Evaluation of the 15-year ROM SAF monthly mean GPS radio occultation climate data record

We here present results from an evaluation of the Radio Occultation Meteorology Satellite Application Facility (ROM SAF) gridded monthly mean climate data record (CDR v1.0), based on Global Positioning System (GPS) radio occultation (RO) data from the CHAMP (CHAllenging Minisatellite Payload), GRACE (Gravity Recovery and Climate Experiment), COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate), and Metop satellite missions. Systematic differences between RO missions, as well as differences of RO data relative to ERA-Interim reanalysis data, are quantified. The methods used to generate gridded monthly mean data are described, and the correction of monthly mean RO climatologies for sampling errors, which is essential for combining data from RO missions with different sampling characteristics, is evaluated. We find good overall agreement between the ROM SAF gridded monthly mean CDR and the ERA-Interim reanalysis, particularly in the 8–30 km height interval. Here, the differences largely reflect time-varying biases in ERA-Interim, suggesting that the RO data record has a better long-term stability than ERA-Interim. Above 30–40 km altitude, the differences are larger, particularly for the pre-COSMIC era. In the 8–30 km altitude region, the observational data record exhibits a high degree of internal consistency between the RO satellite missions, allowing us to combine data into multi-mission records. For global mean bending angle, the consistency is better than 0.04 %, for refractivity it is better than 0.05 %, and for global mean dry temperature the consistency is better than 0.15 K in this height interval. At altitudes up to 40 km, these numbers increase to 0.08 %, 0.11 %, and 0.50 K, respectively. The numbers can be up to a factor of 2 larger for certain latitude bands compared to global means. Below about 8 km, the RO mission differences are larger, reducing the possibilities to generate multi-mission data records. We also find that the residual sampling errors are about one-third of the original and that they include a component most likely related to diurnal or semi-diurnal cycles.

Evaluation of the 15-year ROM SAF monthly mean GPS radio occultation climate data record

We here present results from an evaluation of the ROM SAF gridded monthly-mean climate data record (CDR v1.0), based on GPS radio occultation (RO) data from the CHAMP, GRACE, COSMIC, and Metop satellite missions. Systematic differences between RO missions, as well as differences of RO data relative to ERA-Interim reanalysis data, are quantified. The methods used to generate gridded monthly mean data are described, and the correction of monthly-mean RO climatologies for sampling errors, which is essential for combining data from RO missions with different sampling characteristics, is evaluated. We find a good overall agreement between the ROM SAF gridded monthly-mean CDR and the ERA-Interim reanalysis, particularly in the 8–30 km height interval. Here, the differences largely reflect time-varying biases in ERA-Interim, suggesting that the RO data record has a better long-term stability than ERA-Interim. Above 30–40 km altitude, the differences are larger, particularly for the pre-COSMIC era. In the 8–30 km altitude region, the observational data record exhibits a high degree of internal consistency between the RO satellite missions, allowing us to combine data into multi-mission records. For global mean bending angle the consistency is better than 0.04 %, for refractivity 0.05 %, and for global mean dry temperature the consistency is better than 0.15 K in this height interval. At altitudes up to 40 km, these numbers increase to 0.08 %, 0.11 %, and 0.50 K, respectively. The numbers can be up to a factor of 2 larger for certain latitude bands compared to global means. Below about 6–8 km the RO mission differences are larger, reducing the possibilities to generate multi-mission data records. We also find that the residual sampling errors are about one third of the original and that they include a component most likely related to diurnal or semi-diurnal cycles.

PROLONGED SIGNAL FADE-OUT ON A SHORT MICROWAVE PATH

During a period of 1 year, the incidence of signal fade-out has been observed in 2000 Mc/s transmissions over a 21-mile path near Ottawa. Fade-out durations varied from a few minutes to several hours, with the most frequent occurrence in summer and during the night. The cause of signal fade-out has been studied qualitatively through a ray analysis of the air refractivity profiles at the center of the radio path. This analysis indicates that fade-out is associated with a shallow, horizontal transition zone in air vapor pressure at a level near the antenna heights. Here, the vapor pressure changes through 1 millibar or more in a height interval of 100 ft. The signal fade-out is weak if the transition is from dry air below to moist air above, but is strong if the inverse transition occurs. Weak fade-out (a 'radio hole') is due to ray divergence, and strong fade-out (a 'radio antihole') is due to ray interference at the receiver.

Wind Variability as a Function of Time at Muroc, California

Records of 226 sets of triple theodolite balloon observations taken at Muroc, California, to heights of 38,000 feet have been analyzed to determined magnitudes of horizontal vector wind changes over time intervals from thirty minutes to five hours. The medians of magnitudes of velocity change vary from 2.0 mph over a time interval of 30 minutes to 6.4 mph over a time interval of five hours. The effect of altitude upon time variability of these winds has been studied. The height interval from the surface to 38,000 feet was separated into four altitude zones and it was found that wind variability is greater near the surface than in the zone above it. From the second zone upward, wind variability increased with height.