the earth rotational excitations in a coherent geophysical fluids system jianli chen center for...
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The Earth Rotational Excitations in a Coherent The Earth Rotational Excitations in a Coherent
Geophysical Fluids SystemGeophysical Fluids System
Jianli Chen Jianli Chen
Center for Space Research, University of Texas at Austin, USACenter for Space Research, University of Texas at Austin, USA
http://www.csr.utexas.edu/personal/chenhttp://www.csr.utexas.edu/personal/chen
Email: [email protected]: [email protected]
2006 WPGM, July 24 - 27, Beijing, China G41A-05 Thu. 9:30 AM
I sincerely apologize for being absent due to an unexpected urgency, and am grateful to Richard for his kind help. - Jianli Chen
Atmospheric Contribution to Length-of-day (LOD)
Wind and pressure effects Topographic effects Upper wind effects
Oceanic and Hydrological Contributions
Ocean current and bottom pressure Terrestrial water storage change
Global Mass Balance
Mass conservation of the ocean model Mass balance between land and ocean Mass balance between atmosphere and land/ocean
Objectives
Global Mass Balance
Atmosphere
LandOcean
Precipitation
Preci
pita
tion
Evaporation
Evapo
trans
pira
tion
Runoff
Snow/ice sheet melting
About LOD Excitations
Atmospheric Angular Momentum (AAM) Change
Dominant contributor, 90% of the observed LOD change; Upper winds (above 10 mb) appear important as well;
Oceanic Angular Momentum (OAM)
Hydrological Angular Momentum (HAM)
At period of a few years or shorter,
LOD = AAM + OAM + HAM
orOAM + HAM = LOD - AAM
Can we close the budget yet ?
About LOD Excitations (Cont.)
Unlike polar motion X and Y, LOD is particularly sensitive to zonal wind circulation in the atmosphere -
Errors in wind fields Upper winds (not included in typical atmospheric models)
LOD is also particularly sensitive to mass balance among the atmosphere, land, and ocean -
Mass conservation issue of individual models (e.g., OGCMs); Mass conservation of the entire earth system; How to coherently combine the atmosphere, ocean, and land
About LOD Excitations (Cont.)
LOD excitations can be computed as [Eubanks 1993],
€
χ 3 = χ 3mass + χ 3
motion
χ 3mass =
0.753Re4
Cm gΔP(θ , λ ) cos3 θdθdλ∫∫
χ 3motion =
0.998Re3
Cm Ω 0 gU(θ , λ ) cos2 θ∫∫∫ dpdθdλ
€
χ 3mass =
0.753MRe2
Cm⋅
2
3(C0,0 − 5C2,0 )
Based on the definition of spherical harmonics, the above mass-term excitation can be rewritten as [see details in Chen 2005 - JGR, 110, B08404 10.1029/2004JB003474],
€
C0,0 = ΔMM
Atmospheric Excitations
NCEP reanalysis atmospheric model
Winds of 17 layers from 1000 mb to 10 mb
Surface pressure
Daily intervals, Gaussian grids (~1.904 x 1.875 )
Jan. 1948 to the present
Topography effects are applied - integration from the real
surface to the top of the model (at 10 mb).
Data and Processing
Oceanic Excitations
ECCO data assimilating ocean general circulation model
Ocean current and bottom pressure
1993 - present , telescoping meridional grids, 46 layers
Ocean bottom pressure (OBP) at 12-hourly intervals
Currents at 10-day intervals
Hydrological Excitations
CPC land data assimilation system Terrestrial water storage change
Monthly, 1980 to present, 1 x 1 grids
Data and Processing (cont.)
Water Mass Balance
Step 1: Conserving the total mass of the ECCO model
Step 2: Balancing land and ocean - adding a uniform layer over the oceans that is equal to the total water storage change over land.
Step 3: Balancing atmosphere and land/ocean - adding a uniform layer over the land/ocean that is equal to the total mass change of the atmosphere.
Different strategy in Step 3 - uniform or Gaussian redistribution of total atmospheric mass change.
Data and Processing (cont.)
Results
Observed LOD Variations
AAM Contributions
OAM Contributions
HAM Contributions
Table 1. Amplitude and phase of annual and semiannual LOD changes from spacegeodetic observations, atmosphere (AAM), ocean (OAM), and continental water(HAM) during the period Jan. 1993 to Mar. 2004. The phase is defined as φ in
€
cos(2π(t−t0)+φ), where
t0
refers to h
0 on January 1.
LOD ChangeAnnual
Amp litude Phase (μs) (deg)
Sem iannual Am plitude Phase
(μs) (deg)O bserved (Obs) 353.8 29.6 258.8 -114.9AAM (up to 10 m b) 363.2 32.8 216.2 -110.9AAM (up to 0.3 m b) 343.6 33.6 243.7 -112.2O bs - AAM (up to 10 mb) 15.3 -100.4 47.5 -137.5O bs - AAM (up to 0.3 mb ) 18.3 -27.8 21.6 -156.9OA M 12.4 -159.9 3.5 -149.1OA M 1 (from land mass balance) 23.4 -130.8 3.3 -127.7HA M 17.2 66.7 6.4 71.6OA M +HAM 12.5 -247.1 4.4 -257.0OA M + OAM1 +H AM (land mass balanced) 21.1 -162.9 3.5 -208.3OA M 2+HA M 2 (from air mass balance) 20.1 20.6 5.3 -93.7OA M +HAM (Full mass balanced) 1.4 -228.7 5.0 -132.8
Main Conclusions
Global water mass balance plays an important role in estimating oceanic and hydrological excitations of LOD change.
When total atmospheric mass change is compensated by land and ocean, the combined seasonal oceanic and hydrological contribution is much smaller than before (when full mass balance is not enforced).
Remaining LOD variations unaccounted for by the atmosphere (i.e., LOD - AAM) are more likely caused by errors in atmospheric wind fields.
A full mass balance (or conservation) of the Earth system is mandatory in order to close the LOD budget.
Additional Notes
This research was supported by NASA's Solid Earth and Natural Hazards Program (under grants NNG04G060G, NNG04GP70G) .
Results presented here have been published in
Chen, J.L., Global Mass Balance and the Length-of-day Variation, J. Geophys. Res., 110, B08404 10.1029/2004JB003474, 2005.
Reprints are available at: http://www.csr.utexas.edu/personal/chen/publication.html
Or email request to: [email protected]
Thanks !Thanks !