley axis
Cold-air is transported rapidly up valleySteenburgh and Blazek (2001)
Smith (1986)
Terrain channeling
Terrain-parallel jet may develop in post-frontal environment Contributes to development of frontal nose
Steenburgh and Blazek (2001)
Orographic impacts on frontogenesis/frontolysis
Horizontal winds generally frontogenetical for whole case, but flow splitting around Olympics enhances frontogenesis as front initially approaches windward slopes
Frontolysis over mid and high elevation slopes where tilting generally contributes to frontolysis
Summary
Topography can distort the structure of a low-level cold front in several ways
• Fronts can be retarded by pre-frontal downslope and blocking of the post-frontal airmass windward of the topography
• Fronts may rotate anticyclonically due to development of mountain anticyclone
• Along-valley or gap winds may accelerate fronts through lowland regions
• Low-level and upper-level portions of a front may become decoupled
Mountain-induced horizontal winds and vertical motion can result in frontogenesis or frontolysis
Frontal analysis in complex terrain is difficult, but is being improved over western U.S. by MesoWest
Wave above convection streets
WMO (1993)Wind speed component normal to street axis should exhibit shear greaterthan 3 m/s per 1000 m both above and below the inversion. Then, streetsact line a line of mountain ranges and a wave develops above the street.
Monthly frequency of lee wave clouds in Europe
After Cruette (1973)
Wave climatology of W Europe under NWly flow
After Cruette (1973)
Numbers indicate frequency of observance; orientation and areal extension is as shown; from satellite picturesfrom 1966-1968.
Position of wave in lee of Sierra Nevadas
WMO (1993)