NCAR Advanced Study Program (ASP) Seminar, February 13, 2008 1 Solar Semidiurnal Tide in the...

NCAR Advanced Study Program (ASP) Seminar, February 13, 2008 1

Solar Semidiurnal Tide in the Atmosphere

Jeff ForbesDepartment of Aerospace Engineering Sciences University of Colorado, Boulder, CO 80309-0429

• Forcing of the Semidiurnal Tide

• Vertical Propagation of the Semidiurnal Tide and its Interactions with the Overlying Atmosphere

Distortion by Zonal Mean Winds Modulation by Longitude Variations in Mean Winds (Stationary Planetary Waves) Modulation by Traveling Planetary Waves (e.g., 2-day

wave)

• Solar Semidiurnal Tide in Mars’ Dusty Atmosphere


ITM System

0 km

60 km

400 km

Pole Equator

Mass Transport

Wave Generatio

n

Planetary Waves

ConvectiveGenerationof GravityWaves & Tides

Turbulence

CO2

CH4

CO2 Cooling

Ion Outflow

Solar Heating

The ITM SystemThe ITM System HEscape

Wind Dynamo

BEEnergetic Particles

B

Polar/AuroralDynamics

E

MagnetosphericCoupling

Joule Heating

H2O

solar-driven tides

O3TopographicGenerationof GravityWaves


The semidiurnal tide is just one

example from a whole spectrum of

waves that couple different

atmospheric regions and produce

observable phenomena.


0

50

100

150H

eig

ht

(km

)

UV Absorptionby O3

12local time

0 24

hea

tin

g r

ate

0latitude

+90 -90

hea

tin

g r

ate

Near-IR Absorption by H2O, latent heating

EUV Absorptionby O, O2, N2

Thermal Excitation of the Semidiurnal Tide

mean

diurnal

semidiurnal


Q = Qo + an cosnΩtLTn=1

N

∑ +bnsinnΩtLT

=Qo + An cos(nΩtLTn=1

N

∑ −φ)

12local time

0 24

hea

tin

g r

ate,

Q

In the local time frame, the heating may be represented as

Ω =2π

24

Q

nΩt+nλ −φ = const

⇒ dλdt

=−nΩn

=Cph ⇒ Cph= -Ω

Implying a zonal phase speed

Converting to universal time tLT = t + λ/Ω , we have an expression of the form

An cos(nΩtn=1

N

∑ +nλ −φ)


Solar Heating Distribution from a Space-Based Perspective

Cph= -Ω : Migrating Solar Thermal Tides

To an observer in space, it looks like the heating bulge (and the tides it generates) are fixed with respect to the Sun, and the planet is rotating beneath.

To an observer on the ground, the heating bulge, and the tides it generates, are moving westward or “migrating” at the apparent motion of the Sun.


QuickTime™ and aGIF decompressor

are needed to see this picture.

Meridional wind field at 103 km (April) associated with the semidiurnal tide propagating upward from the lower atmosphere, mainly excited by UV

absorption by O3 in the stratosphere-mesosphere

The tide propagates westward with respect to the surface once per day, and is locally seen as the same semidiurnal tide at all longitudes.

Courtesy M. Hagan


QuickTime™ and a decompressor



0

50

100

150H

eig

ht

(km

)

QuickTime™ and aTIFF (Uncompressed) decompressor


Coupling into higher-order

shorter-wavelength modes





Semidiurnal variation in surface pressure 6oS

Propagation to surface

Distortion by zonal mean zonal winds

UV Absorptionby O3





TIMED/SABER Semidiurnal Tide at 100 km


Eastward Winds over Saskatoon, Canada, 65-100 km

Note thepredominance

of the semidiurnal tide at upper

levels, withdownward

phase progression.

Note the transition from easterlies (westerlies) below ~80-85 km to westerlies (easterlies) above during summer (winter), due to GW filtering and momentum deposition.

Tidal Variability

Courtesy of C. Meek and A. Manson


Longitude variations are taken into account with zonal wavenumbers s n.

A spectrum of tides thus exists, to first order representable as a linear superposition of waves of various frequencies (n) and zonal wavenumbers (s):

Cph= -nΩs

: Non-Migrating Solar Thermal Tides

s=−k

s=+k

∑ An cos(nΩt+ sλn=1

N

∑ −φ)

€

Cph =dλ

dt= −nΩ

s ∴ s > 0 ⇒ westward propagation

The waves with s ≠ n are referred to as non-migrating tides because they do not migrate with respect to the Sun to a planetary-fixed observer.




EQ116 km

50

100

150

Hei

gh

t (k

m)

Interaction with (modulation by) longitude variation in

background wind field(s = 1 only)

UV Absorptionby O3

s = 2(SW2)



Coupling into sum and difference

secondary waves

s = 1(SW1)

s = 1,3(SW1,3)

Interaction with (modulation by) longitude variation in

background wind field(s = 1 only)

cos(λ)cos(2Ωt+ 2λ)→ cos(2Ωt+ 3λ) + cos(2Ωt+ λ)

SW2migrating

semidiurnaltide

SW1“difference”

SW3“sum”



SW1 Observed over South Pole, 92 km 200

Cph =2Ω



EQ116 km

5-year mean Semidiurnal Amplitude Temperatures, TIMED/SABER

SW1 SW3

SW0SW4


Zonal Mean Winds due to Dissipation of Semidiurnal Tides

Angelats i Coll and Forbes, 2002

SW2 +SW1 + SW3

SW2 only

SW1 + SW3


The total atmospheric response to solar forcing is the result of constructive and destructive interference between migrating and

nonmigrating tidal components, giving rise to a different tidal response at each longitude.



TIMED/SABERSemidiurnal

Temperatures110 km

April 2004

Zhang et al., 2006


50

300

350

Hei

gh

t (k

m)

UV Absorptionby O3

Coupling into sum and difference

secondary waves

s = 59.6 h

s = -116 h

400

Interaction with (modulation by) Quasi-Two-Day Wave (QTDW)

with s = 3

s = 2(SW2)

Penetration into upper thermosphere &

ionosphereInteraction with (modulation by) Quasi-Two-Day Wave (QTDW)

with s = 3

cos(0.5Ωt+ 3λ)cos(2Ωt+ 2λ) → cos(2.5Ωt+ 5λ) + cos(1.5Ωt−λ)SW2

migratingsemidiurnal

tide

Eastwards = 1 16.0 h“difference”

Westwards = 5 9.6 h

“sum”

QTDW Modulation of Semidiurnal Meridional Wind AmplitudeIn the E-Region, EISCAT (Huskonen et al., 1995).

NCAR TIME-GCM: QTDW Modulation of Semidiurnal TideGenerates Sideband Waves that Propagate Above 100 km.

Palo, Roble & Hagan, Earth Planets Space, 51, 629-647, 1999

Do these waves beat with the semidiurnal tide generated in-situ in the thermosphere?

QTDW reflected in Critical Plasma Frequency

Are the above results due to modulation of the equatorial fountain by dynamo electric fields?

Is the dynamo driven directly by the 2-day wave, or by a 2-day modulated semidiurnal tide?


Solar Semidiurnal Tide in the DustyMars Atmosphere


The Model

• Time-dependent global model of the mutually-interactive semidiurnal tide and zonal mean circulation;

• Parameterization employed to handle convective instability -- eddy diffusivity introduced to keep wave amplitude at stable limit.

• Heating rates used based on observed dust distributions; validated against surface pressure perturbations measured by Viking-1and Viking-2 landers.


0

50

100

150

200H

eig

ht

(km

)

UV Absorptionby O3

s = 2(SW2)Solar radiation

absorption by dust

s = 2(SW2)

Solar Semidiurnal Tide in Mars’ Atmosphere, Ls = 270, High Dust ( ~ 2.3)

~60% density perturbationsat 122 km in aerobraking regime

~80 K

‘whole atmosphereresponse’

Solar Semidiurnal Tide in Earth’s Atmosphere, Ozone Heating

Solar Semidiurnal Tide in Mars’ Atmosphere, Dust Heating

To what degree does the solar semidiurnal tide contribute to the observed density

changes at aerobraking altitudes in connection with dust storms?


QuickTime™ and aBMP decompressor


Semidiurnal Temperature Perturbation


QuickTime™ and aBMP decompressor


Eddy diffusion Coefficient

due to Breaking Semidiurnal Tide


Zonal Mean Acceleration of the Atmosphere due to the Dissipating Semidurnal Tide





Zonal Mean Zonal Wind, Low Dust Zonal Mean Zonal Wind Difference, = 2.3


• The semidiurnal tide and its effects are pervasive and ubiquitous in Earth’s atmosphere

• There are new things to be learned, and probably, to be discovered

• The semidiurnal tide is just one example from a whole spectrum of waves that couple different atmospheric regions and produce observable phenomena.

• The solar semidiurnal tide is important in vertically-coupling Mars’ atmosphere, with potential importance to aerobraking.

CONCLUDING REMARKS

Solar Semidiurnal Tidein the Atmosphere

NCAR Advanced Study Program (ASP) Seminar, February 13, 2008 1 Solar Semidiurnal Tide in the...

Documents

Transcript of NCAR Advanced Study Program (ASP) Seminar, February 13, 2008 1 Solar Semidiurnal Tide in the...