Atmospheric Angular Momentum Functions Research Articles

The Short-Term Prediction of Length of Day Using 1D Convolutional Neural Networks (1D CNN)

Accurate Earth orientation parameter (EOP) predictions are needed for many applications, e.g., for the tracking and navigation of interplanetary spacecraft missions. One of the most difficult parameters to forecast is the length of day (LOD), which represents the variation in the Earth's rotation rate since it is primarily affected by the torques associated with changes in atmospheric circulation. In this study, a new-generation time-series prediction algorithm is developed. The one-dimensional convolutional neural network (1D CNN), which is one of the deep learning methods, is introduced to model and predict the LOD using the IERS EOP 14 C04 and axial Z component of the atmospheric angular momentum (AAM), which was taken from the German Research Centre for Geosciences (GFZ) since it is strongly correlated with the LOD changes. The prediction procedure operates as follows: first, we detrend the LOD and Z-component series using the LS method, then, we obtain the residual series of each one to be used in the 1D CNN prediction algorithm. Finally, we analyze the results before and after introducing the AAM function. The results prove the potential of the proposed method as an optimal algorithm to successfully reconstruct and predict the LOD for up to 7 days.

Introducing atmospheric angular momentum into prediction of length of day change by generalized regression neural network model

The general regression neural network (GRNN) model was proposed to model and predict the length of day (LOD) change, which has very complicated time-varying characteristics. Meanwhile, considering that the axial atmospheric angular momentum (AAM) function is tightly correlated with the LOD changes, it was introduced into the GRNN prediction model to further improve the accuracy of prediction. Experiments with the observational data of LOD changes show that the prediction accuracy of the GRNN model is 6.1% higher than that of BP network, and after introducing AAM function, the improvement of prediction accuracy further increases to 14.7%. The results show that the GRNN with AAM function is an effective prediction method for LOD changes.

High-resolution atmospheric angular momentum functions related to Earth rotation parameters during CONT08

Due to the temporal resolution of available numerical weather analyses, the effect of the atmosphere on Earth rotation at daily and sub-daily periods is usually investigated using 6-hourly atmospheric angular momentum (AAM) functions. During the period of CONT08, however, atmospheric analysis data were provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) also on an hourly basis. In this paper, we, therefore, determine two sets of AAM functions from ECMWF data—one for CONT08 with hourly resolution and one for the year 2008 with 6-hourly resolution. The comparisons of the AAM functions to high-resolution Earth rotation parameters (ERP) from VLBI and GPS observations are carried out in the frequency domain. Special attention is paid to the preparation of the high-resolution data sets for the geodetic purposes, as there are jump discontinuities at 12 h intervals. Hence, the hourly AAM functions need to be concatenated. The revised functions yield much smaller amplitudes than their 6-hourly counterparts, as can be seen from the equatorial and the axial frequency spectra of atmospheric excitation in Earth rotation. This decrease of spectral power in the hourly AAM functions is found to be associated with a strong counteraction of pressure and wind terms, which originates from atmospheric circulation on short time scales. The results are compared to previous findings published by Brzezinski and Petrov (IERS Tech Note 28:53–60, 2000) based on the data from the U.S. National Centers for Environmental Prediction (NCEP).

Investigation of stronger diurnal ERP signals in summer derived from the VLBI CONT08 campaign

A series of very successful Continuous Very Long Baseline Interferometry Campaigns (CONT) have been carried out at irregular intervals since 1994. One of the goals is to support high-resolution earth rotation studies. The most recent CONT08 campaign was conducted with a network of 11 observatories at an increased recording rate of 512 Mbit per second over a fortnightly time span, from which stronger diurnal variations of the Earth Rotation Parameters (ERP) have been detected. We compared the high-frequency ERP from VLBI and GPS observations during CONT02, CONT05 and CONT08 with the atmospheric angular momentum functions derived from the National Centers for Environmental Prediction/National Center for Atmospheric (NCEP/NCAR) during these periods. We attribute the detected diurnal discrepancies between the theoretical models and the CONT08 observations to atmospheric excitations.

Wind field differences between three meteorological reanalysis data sets detected by evaluating atmospheric excitation of Earth rotation

We compared atmospheric excitations of the varying Earth rotation calculated from three different meteorological reanalysis data sets (NCEP/NCAR, NCEP‐DOE and ERA‐40) over 23 years, from 1979 to 2001. Atmospheric excitations are evaluated by atmospheric angular momentum (AAM) functions. We found that differences in AAM functions are primarily caused by differences in the wind data, especially the northward wind data, between the three reanalysis data sets. Differences between the NCEP‐DOE and ERA‐40 AAM functions are larger than those between the NCEP/NCAR and NCEP‐DOE ones. The northward wind differences that have a large effect on the equatorial AAM functions originate from the upper troposphere at the tropics and midlatitudes in the southern hemisphere, especially in three regions: east Pacific off Latin America, Africa and the Indian Ocean. The eastward wind differences are seen at midlatitudes in the southern hemisphere. These differences are not confined to a single frequency. The difference spectra show a reddish frequency dependence with some peaks in seasonal bands and tidal diurnal bands. Since the eastward and northward winds over the tropics can most efficiently change the Earth's spin rate and polar motion, respectively, the wind accuracy over the tropics directly determines the accuracy in evaluations of the atmospheric excitations.

Atmospheric, hydrological, and ocean current contributions to Earth's annual wobble and length‐of‐day signals based on output from a climate model

The atmospheric, hydrologic, and ocean current contributions to the annual wobble and length‐of‐day change are investigated, based upon 10 years of monthly values simulated by a coupled ocean‐atmosphere general circulation model (COAGCM). Also used are time series of geodetic excitation inferred from the observed wobble in the “SPACE97” data and, for reference, atmospheric angular momentum (AAM) functions from operational objective analysis data of Japan Meteorological Agency (JMA) for the period 1988–1997. The simulated annual variation in equatorial AAM function, which includes pressure and wind contributions, agrees well with that from the JMA operational analysis for 90°E component, but disagrees for 0° component due to differences in both pressure and wind contributions over ocean hemisphere. In the seasonal cycle the simulated hydrologic and ocean current excitation functions plus the AAM function from the JMA operational analysis tend to be in better agreement with the geodetic excitation function than those from the COAGCM output themselves. Because of the small magnitude of the simulated ocean current effects, the simulated hydrological cycle is found to be an important contributor to the nonatmospheric annual variations in polar motion. This confirms the importance of hydrology as a source of the annual wobble.

Atmospheric angular momentum fluctuations and their excitations of earth rotation variations at seasonal scale

We discuss the atmospheric contributions, especially wind contribution, to seasonal variations in the polar wobble and length of day during the period 1985–1995. The atmospheric angular momentum functions, calculated from the output data of GSM9603 GCM of Japan Meteorological Agency (JMA) are compared with those calculated from the reanalysis data of NCEP / NCAR, and from the operational objective analysis data of JMA. The results show that two different vertical wind integration methods lead to different wind contributions to seasonal polar wobble, especially for the component along Greenwich meridian direction in annual variation. However the inter-annual variations of wind contributions estimated by these two methods are almost the same with each other in the two equatorial components. The simulated axial relative AAM function agrees well with the other two as regards the seasonal variation. In addition, its inter-annual amplitude fluctuation of axial relative AAM function at semi-annual scale shows the main characteristics of ENSO evolution better than that estimated from the NCEP reanalysis data, that is, the semi-annual axial relative AAM oscillation becomes weaker during ENSO periods.

The investigation of atmospheric angular momentum as a contributor to polar wobble and length of day change with AMIP II GCM data

The atmospheric angular momentum (AAM) functions in terms of contribution to polar wobble and length of day change, are calculated from the output data of GSM9603 global circulation model (GCM) of Japan Meteorological Agency (JMA), from the reanalysis data of the National Centers for the Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR), and from the operational objective analysis data of JMA, respectively. The comparison shows that during the period from 1985 to 1995, the values of the pressure terms in the equatorial components of AAM functions calculated from three data sets agree with each other better along 90°E longitude than along Greenwich meridian direction. The axial component of relative AAM function estimated from GSM 9603 agrees well with those from the other two data sets in terms of seasonal variations with the moderate amplitudes, but not so well with the composite axial component of relative AAM functions estimated from 23 GCM models anticipating in the first phase of AMIP. In addition, its interannual variation from 1979 to 1996 shows the main characteristics of ENSO evolution, just as does the axial component of relative AAM function estimated from NCEP reanalysis data except for the period of anomalous ENSO from 1991 to 1993.

Atmospheric excitation of the Chandler wobble, 1983–1998

The excitation of the Chandler wobble (CW) by atmospheric wind and pressure variations is studied for the period 1983–1998. The data used here are the atmospheric angular momentum (AAM) functions computed from the operational objective analysis data of the Japan Meteorological Agency, compared with observed Earth wobble data. In the vicinity of the Chandler frequency (0.847 cycle per year), the AAM function has a broad spectral peak with somewhat excessive power and about 30° phase lead to that inferred from the observed wobble, with a coherence exceeding 95% confidence threshold. In addition, the temporal variation in the CW excitation calculated from the shifted 8‐year data of the AAM function is found to have nearly the same pattern as that inferred from observation, in which the wind and inverted barometer (IB) pressure contributions complement each other. They reveal that the atmospheric wind and IB pressure variations by themselves maintain a major part of the observed CW during the analysis period.

Wind contributions to the Earth's angular momentum budgets in seasonal variation

The atmospheric contributions to seasonal variations in length of day (LOD) and wobble are discussed for the period 1988–1997. The data sets used here are SPACE97 and EOP97C04 for LOD and wobble, respectively. Two atmospheric angular momentum (AAM) functions are calculated from the operational objective analysis data of the Japan Meteorological Agency (JMA) and the reanalysis data of the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR). Both the axial AAM functions agree well in annual variation and roughly in semiannual variation, with the function inferred from the observed LOD. The two equatorial AAM functions show the different wind contributions to the function inferred from the observed wobble, in both annual and semiannual variations, which are attributable in part to the different vertical wind integration methods between NCEP/NCAR and JMA. The equatorial tropospheric wind contributions, calculated by the same method, are found to show discrepancies arising from the different tropospheric regional winds associated with the Asian monsoon.

Dynamical analysis of the atmospheric angular momentum short-term fluctuations. Comparison with the Earth

The general behaviour of geophysical flows (atmosphere, oceans) is mainly nonlinear. Nevertheless, to date most of the studies concerning the atmospheric angular momentum functions and the Earth rotation time series have been devoted to their linear characterization and comparison, assuming their stochastic nature. In order to look for an alternative description of that time series, nonlinear time series analysis is applied to the effective atmospheric angular momentum short-term fluctuations (the pressure term of the equatorial functions and the wind term of the axial function) evaluated from the National Center for Environmental Prospect (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis Project, assuming a possible deterministic and nonlinear nature for these systems. The method consists of embedding the time series into a multidimensional space to recover the `attractor' of the system. It is shown that the dynamics of these processes are indeed nonlinear, deterministic and low-dimensional, thanks to their geometrical and topological description in the embedded space. The existence of a positive Lyapunov adds the property of chaoticity to the underlying system. It is found that the atmospheric processes observed through the angular momentum functions are chaotic and we are able to quantify the associated characterizations (D E=5, 4<D Lyap<5, variations of stability). We can also evaluate a theoretical horizon of prediction for the systems of about 6 days. The results suggest that the atmospheric angular momentum time series should no longer be described as purely stochastic processes. Additionally, the various nonlinear quantities evaluated are of the same order as those previously found for the short-term fluctuations of the Earth's rotation, indicating that the observed chaotic dynamics in the Earth's rotation is induced (through forcing processes) and belongs for a large part to the atmospheric dynamics, as expected.

気象庁モデルによってシミュレートされた1955年-1994年の3次元大気角運動量

Axial and equatorial atmospheric angular momentum (AAM) functions for the rotational dynamics of the Earth are calculated monthly from ensemble mean data of three independent 40-year simulations during 1955-1994 by the global model of the Japan Meteorological Agency (JMA) forced by observed near-global sea surface temperature (SST) conditions. The model results are compared with those from the reanalysis data of the National Centers for Environmental Prediction (NCEP) and the operational objective analysis data of JMA and with the functions inferred from the observed length of day (LOD) and polar motion. The annual term of the simulated axial wind AAM function (dimensionless relative angular momentum of atmosphere due to zonal wind) during 1984-1994 agrees well with those from the two analysis data sets and roughly with the inferred function from LOD, while the semi-annual term is considerably over-estimated, suggesting an incompleteness in the simulated subtropical zonal winds. The annual term of the simulated equatorial pressure AAM function (dimensionless atmospheric inertia products due to atmospheric mass redistribution) is considerably over-estimated with respect to those from the two analysis data sets, presumably due to the large simulated redistribution of atmospheric mass between the Eurasian continent and the North Pacific Ocean. For interannual Variations during 1955-1994, only the axial wind AAM function is reasonably simulated and shows good agreement with that from NCEP data as well as the Southern Oscillation Index. The above results lead to an understanding that the SST-forced AGCM simulates reasonably the atmospheric axial modes exciting LOD change but not the equatorial (non-axial) modes exciting the polar motion.

Quasi‐periodic wind signal as a possible excitation of Chandler wobble

The atmospheric contribution to the excitation of the Chandler wobble (CW) is studied for a period of about 11 years period beginning in September 1983, using the wobble data set (SPACE93). Two atmospheric angular momentum (AAM) functions are employed; one is based on Japan Meteorological Agency (JMA) data, and the other is based on U.S. National Meteorological Center (NMC) data. In the vicinity of the Chandler frequency, the excitation power for both AAM functions is comparable with that inferred from the observed wobble. If we separate the AAM excitation into wind and pressure effects, the wind contribution exceeds the pressure contribution around the Chandler frequency. In addition, the JMA wind AAM function reveals a spectral peak around the Chandler frequency. Significant coherence with the inferred excitation exceeding the 99% confidence level can be recognized for both JMA and NMC wind excitation. We propose that the wind contribution to the excitation of polar motion may be somewhat larger than at other frequencies because of the existence of quasi‐periodic atmospheric variations that have been found in other investigations. In a narrow‐band analysis of the excitation using least squares fit sinusoids, we found that both the amplitude and phase of the inferred excitation near the Chandler band indicate their strong dependence on the assumed Chandler period and that an assumed CW period of 431 days caused atmospheric and observed excitations to agree most closely.

Connection of the earth's rotation with the atmospheric angular momentum and the strongest earthquakes

Abstract Quick changes of the effective atmospheric angular momentum (AAM) functions are compared with the occurence of the strong earthquakes (with a body-wave magnitude m b ≥ 6.0) and quick changes of the Earth's rotation parameters (ERP). A strong correlation exists between these changes of the AAM functions and the times of earthquakes. The quick variations of the length-of-day (LOD) (up to 0.2 msec) are induced by the variations of the axial component of the AAM functions and strongly correlate with the times of earthquakes too. Delta-like changes of the equatorial components of the AAM functions correlate with the times of earthquakes and do not correlate with quick variations of polar motion. Common periodicities exist both in the AAM functions and earthquake series.