CHAllenging Minisatellite Payload Radio Occultation Research Articles

Comparison of the CHAMP radio occultation data with the Canadian advanced digital ionosonde in the Polar Regions

In this paper we present the results of the comparison of the retrieved electron density profiles of the Ionospheric Radio Occultation (IRO) experiment on board CHAMP (CHAllenging Minisatellite Payload), with the ground ionosonde profiles for the Polar Regions. IRO retrieved electron density profiles from CHAMP are compared with Canadian Advanced Digital Ionosonde (CADI) measurements at two vertical sounding stations well within the Polar Cap, Eureka (geog. 80°13′ N; 86°11′ W) and Resolute Bay (geog. 74°41′ N; 94°54′ W). We compared the ionospheric parameters such as the peak electron density of the F-layer (NmF 2) and the peak height of the F-layer (h mF 2) for a 3-year period, 2004–2006. CHAMP derived NmF 2 shows reasonable agreement with the ionosonde retrieved NmF 2 for both the stations (0.76 and 0.71 correlation coefficient, for Eureka and Resolute Bay, respectively) whereas the h mF 2 agreement is not that acceptable (0.25 and 0.37 correlation coefficient, respectively). The h mF 2 from vertical sounding showed less spread than the CHAMP h mF 2.

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.

CHAMP observations of global gravity wave fields in the troposphere and stratosphere

This paper presents the analysis of 56 months of CHAMP (Challenging Minisatellite Payload) RO (radio occultation) temperature profiles to study the global gravity wave fields in the troposphere and lower stratosphere. The relatively new GPS (Global Positioning System) RO technique is suitable for global monitoring and provides high vertical resolution, long‐term stability, and opportunity of all‐weather viewing. The CHAMP data set analyzed in this paper is the largest RO data set used for the investigation of global structure of gravity waves. The analysis also makes use of the global radiosonde data provided by the British Atmospheric Data Centre (BADC). First, the global winter temperature pattern and temperature fluctuations in the troposphere and lower stratosphere are discussed. The global structure of gravity wave variances in the upper troposphere and lower stratosphere for the summer and winter seasons indicate the dominance in the tropical region. The analysis reveals that the maximum potential energy (Ep) is observed around the equator with considerable longitudinal variation. Comparison of the wave energy derived from the CHAMP and radiosonde data sets for the low‐latitude region (20°S–20°N) shows good similarity. The observed vertical wave number power spectra were compared with a model spectrum. The low‐latitude power spectra show considerable similarities with the model spectrum. However, the disparity increases in the midlatitude or high‐latitude sectors; particularly, in the high latitudes the observed spectral power is much underestimated for longer wavelengths. The results demonstrate the usefulness of GPS RO data in determining the global gravity wave fields.

Evaluation of a Nonlocal Quasi-Phase Observation Operator in Assimilation of CHAMP Radio Occultation Refractivity with WRF

Abstract A nonlocal quasi-phase radio occultation (RO) observation operator is evaluated in the assimilation of Challenging Minisatellite Payload (CHAMP) radio occultation refractivity using a Weather Research and Forecasting (WRF) ensemble data assimilation system at 50-km resolution. The nonlocal operator calculates the quasi phase through integration of the model refractivity along the observed ray paths. As a comparison, a local refractivity operator that calculates the model refractivity at the observed ray perigee points is also evaluated. The assimilation is done over North America during January 2003 in two different situations: in conjunction with dense, high-quality radiosonde observations and with only satellite cloud drift wind observations. Analyses of temperature and water vapor with the RO refractivity assimilated using the local and nonlocal operator are verified against nearby withheld radiosonde observations. The bias and RMS errors of the analyses of water vapor and temperature using the nonlocal operator are significantly reduced compared with those using the local operator in the troposphere when the only additional observations are satellite cloud drift winds. The reduction of the bias and RMS errors is reduced when radiosonde observations are assimilated.

Temperature comparison between CHAMP radio occultation and TIMED/SABER measurements in the lower stratosphere

Global Positioning System (GPS) receiver on the CHAllenging Mini-satellite Payload (CHAMP) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument, one of four on board the TIMED satellite, provide middle atmosphere temperature profiles by Radio Occultation (RO) and limb viewing infrared emission measurements, respectively. These temperature profiles retrieved by two different techniques in the stratosphere are compared with each other using more than 1300 correlative profiles in March, September and December 2005. The over-all mean differences averaged over 15 and 35 km are approximately −2 K and standard deviation is less than 3 K. Below 20 km of altitude, relatively small mean temperature differences ∼1 K are observed in wide latitudinal range except for June (during the SABER nighttime observation). In the middle to low latitudes, between 30°S and 30°N, the temperature difference increases with height from ∼0–1 K at 15 km, to ∼−4 K at 35 km of altitude. Large temperature differences about −4 to −6 K are observed between 60°S and 30°N and 31–35 km of altitude for all months and between 0° and 30°N below 16 km during June (nighttime).

Application of Principal Component Analysis to CHAMP Radio Occultation Data for Quality Control and a Diagnostic Study

Abstract A principal component analysis (PCA) method is applied to Challenging Minisatellite Payload (CHAMP) level-2 radio occultation (RO) observations and the corresponding global analyses from the National Centers for Environmental Prediction (NCEP) in March 2004. The PCA is performed on a square symmetric vertical correlation matrix of observed or modeled RO profiles. By decomposing the matrix into pairs of loadings (EOFs) and associated principal components (PCs), outliers are identified and important modes that explain most variances of the vertical variability of the atmosphere as represented by the GPS RO data and the NCEP analyses are extracted and compared. Specifically, a quality control of RO data based on Hotelling’s T 2 index is applied first, which removes 255 RO profiles from 4884 total profiles (about 5%) and smoothes the distributions of PC modes, making the remaining GPS RO dataset much more meaningful. The leading PC mode for global refractivity explains 60% of the total variance and is associated with a symmetric zonal pattern, with positive anomalies in the Tropics and negative anomalies at the two poles. The second PC mode explains an additional 16% of the total variance and shows a dipole pattern with positive anomalies in the North Pole and negative anomalies in the South Pole. Three significant positive anomalies are also found in the second and third PC modes over three predominant convective areas in the western Pacific, South America, and Africa in the Tropics. The first leading PC mode calculated from global NCEP analyses compared favorably with that from CHAMP observations, which proves that NCEP analyses are capable of representing most of the variance of the atmospheric profiles. However, disagreements between CHAMP observations and NCEP analyses are noticed in the second EOF over the Tropics and the Southern Hemisphere (SH). It is also found that the NCEP analyses describe CHAMP-observed larger vertical scale features better than smaller-scale features, captures features of more leading EOF modes in the Northern Hemisphere than in the SH and the Tropics, and does not capture the vertical structures revealed by the EOFs in CHAMP observations near and above the tropopause in the Tropics.

Evaluation of a Linear Phase Observation Operator with CHAMP Radio Occultation Data and High-Resolution Regional Analysis

Abstract In this study a nonlocal, linear observation operator for assimilating radio occultation data is evaluated. The operator consists of modeling the excess phase, that is, integrating the refractivity along straight lines tangent to rays, below a certain height. The corresponding observable is the excess phase integrated through the Abel-retrieved refractivity, along the same lines, below the same height. The operator allows very simple implementation (computationally efficient) while accurately accounting for the horizontal refractivity gradients. This is due to significant cancellation of the linearization and discretization errors when modeling the observable. Evaluation of the operator with Challenging Minisatellite Payload (CHAMP) radio occultation data and grid refractivity fields from high-resolution regional analysis over the continental United States showed reduction of the observation error in the troposphere (below 7 km) 1.5–2 times, compared to the error of local refractivity. The operator is useful for the assimilation of radio occultation data by high-resolution weather models in the troposphere.

Using CHAMP radio occultation data to determine the top altitude of the Planetary Boundary Layer

A simple approach to derive the Planetary Boundary Layer (PBL) top altitude from CHAMP (CHAllenging Minisatellite Payload) radio occultation (RO) data is presented. Our RO processing cuts off at an altitude, typically ≤4 km, below which the GPS signals are affected by tracking errors. This lowest processed altitude (LPA) is assumed to coincide with the PBL top. We average LPAs for the years 2001 to 2004 over 5 Degree latitude longitude boxes and compare them to ECMWF analysis data. The ECMWF PBL top was calculated from the relative humidity gradient with respect to altitude. Agreement between the data sets is good in terms of mean PBL height, especially over sea. The CHAMP data shows the major features of PBL height with a realistic transition from stratocumulus regions to shallow and deep cumulus areas. CHAMP also shows a substantial amount of PBL height variability that may prove useful to study PBL dynamics.

Comparison of GPS radio occultation soundings with radiosondes

The radio occultation (RO) sounding technique that uses signals transmitted by the Global Positioning System (GPS) has evolved as an important global observing technology. In this paper, we compare RO refractivity profiles from the CHAMP (CHAllenging Minisatellite Payload) satellite mission with those calculated from radiosonde soundings over five geographical areas, each of which uses a different type of radiosonde. CHAMP RO soundings that occur within 2 hours and 300 km of radiosonde soundings are used during the period from June 2001 through March 2004. The comparison was made between the altitudes of 5–25 km, where RO soundings are most accurate. These results indicate that the RO soundings are of sufficiently high accuracy to differentiate performance of various types of radiosonde. The differences in performance among various types of radiosonde present a challenge for climate analysis. In this regard, RO is a considerably more robust measurement technique for climate monitoring.

Tropical tropopause parameters derived from GPS radio occultation measurements with CHAMP

The temperature structure in the tropical upper troposphere and lower stratosphere (UTLS) region is discussed based on Global Positioning System (GPS) radio occultation (RO) data from the German CHAMP (CHAllenging Minisatellite Payload) satellite mission. Several climatologies for tropopause parameters based on radiosonde data and model analyses have been published in recent years. Both data sources suffer either from low global coverage or poor vertical resolution. The GPS RO technique, on the other hand, is characterized by global coverage, high vertical resolution, all‐weather viewing, and long‐term stability. CHAMP RO data are available since February 2001. Since May 2001, up to 200 high‐ resolution temperature profiles per day are available. The temperature bias between CHAMP temperature profiles and radiosonde data as well as ECMWF analyses is less than 0.5 K between 300–30 hPa. On the basis of the May 2001 to November 2003 data set of CHAMP RO data the structure and temporal and spatial variability of the tropical tropopause based on several tropopause definitions (thermal and cold‐point tropopause) are discussed. This includes an overview of the global tropopause characteristics, the discussion of the annual cycle and the latitudinal‐longitudinal structure of the tropical tropopause. In the CHAMP RO temperature data, clear evidence of the stratospheric quasi‐biennial oscillation (QBO) was found. The goal of this study is to show the potential of GPS RO for global monitoring of the temperature demonstrated exemplarily for the tropical UTLS region and based on the first 31 months (as of November 2003) of CHAMP RO data.

Impact of CHAMP Radio Occultation Observations on Global Analysis and Forecasts in the Absence of AMSU Radiance Data

Challenging Minisatellite Payload (CHAMP) radio occultation (RO) observations during a two-week period are assimilated into global analyses using the National Center for Environmental Prediction (NCEP) three-dimensional variational (3D-Var) system with a recently improved observation operator for assimilating GPS bending angle data. The NCEP 3D-Var system used in this research is suboptimal since Advanced Microwave Unit (AMSU) radiances are not included in our experiments. Analyses with and without CHAMP observations are compared with each other and with collocated conventional radiosonde and dropsonde data, which are excluded from both experiments. Zonal mean temperature, humidity and surface pressure differences between the GPS analyses and NO-GPS analyses are examined. The GPS analyses in the Southern Hemisphere show higher temperatures than the NO-GPS analyses, particularly in the mid- and high latitudes. The GPS analyses show drier air in the lower troposphere and more moist air in the middle troposphere compared to the NO-GPS analyses. The surface pressure is slightly increased (maximum 0.8 hPa) in the Southern Hemisphere and decreased (maximum 0.25 hPa) in the Northern Hemisphere due to the inclusion of GPS observations. Compared with the collocated independent soundings, the large cold bias (as large as 2.5 K) in the NCEP Southern Hemisphere analyses produced without CHAMP observations is significantly reduced. On average, a 20% mean error reduction in the temperature analysis is obtained in the Southern Hemisphere when CHAMP data are included. Degradations in the surface pressure analysis found from previous the GPS/Meteorology data assimilation studies are greatly reduced. The differences between the surface pressure analysis errors with and without CHAMP data are less than 0.8±1.5 hPa. Comparisons of numerical forecasts initialized with analyses produced with and without CHAMP occultations display a small improvement in the forecasts in the tropics and the Southern Hemisphere associated with the use of the CHAMP observations.

GPS radio occultation with CHAMP: an innovative remote sensing method of the atmosphere

The availability of GPS (Global Positioning System) radio signals has introduced a new promising remote sensing technique for the Earth’s atmosphere. GPS-based radio occultation exploits GPS signals received onboard a Low Earth Orbiting (LEO) satellite for atmospheric limb sounding. Temperature and water vapour profiles with high accuracy and vertical resolution can be derived from these measurements. The GPS radio occultation technique requires no calibration, is not affected by clouds, aerosols or precipitation, and the occultations are almost uniformly distributed over the globe. The radio occultation experiment aboard the German CHAMP (CHAllenging Minisatellite Payload) satellite provided its first measurements in February 2001. Since then up to 250 daily occultation measurements were performed. About 70% of these were successfully processed to yield atmospheric temperature and water vapour profiles. The CHAMP radio occultation experiment demonstrated the immense potential of this technique for the global monitoring of stratospheric temperature, the tropopause region and the global humidity distribution in the troposphere. All of these are important variables of the climate system. In this paper the GPS radio occultation experiment onboard CHAMP will be described. At GeoForschungsZentrum (GFZ) Potsdam an operational system was established to process CHAMP radio occultation data, orbit data, and GPS ground station observations in an automatic and operational way to derive profiles of atmospheric parameters. The CHAMP temperature bias compared to ECMWF analyses is less than 1.5 K between the tropopause layer and the 30 hPa level, and less than 0.5 K between 12 and 20 km at mid and high latitudes. Further comparisons between CHAMP atmospheric profiles and radiosonde data as well as applications of CHAMP radio occultation data for atmospheric research and climate monitoring are given.