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: chen@csr.utexas.eduEmail: chen@csr.utexas.edu

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: chen@csr.utexas.edu

Thanks !Thanks !