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Estimation of the free core nutation (FCN) period is a challenging prospect. Mostly, two methods, one direct and one indirect, have been applied in the past to address the problem by analyzing the Earth orientation parameters observed by the very long baseline interferometry. The indirect method estimates the FCN period from resonance effects of the FCN on forced nutation terms, whereas the direct method estimates the FCN period using the Fourier Transform (FT) approach. However, the FCN period estimated by the direct FT technique suffers from the non-stationary characteristics of celestial pole offsets (CPO). In this study, the FCN period is estimated by another direct method, i.e., the sliding-window complex least-squares fit method (SCLF). The estimated values of the FCN period for the full set of 1984.0–2014.0 and four subsets (1984.0–2000.0, 2000.0–2014.0, 1984.0–1990.0, 1990.0–2014.0) range from −428.8 to −434.3 mean solar days. From the FT to the SCLF method, the estimate uncertainty of the FCN period falls from several tens of days to several days. Thus, the SCLF method may serve as an independent direct way to estimate the FCN period, complementing and validating the indirect resonance method that has been frequently used before.

[1] The meteorological excitation of Mars' rotation refers to the change of Mars' orientation parameters caused by meteorological processes, e.g., that portion due to variations of atmospheric circulation and surface pressure, and a cryospheric part due to the variable Martian polar ice caps associated with the CO2 sublimation/condensation effects. Global dust storms (GDSs) are unique meteorological events on Mars that cause changes in surface pressure, polar caps extent, and circulation patterns on a global scale, which in turn alter Mars' rotation under the conservation of Mars' total angular momentum. In this paper, the GDS signal in the meteorological excitation of Mars' rotation is investigated on seasonal, diurnal, and semidiurnal time scales, based on the Martian Climate Database. On seasonal scales, the cryospheric excitation plays a less important role in general, due to its location at high latitudes, than its atmospheric counterpart. The GDS brings (−3%, +15%) changes in the amplitude of annual polar motion and length-of-day (LOD) variation and (+22%, −34%) changes in the amplitude of semiannual polar motion and LOD variation. On diurnal and semidiurnal scales, the cryospheric excitation can be fully neglected in comparison to its atmospheric counterpart. A significant GDS signal is found with respect to the diurnal polar motion and semidiurnal LOD excitation variation, because of strong atmospheric tides arising from GDS effects. The amplitude increases in diurnal polar motion and semidiurnal LOD variation during some periods of the Martian year may reach 3–5 times as large as those during the same stage in a typical Martian year.

We use the latest time-variable Earth gravity field harmonics from the GRACE satellite mission (Release 5 – RL05) to determine seasonal and nonseasonal scales of polar motion excitation functions from global geophysical fluids, and particularly from the portion from land-based hydrology. Hydrological excitation functions of polar motion from the mass of equivalent water thicknesses (EWT) derived gravimetrically from the solutions of three GRACE processing centers, the Center for Space Research (CSR), the Jet Propulsion Laboratory (JPL) and the GeoforschungsZentrum (GFZ), are intercompared. Additionally, we estimate the hydrological signal as well in a different manner, as a residual from geodetically observed polar motion, by subtracting atmospheric (pressure + wind) and oceanic (bottom pressure + currents) contributions. Among the gravimetric excitation functions based on RL05 there are still significant differences between center results which we attribute mainly to residual signals over ocean areas. It appears that the CSR processing may lead to stronger agreement between the hydrological signal and residual values, surely due to details in the processing, like resolutions to different degree and order used in the static background gravity model.

To assess whether trends in the troposphere's circulation have occurred this century in concert with changes at the surface, we consider the low-frequency behavior in the global angular momentum of the atmosphere. Values of this index have been computed each month since 1870 from model experiments conducted by the UK Hadley Centre for Climate Prediction and Research, in which the model atmosphere is driven by prescribed boundary forcings, most especially sea surface temperature (SST) fields. The interannual variability of the model's atmospheric angular momentum compares favorably with that derived from the NCEP/NCAR reanalysis for the period of overlap (the second half of the twentieth century). Evidence is found for a notable increase in interannual variability since around 1970, but this recent high level of variability may not be significantly greater than that during 1900–1920. This latter result is strongly dependent on the character of the SST field used in an experiment, because low-frequency signals in atmospheric momentum are closely linked to those in the El Nino/Southern Oscillation. Large differences exist, however, among SST estimates in the tropical Pacific for the early part of the century.

SUMMARY Fluctuations in the Earth's rotation since 1830, as evidenced by changes in the length of the day, are derived from astronomical data having subannual resolution. Before 1955.5, timings of linar occultations are used; after 1955.5, the data are taken from the time series TAI-UT1. Although the data in the earliest period, 1830–90, display decade fluctuations in the length of the day, they are not accurate enough to reveal interannual variations. In this regard, also, the results from 1890–1925 are somewhat dubious. The quality of the data after 1925, though, is such that the temporal behaviour of the interannual fluctuations in the length of the day can be traced with confidence. We present plots of the interannual fluctuations in the period 1890–1987 and the longer-term decade fluctuations in the period 1830–1983. The interannual fluctuations in the length of the day since 1925 are compared with an index of the El Nino/Southern Oscillation (ENSO) phenomenon in the ocean-atmosphere system and are subjected to spectral analysis. The results support the conclusions reached by other authors that these fluctuations are linked to circulation changes in the atmosphere associated with ENSO, and in part to the quasi-biennial oscillation in the equatorial stratosphere's zonal winds. A spectral analysis of our 62 yr series of length of day values since 1925 reveals two significant peaks in the interannual range 2–4 yr. One is roughly biennial and the other is about twice this period, broadly supporting results obtained previously from shorter records. Our analysis of high-resolution data, therefore, contributes to ongoing efforts to establish a close relationship between the length of the day and aspects of the global climate system in the period before modern data became available in 1955.5.

Changes in the atmosphere's horizontal wind and surface pressure fields contribute strongly to variations in Earth rotation and polar motion on many time scales. The analysis-forecast systems of the world's major weather centers utilize a mix of space-based, in situ , and ground-based observations to produce wind and pressure fields from which Earth orientation excitation functions are calculated. At the same time, the torques that dynamically link Earth and atmosphere can be calculated at the meteorological centers. Elements of meteorological data assimilation are reviewed, as well as the current state of the atmospheric data base. New opportunities for wind measurements, in particular, and related surface stresses, used in calculating torques, are addressed.

[1] The atmospheric angular momentum is closely related to variations in the Earth rotation. The atmospheric excitation function (AEF), known also as the atmospheric effective angular momentum function, is introduced in studying the atmospheric excitation of the Earth's variable rotation. It may be separated into two portions, i.e., the “wind” terms due to the atmospheric motion relative to the mantle and the “pressure” terms due to the variations of atmospheric mass distribution evident through surface pressure changes. The AEF wind terms during the period of 1948–2004 are reprocessed from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP/NCAR) reanalysis 6-hourly wind and pressure fields. Some previous calculations were approximate, in that the wind terms were integrated from an isobaric lower boundary of 1000 hPa. To consider the surface topography effect, however, the AEF is computed by integration using the winds from the Earth's surface to 10 hPa, the top atmospheric model level, instead of from 1000 hPa. For these two cases, only a minor difference, equivalent to ∼0.004 ms in length-of-day variation, exists with respect to the axial wind term. However, considerable differences, equivalent to 5∼6 milliseconds of arc in polar motion, are found regarding equatorial wind terms. We further compare the total equatorial AEF (with and without the topography effect) with the polar motion excitation function (PMEF) during the period of 1980–2003. The equatorial AEF gets generally closer to the PMEF, and improved coherences are found between them when the topography effect is included.

The effectiveness of controlled atmospheric convection is analyzed in comparison to natural precipitation at five different locations on the globe. The aerological accelerator, a vertical tube of large height and diameter, presents a method for removal of water vapor from air by vertical convection. The question of whether a period of low precipitation is a period of favorable or unfavorable conditions for production of water by the aerological accelerator is examined. In addition, a prototype of the device is presented, namely, a deep mine shaft that produces water upon condensation in forced vertically rising air.

Analyses of the global fields of moisture transport have been decomposed into their transient eddy, standing eddy and mean cell components for each of two semester seasons during the IGY. Maps of the vertically integrated transient eddy fluxes in both the zonal and meridional directions are presented. Further consideration is given to the zonally averaged fields of moisture transport by the different modes. In the zonal direction, the influence of the mean cellular mode is dominant. In the meridional direction, the transient eddies play a significant role in the seasonal water balance of mid-latitudes, whereas the mean meridional circulation is the determining mode in the tropics. Standing eddy contributions are generally small. Differences in the water vapor fluxes associated with each mode from one season to the next are depicted and related to baroclinic effects and monsoonal behavior where they occur.

We have used lunar laser ranging (LLR) observations from the McDonald Observatory in conjunction with pole positions from the Bureau International de l’Heure (BIH) to determine the length of day (l.o.d.) at five-day intervals over the period 1971 through 1979. We compared the changes in these l.o.d. values to those inferred from determinations of the angular momentum of the global atmosphere, for the years 1976–1979, computed from twice-daily analyses of the zonal wind. These latter analyses were carried out by the National Meteorological Center, Camp Springs, MD. In both the observed (LLR) and the inferred (meteorological) values of changes in l.o.d., we have identified the near 50-day oscillation recently noted by Feissel and Gambis at the BIH. For the period of overlapping dates, the two oscillations agree closely in both amplitude and phase. The typical amplitude is about 0.2 milliseconds or approximately half the amplitude of the annual and semiannual variations in l.o.d.

This review critically surveys types and accuracies of available satellite data, satellite data retrieval methodologies, and data requirements of numerical weather prediction (NWP) models. NWP modeling, parameterization and data assimilation methods are reviewed with particular attention to the hydrological variables. It is concluded that the means exist to obtain improved retrievals of cloud and moisture parameters and to better parameterize the hydrological cycle. Further advances in the retrieval of humidity profiles, the parameterizations of clouds and the objective analysis of humidity are required.

We have used lunar laser ranging (LLR) observations from the McDonald Observatory in conjunction with pole positions from the Bureau International de l’Heure (BIH) to determine the length of day (l.o.d.) at five-day intervals over the period 1971 through 1979. We compared the changes in these l.o.d. values to those inferred from determinations of the angular momentum of the global atmosphere, for the years 1976-1979, computed from twice-daily analyses of the zonal wind. These latter analyses were carried out by the National Meteorological Center, Camp Springs, MD. In both the observed (LLR) and the inferred (meteorological) values of changes in l.o.d., we have identified the near 50-day oscillation recently noted by Feissel and Gambis at the BIH. For the period of overlapping dates, the two oscillations agree closely in both amplitude and phase. The typical amplitude is about 0.2 milliseconds or approximately half the amplitude of the annual and semiannual variations in l.o.d.