Chi Ao

Estimating the uncertainty of using GPS radio occultation data for climate monitoring: Intercomparison of CHAMP refractivity climate records from 2002 to 2006 from different data centers

[1]xa0To examine the suitability of GPS radio occultation (RO) observations as a climate benchmark data set, this study aims at quantifying the structural uncertainty in GPS RO-derived vertical profiles of refractivity and measured refractivity trends obtained from atmospheric excess phase processing and inversion procedures. Five years (2002–2006) of monthly mean climatologies (MMC) of retrieved refractivity from the experiment aboard the German satellite CHAMP generated by four RO operational centers were compared. Results show that the absolute values of fractional refractivity anomalies among the centers are, in general, ≤0.2% from 8 to 25 km altitude. The median absolute deviations among the centers are less than 0.2% globally. Because the differences in fractional refractivity produced by the four centers are, in general, unchanging with time, the uncertainty of the trend for fractional refractivity anomalies among centers is ±0.04% per 5 years globally. The primary cause of the trend uncertainty is due to different quality control methods used by the four centers, which yield different sampling errors for different centers. We used the National Centers for Environmental Prediction reanalysis in the same period to estimate sampling errors. After removing the sampling errors, the uncertainty of the trend for fractional refractivity anomalies among centers is between −0.03 and 0.01% per 5 years. Thus 0.03% per 5 years can be considered an upper bound in the processing scheme–induced uncertainty for global refractivity trend monitoring. Systematic errors common to all centers are not discussed in this article but are generally believed to be small.

Reproducibility of GPS radio occultation data for climate monitoring: Profile-to-profile inter-comparison of CHAMP climate records 2002 to 2008 from six data centers

[1]xa0To examine the claim that Global Positioning System (GPS) radio occultation (RO) data are useful as a benchmark data set for climate monitoring, the structural uncertainties of retrieved profiles that result from different processing methods are quantified. Profile-to-profile comparisons of CHAMP (CHAllenging Minisatellite Payload) data from January 2002 to August 2008 retrieved by six RO processing centers are presented. Differences and standard deviations of the individual centers relative to the inter-center mean are used to quantify the structural uncertainty. Uncertainties accumulate in derived variables due to propagation through the RO retrieval chain. This is reflected in the inter-center differences, which are small for bending angle and refractivity increasing to dry temperature, dry pressure, and dry geopotential height. The mean differences of the time series in the 8xa0km to 30xa0km layer range from −0.08% to 0.12% for bending angle, −0.03% to 0.02% for refractivity, −0.27xa0K to 0.15xa0K for dry temperature, −0.04% to 0.04% for dry pressure, and −7.6xa0m to 6.8xa0m for dry geopotential height. The corresponding standard deviations are within 0.02%, 0.01%, 0.06xa0K, 0.02%, and 2.0xa0m, respectively. The mean trend differences from 8xa0km to 30xa0km for bending angle, refractivity, dry temperature, dry pressure, and dry geopotential height are within ±0.02%/5xa0yrs, ±0.02%/5xa0yrs, ±0.06xa0K/5xa0yrs, ±0.02%/5xa0yrs, and ±2.3xa0m/5xa0yrs, respectively. Although the RO-derived variables are not readily traceable to the international system of units, the high precision nature of the raw RO observables is preserved in the inversion chain.

Sensitivity of GPS occultation to the stratopause height

[1]xa0We scrutinize temperature profiles collected with radio occultation measurement for an imprint of the stratopause. In the retrieval step that integrates bending angle data to atmospheric refractivity, the falloff toward infinite altitude is constrained in a boundary condition with statistical optimization or extrapolation. We point to the coherence between temperature and density scale height as advocator for a fitting procedure that exploits data down to the stratopause. Data below have higher signal-to-noise ratio, but fitting them in costs additional parameters for the shifted sign in scale height gradient. On the basis of noise free simulation using a climatology covering all latitudes, seasons, and hours and on the basis of validation against data collected with weather balloons, laser imaging, and limb sounding, we find that adaptation to the fluctuating stratopause is crucial for the accuracy of the retrieved temperature. To facilitate investigation, the stratopause altitude was preset in the boundary condition according to climatological temperature. Biases between 10 and 50 hPa cancel out when the fitting interval is extended to 7.2 km above the stratopause, almost the average displacement of the maximum density scale height caused by nonlinearity in the temperature profile. This is a reminder that the stratopause should be read from bending angle, not temperature. Moreover, bias would be approaching zero at all levels if the a priori information for extrapolation had been lapse rate, not isothermal conditions. Keeping the model seed for temperature conversion to subsequent retrieval steps eliminates external information from the deconvolved refractivity. It will help argue for radio occultation as independent vehicle for climate monitoring.

Demonstration of Mars crosslink occultation measurements for future small spacecraft constellations

Future planetary atmospheric profiling via radio occultations (RO) benefits significantly from increased received signal-to-noise ratio as well as geometrical coverage of links between two or more spacecraft in orbit around a target planet. These can be small spacecraft, possibly dedicated to quality radio-metrics with precision clock reference. Motivated by long-term research investigating the optimum SNR for the science performance, the optimum reference clock stability, the number and combination of wavelengths driving the number of transmitters and antennas, the optimum number and orbital spacecraft configuration, and design modifications to existing radio communication systems that would allow these new science objectives on future missions, three crosslink occultation experiments have been acquired between the Mars Odyssey and Mars Reconnaissance Orbiter spacecraft to probe the Martian atmosphere. While crosslink occultations between Earth orbiting satellites have long been used to profile the Earths atmosphere, this represents the first demonstration of crosslink occultation measurements at another planet. These measurements leverage the proximity link telecommunication payloads on each orbiter, which were designed to provide relay communication and navigation services to Mars landers and rovers. Analysis of the observed Doppler shift on each crosslink measurement reveals a clear signature of the Martian atmosphere, primarily the ionosphere. Inversion of the observed Doppler data yields vertical profiles of the Martian refractivity and electron density. The electron density profiles show the presence of two layers with peak densities and peak heights that are consistent with empirical models.