Georg Beyerle

Atmosphere sounding by GPS radio occultation: First results from CHAMP

The first radio occultation measurements of the CHAMP (CHAllenging Minisatellite Payload) satellite using Global Positioning System (GPS) signals have been performed on February 11, 2001. By the end of April 2001 more than 3000 occultations were recorded. Globally distributed vertical profiles of dry temperature and specific humidity are derived, of which a set of 438 vertical dry temperature profiles is compared with corresponding global weather analyses. The observed temperature bias is less than ∼1 K above the tropopause and even less than 0.5 K in the altitude interval from 12 to 20 km at latitudes >30°N. About 55% of the compared profiles reached the last kilometer above the Earths surface. In spite of the activated anti-spoofing mode of the GPS system the state-of-the-art GPS flight receiver aboard CHAMP combined with favorable antenna characteristics allows for atmospheric sounding with high accuracy and vertical resolution.

Observation and simulation of direct and reflected GPS signals in Radio Occultation Experiments

GPS/MET radio occultation observations are analyzed with the radio holographic method. Using the Multiple Signal Classification algorithm power spectra of 2038 radio holograms observed between 2 February and 16 February 1997 are calculated. In about 28% of the analyzed L1-band spectra secondary frequency components are found at power levels of about 10–20 dB below the main component. These secondary components are interpreted as signatures of GPS signals reflected at Earths surface. Results from geometrical ray tracing simulations support our interpretation.

Multiwavelength lidar measurements of stratospheric aerosols above Spitsbergen during winter 1992/93

Within the period of December 1992 to March 1993 lidar investigations of stratospheric aerosols were performed at Ny-Alesund, Spitsbergen (79°N, 12°E). Backscatter signals at wavelengths of 353, 532 and 1064 nm and depolarization signals at 532 nm in altitudes ranging from the tropopause up to 30 km were analyzed. Throughout the whole measurement period we observed an aerosol layer of volcanic origin in the lower stratosphere. Depolarization profiles suggest that the volcanic aerosol layer consisted mainly of liquid droplets. Comparison with model calculations indicate median particle radii between 0.1 and 0.2 µm. Surface densities exceeded approximately 40 µm²/cm³ in the lower part of the layer around 12 km. In January 1993 polar stratospheric clouds (PSCs) were frequently observed at altitudes up to 22 km. We analyzed the backscatter and depolarization data with respect to the temperature history of backward trajectories reaching Ny-Alesund. Signatures for micron sized crystalline PSC particles appear for cooling rates of −1 to −3 K day−1. Larger cooling rates of −4 to −10 K day −1 produced submicron sized aerosols, presumably supercooled droplets, characterized by enhanced backscatter ratios and reduced depolarization.

Nonequilibrium coexistence of solid and liquid particles in Arctic stratospheric clouds

Observations of polar stratospheric clouds (PSCs) from Ny Alesund, Spitsbergen, have been examined to quantify the occurrence of solid particles. The polarized backscatter ratio was found to be a more sensitive indicator of the presence of non-spherical (solid) particles in a PSC than the aerosol depolarization, which approaches zero for clouds containing a large volume of liquid droplets. The analysis corroborates our previous finding that type Ia PSCs cannot be composed of nitric acid trihydrate particles in equilibrium with the gas phase, which would lead to a much too high backscatter ratio. Conversely, type Ib PSCs, previously thought to be composed solely of liquid droplets, often contain a very small fraction of solid particles. These clouds develop a high parallel backscatter, arising from Mie scattering by droplets, and a nonvanishing perpendicular signal due to the few solid particles. Other type Ib PSCs appear to contain only liquid particles. The mixed liquid/solid clouds are observed on the smallest spatial and temporal scale resolvable by the lidar instrument (30 m, l min), implying that their properties are not a result of spatial averaging of different cloud types. Comparison of the observations with optical calculations shows that such nonequilibrium particle distributions are to be expected, as temperature changes are sufficiently rapid to prevent the particles from assuming equilibrium sizes. The observed optical characteristics of type Ia and type Ib clouds can be reproduced in a model by assuming that a very small fraction of the particles are composed of nitric acid hydrate, with the majority being binary H2SO4/H2O or ternary HNO3/H2SO4/H2O droplets.

GPS Ground Station Data for CHAMP Radio Occultation Measurements

Abstract The role of GPS ground station data for the CHAMP Atmospheric Profiling experiment is discussed. The profiling activities at GeoForschungZentrum Potsdam (GFZ) are briefly described and first results of the occultation processing system are presented. The atmospheric excess phase of the occultation link between the CHAMP and the GPS satellite is derived by a double difference method using ground station data. The influence of ground station data on the accuracy of the derived atmospheric profiles is discussed. A relationship between the temperature errors at several heights resulting from errors in atmospheric excess phase is given, e.g. an error of 1mm/s in the time derivative of the excess phase results in a temperature error of 1 K at 30 km. Furthermore, a study was performed to estimate the required data rate of the ground station measurements. In particular the influence of the Selective Availability (SA) termination on May 2, 2000 is emphasized. We conclude that the acquisition rate can be reduced from 1 Hz to 0.1 Hz without loss of information. Simulations of the CHAMP occultation experiment show that the joint Jet Propulsion Laboratory, Pasadena (JPL) & GFZ “high rate & low latency” network enables the processing of globally distributed occultation events with a highly redundant number of ground stations. Latency aspects are studied in view of the future task of providing atmospheric occultation data for operational weather forecast.

Temperature dependence of ternary solution particle volumes as observed by lidar in the Arctic stratosphere during winter 1992/1993

Multiwavelength lidar measurements of stratospheric aerosols performed at the Arctic Network for the Detection of Stratospheric Change station on Spitsbergen during winter 1992/1993 are analyzed. Altitude profiles of particle median radius and volume density are derived for measurements with aerosol depolarization smaller than 0.01. Below an altitude corresponding to 450 K potential temperature the Pinatubo aerosol layer dominated the stratospheric aerosol content with volume densities of more than 5 μm 3 cm -3 , whereas above 450 K, volume densities were close to background values of 0.1 μm 3 cm -3 . However, at all altitude levels between 350 and 550 K, volume densities consistently increased by a factor of 2-30 when temperatures approached the frost point. The observations are compared to results from thermodynamic model calculations at altitude levels of 400, 440, and 480 K. Good agreement between the observed and theoretically derived temperature dependencies of volume density suggests that nondepolarizing polar stratospheric cloud particles, as well as volcanic aerosols, at low temperatures are composed of a ternary liquid solution of sulfuric and nitric acid. At all altitude levels, model results indicated more than 90% HNO 3 gas phase depletion as temperatures approached the frost point. A mean profile of total H 2 SO 4 volume mixing ratio is derived, decreasing from about 4 parts per billion by volume (ppbv) at 350 K to about 0.5 ppbv above 450 K.

Validation and Data Quality of CHAMP Radio Occultation Data

Since the first successful radio occultation measurement taken by the GPS receiver onboard CHAMP, more than 40,000 vertical soundings of stratospheric temperature and tropospheric humidity have been processed at GFZ Potsdam. During the first year of the radio occultation experiment, several multi-week batches of quasi-continuous measurements have been collected, allowing for detailed validation against other meteorological measurements. This paper presents the first validation results and discusses some conclusions regarding the data quality of CHAMP radio occultation data drawn from these.

A Lidar and Backscatter Sonde Measurement Campaign at Table Mountain during February–March 1997: Observations of Cirrus Clouds

Results from a measurement campaign performed at Table Mountain Facility/Jet Propulsion Laboratory/ California Institute of Technology (34.388N, 117.688W, 2280 m ASL) are presented. Between 19 February and 18 March 1997 more than 400 h worth of lidar data were acquired and four backscatter sondes were launched. About 50% of the observations show the presence of cirrus clouds at altitudes close to and below the tropopause. Of these clouds 80% are characterized as subvisual with optical depths below 0.03 at a wavelength of 532 nm. Simple geometrical considerations lead to cloud spatial scales of 0.31 km vertically and 7.5 km horizontally, respectively. Deviations from color ratio values derived on the basis of geometrical optics are interpreted as small particle signatures. Comparing backscatter ratio profiles observed concurrently by three aerosol lidars, mean deviations of about 10% are found.

Comparison of Water Vapor and Temperature Results From GPS Radio Occultation Aboard CHAMP With MOZAIC Aircraft Measurements

Global positioning system (GPS) radio occultation (RO) observations aboard low earth orbiting (LEO) satellites provide a powerful tool for global atmospheric sounding. Almost continuously activated since mid-2001, the challenging minisatellite payload (CHAMP) GPS RO experiment provides up to 200 vertical atmospheric profiles per day. In this paper, we intercompare CHAMP RO humidity results and analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) with coinciding measurement of ozone and water vapor by airbus in-service aircraft (MOZAIC) data collected during aircraft ascents and descents. About 320 coinciding profiles with CHAMP were found from 2001 to 2006 (coincidence radius: 3 h, 300 km). Between about 650 and 300 hPa, the CHAMP-MOZAIC humidity bias is smaller than the ECMWF-MOZAIC bias. On the other hand, the standard deviation between MOZAIC and CHAMP humidity is slightly higher than that between MOZAIC and ECMWF through the entire altitude range. Apart from the water vapor validation (ascent and descent data), we also compare MOZAIC cruise data at an altitude of typically 10-11 km with CHAMP refractivity and temperature results (dry retrieval), and corresponding ECMWF analysis data. Whereas refractivity data from MOZAIC, CHAMP, and ECMWF show excellent agreement, the CHAMP temperature exhibits a cold bias of about 0.9 K in comparison to MOZAIC and ECMWF.

Dual wavelength lidar observation of tropical high‐altitude cirrus clouds during the ALBATROSS 1996 Campaign

Dual wavelength aerosol lidar observations of tropical high-altitude cirrus clouds were performed during the ALBATROSS 1996 campaign aboard the research vessel “POLARSTERN” on the Atlantic ocean in October–November 1996. On the basis of 57 hours of night-time observations between 23.5°N and 23.5°S we find in 72% of the altitude profiles indications of the presence of cirrus cloud layers. This percentage drops to 32% at subtropical latitudes (23.5°–30°) based on 15 hours of data. About one-half of the subtropical and tropical cirrus layers are subvisual with an optical depth of less than 0.03 at a wavelength of 532 nm. In general the clouds exhibit high spatial and temporal variability on scales of a few tens of meters vertically and a few hundred meters horizontally. No clouds are observed above the tropopause. An abrupt change in the relation between the color ratios of the parallel and perpendicular backscatter coefficients at about 240 K is interpreted in terms of changes of particle shape and/or size distribution. At temperatures between 195 and 255 K only a small fraction of the observations are consistent with the presence of small particles with dimensions of less than 0.1 µm.