Shailen Desai

Empirical ocean tide models estimated from TOPEX/POSEIDON altimetry

Three empirical ocean tide models are determined from repeat cycles 10 to 78 of the TOPEX/POSEIDON (T/P) altimeter mission. The three models investigate the effects of the satellite orbit ephemeris on the ocean tides determined from T/P altimetry and the effect of extracting the free core nutation resonance in the definition of the diurnal ocean tide admittance. The altimetric data series use the Joint Gravity Model JGM-2 geopotential orbit ephemeris and the preliminary JGM-3 orbit ephemeris computed at the University of Texas, Center for Space Research. The altimetric data from the T/P mission are shown to have the precision necessary to estimate the monthly and fortnightly ocean tides in each bin. Inclusion of existing models of the ocean tides in the polar latitudes not sampled by the altimeter demonstrates the importance of these latitudes on spherical harmonic representations of the ocean tides. The ocean tides are first estimated in bins of size 2.834° in longitude by 1° in latitude and then smoothed to 1° by 1° grids within ±66° latitude. The orthotide response formalism of Groves and Reynolds (1975) is used to represent the diurnal and semidiurnal ocean tides, while a constant admittance is assumed across narrow bandwidths around each of the monthly (Mm), fortnightly (Mf), and termensual (Mt) tidal components. Comparisons of the T/P ocean tide models to tide gauge observations indicate their accuracies to be of the order of 2–3 cm. The T/P-derived ocean tide models remove approximately 20 cm2 more of the T/P measured sea surface variance than the Cartwright and Ray (1991) tide model and show a 19 cm2 and 15 cm2 improvement over the Schwiderski (1980a, b) and Cartwright and Ray (1991) tide models, respectively, when compared to tide gauge estimates of the ocean tides.

Long‐period lunar fortnightly and monthly ocean tides

Long-period lunar fortnightly (Mf) and monthly (Mm) tides in the global oceans are of particular interest to geophysicists because of their impact on the Earths rotation, and they are of interest to oceanographers from the point of view of the response characteristics of the global oceans to low-frequency forcing. Long-period tides have long been quite controversial, but precision altimetry is providing an accurate means of measuring and modeling them in the global oceans. Here we describe Mf and Mm tides in the global oceans extracted from a 1° barotropic global hydrodynamic tidal model, which assimilates ocean tides estimated from cycles 10-130 of TOPEX altimetric data by an empirical tide model. The model results enable a better understanding of the energetics of these tides. Compared to the short-period ocean tides, which dissipate a total of roughly 3490 GW of lunisolar gravitational energy, the dissipation rates for Mf and Mm, 0.369 and 0.023 GW, respectively, are quite insignificant from the point of view of the dissipation of tidal energy in the global oceans. Their energy content is also low, 0.381 and 0.049 PJ, respectively (compared to 580 PJ in short-period tides). Also compared to the high quality factor Q values for short-period tides (∼23 for semidiurnal and ∼13 for diurnal), the Q values for these tides are 5.9 and 6.2 for Mf and Mm, respectively. The corresponding unassimilated model Q value is 5.8 for both. These Q values correspond to a decay timescale of the order of a period and hence indicate that unlike its response to short-period tidal forcing, the oceanic response to barotropic forcing at these low frequencies is like a heavily damped system. They are also consistent with the low values of Q (∼5.5) observed in the global oceans on timescales of several days. The relative potential energy to kinetic energy ratios for Mf and Mm tides (0.59 and 2.91 for assimilated and 0.83 and 3.20 for unassimilated, for Mf and Mm, respectively) are consistent with the dynamics of long-wavelength Rossby waves. At amplitudes of ∼0,115 ms in universal time, UT1, for both Mf and Mm, the impact of these long-period tides on the length of day fluctuations of the Earth is considerable.

Topex/Jason combined GPS/DORIS orbit determination in the tandem phase

Abstract In December 2001, the Jason-1 satellite was launched to extend the long-term success of the TOPEX/POSEIDON (T/P) oceanographic mission. The goals for the Jason-1 mission represent both a significant challenge and a rare opportunity for precise orbit determination (POD) analysts. Like its predecessor, Jason-1 carries three types of POD systems: a GPS receiver, a DORIS receiver and a laser retro-reflector. In view of the 1-cm goal for radial orbit accuracy, several major improvements have been made to the POD systems: 1) the GPS “BlackJack” TurboRogue Space Receiver (TRSR) tracks up to 12 GPS spacecraft using advanced codeless tracking techniques; 2) a newly developed DORIS receiver can track two ground beacons simultaneously with lower noise. In addition, the satellite itself features more straightforward attitude behavior, and a symmetric shape, simplifying the orbit determination models compared to T/P. On the other hand, the area-to-mass ratio for Jason-1 is larger, implying larger potential surface-force errors. This paper presents Jason-1 POD results obtained at JPL using the GIPSY-OASIS II (GOA) software package. Results from standard tests (orbit overlaps, laser control points) suggest that 1 to 2 cm radial orbit precision is already being achieved using the JPL reduced-dynamic estimation approach. We also report new DORIS POD strategies that make full profit of the additional number of common DORIS observations due to the T/P·Jason-1 tandem mode of orbit as well the additional dual-channel capability of the upgraded JASON receiver (allowing simultaneous tracking of two ground stations). New information on the satellites time scale is availed through this new estimation strategy. Results show that a significant improvement to DORIS-based orbits could be gained using this strategy. Building on these results, we have extended the GIPSY/OASIS 11 software capability to more fully exploit the combined benefit of both GPS and DORIS measurements from T/P and Jason-1 in their preliminary tandem mode. POD test results are used to demonstrate the accuracy of these orbits and to compare results in different cases: DORIS-alone, and GPS and DORIS together in both single- and multi-satellite modes. On the other, we have demonstrated and explained an anomalous behavior of the on-board oscillator when crossing the South Atlantic Anomaly region. Finally, plans for future software enhancements, processing strategies and modeling improvements are presented.

On the Long-Term Stability of Microwave Radiometers Using Noise Diodes for Calibration

Results are presented from the long-term monitoring and calibration of the National Aeronautics and Space Administration Jason Microwave Radiometer (JMR) on the Jason-1 ocean altimetry satellite and the ground-based Advanced Water Vapor Radiometers (AWVRs) developed for the Cassini Gravity Wave Experiment. Both radiometers retrieve the wet tropospheric path delay (PD) of the atmosphere and use internal noise diodes (NDs) for gain calibration. The JMR is the first radiometer to be flown in space that uses NDs for calibration. External calibration techniques are used to derive a time series of ND brightness for both instruments that is greater than four years. For the JMR, an optimal estimator is used to find the set of calibration coefficients that minimize the root-mean-square difference between the JMR brightness temperatures and the on-Earth hot and cold references. For the AWVR, continuous tip curves are used to derive the ND brightness. For the JMR and AWVR, both of which contain three redundant NDs per channel, it was observed that some NDs were very stable, whereas others experienced jumps and drifts in their effective brightness. Over the four-year time period, the ND stability ranged from 0.2% to 3% among the diodes for both instruments. The presented recalibration methodology demonstrates that long-term calibration stability can be achieved with frequent recalibration of the diodes using external calibration techniques. The JMR PD drift compared to ground truth over the four years since the launch was reduced from 3.9 to -0.01 mm/year with the recalibrated ND time series. The JMR brightness temperature calibration stability is estimated to be 0.25 K over ten days.

Barotropic tides in the global oceans from a nonlinear tidal model assimilating altimetric tides: 2. Altimetric and geophysical implications

In this second part, we explore the implications of the tides derived from the high-resolution, data-assimilative, nonlinear barotropic global ocean tidal model described by Kantha (this issue) in altimetric analysis and geophysical applications. It is shown that when applied to the task of removing tidal sea surface height from TOPEX altimetric records, the model performance is comparable to other global tidal models in the open ocean as measured by the reduction in crossover variances. The performance is slightly better than that of the only other high-resolution global tidal model from Grenoble (Le Provost et al., 1994). The results are however mixed in regions shallower than 1000 m and in semienclosed seas such as the Bering Sea, with the model performance slightly worse overall than the Grenoble model. Computations of total power input (and hence total tidal dissipation rate) are shown to be in excellent agreement with recent analyses of TOPEX data and geophysical observations. In addition, distributions of the tidal power input, tidal dissipation, and the power fluxes in the global oceans are shown for the two primary constituents, M2 and K1. Load tides in solid Earth due to ocean tidal loading fluctuations are also computed for the major semidiurnal and diurnal constituents. The load tides are shown to be large in the shallow seas adjacent to the coasts with high tides such as the Patagonian shelf, because of the higher resolution of this global tide model. This has potential implications in geophysical applications.

Microwave Radiometer Calibration on Decadal Time Scales Using On-Earth Brightness Temperature References: Application to the TOPEX Microwave Radiometer

Abstract A method is described to calibrate a satellite microwave radiometer operating near 18–37 GHz on decadal time scales for the purposes of climate studies. The method uses stable on-earth brightness temperature references over the full dynamic range of on-earth brightness temperatures to stabilize the radiometer calibration and is applied to the Ocean Topography Experiment (TOPEX) Microwave Radiometer (TMR). These references are a vicarious cold reference, which is a statistical lower bound on ocean surface brightness temperature, and heavily vegetated, pseudoblackbody regions in the Amazon rain forest. The sensitivity of the on-earth references to climate variability is assessed. No significant climate sensitivity is found in the cold reference, as it is not sensitive to a climate minimum (e.g., coldest sea surface temperature or driest atmosphere) but arises because of a minimum in the sea surface radio brightness that occurs in the middle of the climatic distribution of sea surface temperatures (...

Error analysis of empirical ocean tide models estimated from TOPEX/POSEIDON altimetry

An error budget is proposed for the TOPEX/POSEIDON (T/P) empirical ocean tide models estimated during the primary mission. The error budget evaluates the individual contribution of errors in each of the altimetric range corrections, orbit errors caused by errors in the background ocean tide potential, and errors caused by the general circulation of the oceans, to errors in the ocean tide models of the eight principal diurnal and semidiurnal tidal components, and the two principal long-period tidal components. The effect of continually updating the T/P empirical ocean tide models during the primary T/P mission is illustrated through tide gauge comparisons and then used to predict the impact of further updates during the extended mission. Both the tide gauge comparisons and the error analysis predict errors in the tide models for the eight principal diurnal and semidiurnal constituents to be of the order of 2–3 cm root-sum-square. The dominant source of errors in the T/P ocean tide models appears to be caused by the general circulation of the oceans observed by the T/P altimeter. Further updates of the T/P empirical ocean tide models during the extended mission should not provide significant improvements in the diurnal and semidiurnal ocean tide models but should provide significant improvements in the long-period ocean tide models, particularly in the monthly (Mm) tidal component.

Ocean water vapor and cloud burden trends derived from the topex microwave radiometer

An end-of-mission recalibration effort was recently completed for Topex Microwave Radiometer to generate climate data records of precipitable water vapor and cloud liquid water for 1992-2005. The TMR climate data is analysed for trends. The global trend in precipitable water vapor is found to be 0.9 plusmn 0.06 mm/decade. Regional precipitable water vapor trends are found to be highly correlated with regional sea surface temperature trends. The cloud liquid water trends are observed to be generally negative outside the tropics and positive in the tropics.

Stacking global GPS verticals and horizontals to solve for the fortnightly and monthly body tides: Implications for mantle anelasticity

The availability of long-term position measurements from permanent GPS stations distributed around the Earth makes it feasible to extract small, globally coherent, long-period geophysical signals from the data. Using 11u2009years of daily vertical and horizontal positions from over 600 permanent GPS stations worldwide, we solve for the amplitude and phase of the fortnightly and monthly body tides by stacking the surface displacements against spherical harmonics and isolating the fortnightly and monthly signals in the stacks. We use our solutions to help constrain the Earths anelastic properties, which are not well understood within the tidal frequency band. Our error estimates include the effects of the following: (1) random noise across the fortnightly and monthly tidal bands; (2) fortnightly and monthly ocean tide model errors; (3) errors in the diurnal and semidiurnal ocean tide models that alias into the fortnightly and monthly frequency bands; (4) errors in the solid Earth Greens functions used to compute ocean tidal loading corrections; and (5) leakage from GPS draconitic signals at periods close to the fortnightly and monthly frequencies. The mantle anelasticity coefficients we infer from our solutions are consistent with a ωα frequency dependence of mantle Q with α in the range 0.1–0.3.

JMR Noise Diode Stability and Recalibration Methodology after Three Years On-Orbit

Long term drifts and shifts in the Jason Microwave Radiometer (JMR) geophysical retrievals, relative to ground truth, have been attributed to long term changes in the JMR noise diode (ND) brightness, which is used for gain calibration. An optimal estimation based calibration system is developed to find an optimal set of calibration coefficients which minimize the root-mean-square (RMS) difference between the JMR brightness temperatures (TBs) and on-Earth references. This calibration system is used to derive a time series of the ND brightness. Changes in the ND brightness, on the order of 1-2%, are observed over the first three years of the mission. Results of the recalibration effort and validation of the retrieved ND time series are presented