Constellation Observing System For Meteorology, Ionosphere, And Climate Radio Occultation Research Articles

Singular spectrum analysis and modeling of the temporal variations of the global ionospheric F2-layer peak electron density retrieved from COSMIC radio occultation measurements

The singular spectrum analysis (SSA) is a relatively new method for decomposing time series into separable and interpretable series (i.e., trend, periodic, and irregular components). Here, the ionospheric variability in the peak electron density (NmF2) observations provided by FORMOSAT-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation measurements during 2009–2019, which completely covers solar-cycle 24, was examined using the nonparametric method (SSA). This study identified different temporal patterns inherent in the COSMIC data, which can be employed to reconstruct/model noise-free time series. Based on a proper choice of SSA parameters, it was observed that SSA can be employed to extract various components of these datasets. Additionally, the results showed that the slowly varying component (trend) was largely influenced by solar activity and closely followed the behavior of the sunspot cycle. The SSA method demonstrated seasonal patterns (the semiannual component). Based on the patterns, the reconstructed series for each of the selected components were separately modeled with a corresponding fit function using the 10.7 cm solar proxy, which prior be smoothed by Spline fitting. It was observed that the polynomial function efficiently modeled the trend component with R2 = 0.95, while the harmonic function adequately represented the semiannual component with R2 = 0.97. To validate the proposed model, the data were estimated for the time interval from January 2020 to August 2022, which represented the ascending phase of the ongoing solar-cycle 25. The results revealed that the modeled data correlated with the observed COSMIC-2 data with a high correlation coefficient (0.95). These findings indicate that the underlying model can effectively describe the behavior of the electron density during the investigated period, and the SSA method is particularly powerful for analyzing nonstationary time series and capable of identifying sub-time series (trends along with seasonal periodic components).

The Stratospheric Diurnal Cycle in COSMIC GPS Radio Occultation Data: Scientific Applications

AbstractThe diurnal cycle throughout the stratosphere is analyzed by applying Bayesian interpolation to Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Global Positioning System radio occultation (RO) data and three scientific applications of the analysis are introduced. First, the migrating thermal tides are analyzed with unprecedented accuracy and precision, with an uncertainty in the analysis of the vertically propagating tides ranging from 0.1 in the lower stratosphere to 0.6 K in the upper stratosphere for an individual month of RO data and with an uncertainty in a 10‐year climatological diurnal cycle a factor of 10 less. Moreover, the midlatitude trapped tide is found to be smaller than what is produced by an atmospheric model and lags the model in phase, a likely consequence of a faulty parameterization of eddy diffusivity in the upper stratosphere. Second, a clear signal of solar cycle influence on the diurnal cycle is evident in this analysis, but whether the cause is the systematic bias of ionospheric residual associated with RO retrieval or it is an actual atmospheric phenomenon is less clear. Third, RO satellites and missions that obtain inadequate coverage of the diurnal cycle will be biased by under‐sampling it, whether or not subsampling weather forecasts is used to removal sampling error. The analysis of the diurnal cycle in COSMIC RO data can be used to diagnose the systematic sampling error incurred by incomplete coverage of the diurnal cycle, which is of the order of 0.2 K for a Metop‐based RO climatology.

Diurnal Variation of the Planetary Boundary Layer Height Observed from GNSS Radio Occultation and Radiosonde Soundings over the Southern Great Plains

Abstract The planetary boundary layer (PBL) height (PBLH) is a key physical parameter of the PBL affected by numerous physical processes within the boundary layer. Specifically, the PBLH over land exhibits large spatial and temporal variation across different geographical regions. In this study, the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) radio occultation (RO) profiles and high-resolution radiosonde profiles from 2007 to 2013 were analyzed to estimate the diurnal cycle of the PBLH over the Southern Great Plains (SGP) in the United States. Large variations in PBLH derived from radiosonde temperature, moisture, and refractivity are observed on seasonal scales. COSMIC RO is capable of observing diurnal and seasonal variations in the terrestrial PBLH over the SGP region. Annual mean diurnal amplitude of approximately 250 m in the terrestrial PBLH was observed, with maxima occurring at around 1500 local solar time (LST) in both the collocated radiosondes and COSMIC RO profiles. Seasonal changes in the PBLH diurnal cycles ranging from approximately 100 to 400 m were also observed. Such PBL diurnal and seasonal changes can be further incorporated into PBL parameterizations to help improve weather and climate model prediction. Significance Statement The atmospheric planetary boundary layer (PBL) and its height (PBLH) control many atmospheric processes that affect our everyday lives. Observations of the PBL are usually limited to radiosondes at limited time intervals. GNSS radio occultation (RO) provide high-vertical-resolution atmospheric observations that are ideal for PBL study. This study demonstrates that the GNSS RO is capable of capturing the diurnal and seasonal variations of the PBLH over the southern Great Plains (SGP) well as compared to the collocated radiosonde observations.

Intercalibration of the Plasma Density Measurements in Earth's Topside Ionosphere

AbstractOver the last 20 years, a large number of instruments have provided plasma density measurements in Earth's topside ionosphere. To utilize all of the collected observations for empirical modeling, it is necessary to ensure that they do not exhibit systematic differences and are adjusted to the same reference frame. In this study, we compare satellite plasma density observations from Gravity Recovery and Climate Experiment (GRACE), Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), CHAllenging Minisatellite Payload (CHAMP), Swarm, and Communications/Navigation Outage Forecasting System (C/NOFS) missions. Electron densities retrieved from GRACE K‐Band Ranging (KBR) system, previously shown to be in excellent agreement with incoherent scatter radar (ISR) measurements, are used as a reference. We find that COSMIC radio occultation (RO) densities are highly consistent with GRACE‐KBR observations showing a mean relative difference of <, and therefore no calibration factors between them are necessary. We utilize the outstanding three‐dimensional coverage of the topside ionosphere by the COSMIC mission to perform conjunction analysis with in situ density observations from CHAMP, C/NOFS, and Swarm missions. CHAMP measurements are lower than COSMIC by ∼. Swarm densities are generally lower at daytime and higher at nighttime compared to COSMIC. C/NOFS ion densities agree well with COSMIC, with a relative bias of ∼. The resulting cross‐calibration factors, derived from the probability distribution functions, help to eliminate the systematic leveling differences between the data sets, and allow using these data jointly in a large number of ionospheric applications.

The Importance of Counting: A New Index to Correctly Quantify Equatorial Plasma Bubble Occurrence in COSMIC Radio Occultation Data

AbstractThe Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Radio Occultation data has been previously used as a way of investigating the climatology of Equatorial Plasma Bubbles (EPB). These low‐density regions can cause random phase and amplitude scintillation of satellite signals and forecasting these is an important priority in the space physics community. The ability to build useful forecasting models depends on the underlying data sets used in testing and validating these models. Correctly identifying days of EPB activity is important, but statistical studies using large data sets require the use of automated detection methods. Many of these detection methods heavily reduce data sets by removing unfavorable data, such as COSMIC profiles where the maximum scintillation occurs in the E region. This study presents a new F region s4max9sec index that can be used to study the presence of F region scintillation without excluding profiles that exhibit E region scintillation. The new index is shown to decrease EPB occurrence through the increase in available data, and is also shown to detect EPB events previously excluded from climatological studies. Calculation of the climatology using the new index is shown to resolve some differences that exist in the EPB climatology literature, particularly the location of the maximum scintillation occurrence during the equinox seasons. The use of this index is highlighted as a potential for studying F region scintillation in the presence of E region scintillation; an area of research that needs to be expanded for a complete understanding of EPBs.

Analysis of ionospheric parameters retrieved from Feng-Yun 3C and COSMIC radio occultation

Electron density profiles observed using Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and Feng-Yun 3C (FY3C) radio occultation (RO) missions from 2014 to 2018 were quality inspected and analysed. After eliminating observations that did not meet the quality requirements, we compared the critical frequency of the F2 layer (foF2) and total electron content (TEC) with data retrieved from ionosondes and global ionospheric maps (GIM). Results showed that the quality of the COSMIC and FY3C RO electron density profiles was higher during the daytime compared to that overnight and was better in summer and autumn compared to spring and winter. Furthermore, the electron density profile qualification rate was higher for FY3C RO than that for COSMIC RO in low latitude regions. The foF2 RO data correlated well with ionosonde data (0.94 and 0.90 for FY3C and COSMIC, respectively). Similarly, TEC RO data correlated well with data from GIM (0.95 and 0.92 for FY3C and COSMIC, respectively). The observation accuracy of the RO data was affected by both solar and geomagnetic activity, and the influence of solar activity was greater than that of geomagnetic activity. During periods of reduced solar and geomagnetic activity, the accuracy of foF2 and TEC retrieved from RO was dependent on the local time, season, and latitude.

Plasmaspheric scale height modeling based on COSMIC radio occultation data

The scale height in the plasmasphere (HP) is an important factor to present the dynamic nature and variations of the plasmasphere. It is challenging to build reliable plasmaspheric scale height model because of poor coverage and limited continuity of plasmaspheric sounder data. In this paper, an empirical orthogonal function (EOF) analysis method is applied to organize the HP obtained from total electron content (TEC) measurements of precise orbit determination (POD) antennas and electron density profiles of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission, from the year 2007–2014, to reconstruct a global plasmaspheric scale height model. The model depends on parameters including the month of the year, the local time, the geographic latitude and the solar radio flux index F10.7. Comparison of this model and the scale height deduced from the International Satellites for Ionospheric Studies (ISIS) data shows remarkable consistency in HP values and variations.

Climate Variability of Atmospheric Rivers and Droughts over the West Coast of the United States from 2006 to 2019

Water resources are crucial to the livelihood and sustainability of the general public across the western United States. This study covers the timespan of both the third driest drought in Californian history between 2012 and 2015 as well as the extreme atmospheric river year in 2016–2017. The evaluation of vertical moisture profiles using Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Radio Occultation (RO) data, National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis of 500 hPa geopotential heights, 1000–500 hPa thickness, Optimum Interpolation (OI) Sea Surface Temperature (SST), NOAA/NDBC buoy data, and NASA, MEaSUREs, Gridded Sea Surface Height Anomalies (SSHA) were performed. The daily COSMIC time evolution from 2006 through 2015 showed a flat to slightly upward trend of both temperature and water vapor profiles through the entirety of the western US drought. Subsequently, a significant increase of temperatures and water vapor were recorded in early 2016 before the extreme Atmospheric River (AR) season of 2016–2017. The quantitative analyses suggest that warmer SST and higher SSHA lead to an increase of heat fluxes from the ocean into the troposphere, which forces thickness changes and thus the position of troughs in the geopotential height field changes afterwards, consequently pushing the trough eastward over the Pacific Northwest and potentially leading to an active AR year in the western US. It appears that regional COSMIC RO moisture profiles, seasonal SST, and SLH anomalies may serve as a precursor for seasonal or sub-seasonal precipitation outlook along the western US.

Tropical tropopause layer evolution during 2015–16 El Niño event inferred from COSMIC RO measurements

The detailed evolution of different tropical tropopause layer (TTL) parameters, such as tropopause height (cold point (CPH)/lapse rate (LRH)/convective (COH)), its corresponding temperature (CPT/LRT/COT) and TTL thickness in response to recent 2015–16 El Niño event are delineated using high-resolution Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Radio Occultation (RO) measurements. Distinct TTL anomalies are noticed between June-July-August (JJA), September-October-November (SON), and December -January-February (DJF) seasons during 2015–16 El Niño event. A pronounced positive anomaly in the CPH and LRH (~0.8 km) is noticed in JJA over most of the tropical equatorial region, whereas, in DJF, positive anomalies mostly concentrate over off equatorial central Pacific (CP) and eastern Pacific (EP) regions. Interestingly, prominent higher positive anomalies are evident in the convective outflow level altitude (~1 km) particularly over the tropical CP and EP regions in JJA and DJF. The CPT and LRT exhibit strong negative anomalies (>4 K) over CP and EP regions and positive anomalies over the western Pacific region in all the seasons. However, significant positive CPT and LRT anomalies are evident over the Atlantic region in DJF. In contrast, the COH (COT) and TTL thickness anomalies are well-matched with the outgoing long wave radiation anomalies compared to the CPH and LRH (CPT and LRT) anomalies. Overall, the observed TTL parameters exhibit similar El Niño signatures as observed for the other events. However, the 2015–16 El Niño event was one of strongest summer ENSO events in the 21st century compared to the previous events. As a result, prominent warm tropopause temperatures are evident over the WP region during JJA and these anomalies reaches maximum in DJF. The observed changes in the TTL parameters are explained in the context of circulation and convection changes over the tropics.

Analysis bias induced in assimilation of the radio occultation bending angle with complex structures in the tropical troposphere

AbstractIn the tropical lower troposphere, the radio occultation (RO) bending angle can have complex and sharp structures. In current numerical weather prediction data assimilation, the estimated observation uncertainty (or error) of the RO bending angle is typically derived from a fixed‐fraction uncertainty model of the RO bending angle. Therefore, the complex and sharp structures can exist in the observation uncertainty as well. This study investigates a potential analysis bias induced in the assimilation of COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) RO bending angles as a result of these complex and sharp structures in the observation uncertainty in the tropical troposphere. It is demonstrated that the complex structures can create a clear negative bias, up to −0.7%, in the bending angle space in the regional analysis with the Weather Research and Forecasting (WRF) model. The WRF analyses and backgrounds also show a larger bias from the COSMIC RO bending angles.

Multi‐Instrumental Observation of Storm‐Induced Ionospheric Plasma Bubbles at Equatorial and Middle Latitudes

AbstractJune solstice is considered as a period with the lowest probability to observe typical equatorial plasma bubbles (EPBs) in the postsunset period. The severe geomagnetic storm on 22–23 June 2015 has drastically changed the situation. Penetrating electric fields associated with a long‐lasting southward IMF support favorable conditions for postsunset EPBs generation in the dusk equatorial ionosphere for several hours. As a result, the storm‐induced EPBs were progressively developed over a great longitudinal range following the sunset terminator. The affected area has a large longitudinal range of ~100° in the American sector and a rather localized zone of ~20° in longitude in the African sector. Plasma depletions of equatorial origin were registered at midlatitudes (30°–40° magnetic latitude) of both hemispheres in the African and American longitudinal sectors. We examine global features of the large‐scale plasma depletion by using a combination of ground‐based and space‐borne measurements—ground‐based Global Positioning System/Global Navigation Satellite System (GPS/GNSS) networks, Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) GPS Radio Occultation (RO), Swarm upward looking GPS data, and in situ plasma density observations provided by Swarm, Communications/Navigation Outage Forecasting System (C/NOFS), and Defense Meteorological Satellite Program (DMSP) missions. Joint analysis of the satellite observations revealed that these storm‐induced EPBs structures had extended over 500 km in altitude, at least from ~350 to ~850 km. These irregularities caused strong amplitude and phase scintillations of GPS/GNSS signals for ground‐based and space‐borne (COSMIC RO) measurements and seriously affected performance of navigation‐based services.

The influence of the spatial and temporal collocation windows on the comparisons of the ionospheric characteristic parameters derived from COSMIC radio occultation and digisondes

GPS radio occultation (RO) ionospheric products obtained by Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission during the year of 2014 and the observations from 3 digisonde stations which are located at different latitudes are used to study the influence of different time and space collocation windows on the comparisons of the ionospheric characteristic parameters (ICPs), including the peak density and peak height, derived from the two techniques. The results show that the correlation coefficients (CC) and the standard deviation of the absolute biases (SDAB) between the ICPs derived from the two techniques vary distinctly under different spatial and time collocation windows. Generally, the CC (SDAB) of the ICPs decrease (increase) as the size of the collocation window increases in time dimension or in space dimension. The rate of change of the statistic parameters with the increase in the size of the collocation window in time dimension and space dimension is analyzed for each digisonde station. It is found that within the collocation window of 60min,20°,20°, the influence of the increase of 1° in the space window on the statistical comparison is much more significant than that of the increase of 1 min in the time window, and it is suggested that there can be appropriate relaxation on the time window within the threshold of 60 min to get a balance between the quality of the comparison results and the number of the matched pairs. In addition, it is found that the same variations in the longitude window and in the latitude window may have different influences on the comparison results when the horizontal gradients in electron density are distinctly different along different directions at the digisonde station, and strict space collocation window is preferred when comparing the observations from COSMIC RO with those from the digisonde station in such cases.

Climatology and Characteristics of Medium‐Scale F Region Ionospheric Plasma Irregularities Observed by COSMIC Radio Occultation Receivers

AbstractMedium‐scale ionospheric ionization structures are a persistent global feature of the Earth's ionosphere. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation measurements are well suited to address the incomplete global observational picture of plasma density irregularities, including the global climatology in both bottomside and topside F region layers, and their structure in the vertical dimension. A climatological database of F region ionospheric irregularities and their characteristics has been developed through detection of total electron content perturbations by Global Positioning System receivers onboard COSMIC satellites. This paper presents global occurrence rates and detailed characteristics of equatorial to midlatitude medium‐scale irregularities under quiet geomagnetic conditions. The study covers 4 years, two during solar minimum (2008–2009) and two during the ascending phase of solar cycle 24 (2012–2013). Irregularities were found to occur frequently at high latitudes and during nighttime in equatorial to midlatitude regions in both bottom and topside F region layers. Longitudinal‐seasonal occurrence trends at equatorial and midlatitudes are consistent with previous irregularity climatology, which reaffirms that localized enhancements in plasma instability growth rates contribute to irregularity occurrence. Seasonal occurrence patterns also indicate a high occurrence of irregularities in regions corresponding to the solar terminator, confined primarily to altitudes below ~300 km. The local time‐altitude distributions of equatorial and midlatitude irregularity occurrence, amplitude, and scale size provide further insight into irregularity generation mechanisms, and include features consistent with “spread F” irregularities and traveling ionospheric disturbances.

Comparison study of COSMIC RO dry-air climatologies based on average profile inversion

Abstract. Global Navigation Satellite System (GNSS) radio occultation (RO) data enable the retrieval of near-vertical profiles of atmospheric parameters like bending angle, refractivity, pressure, and temperature. The retrieval step from bending angle to refractivity, however, involves an Abel integral with an upper limit of infinity. RO data are practically limited to altitudes below about 80 km and the observed bending angle profiles show decreasing signal-to-noise ratio with increasing altitude. Some kind of high-altitude background data are therefore needed in order to perform this retrieval step (this approach is known as high-altitude initialization). Any bias in the background data will affect all RO data products beyond bending angle. A reduction of the influence of the background is therefore desirable – in particular for climate applications. Recently a new approach for the production of GNSS radio occultation climatologies has been proposed. The idea is to perform the averaging of individual profiles in bending angle space and then propagate the mean bending angle profiles through the Abel transform. Climatological products of refractivity, density, pressure, and temperature are directly retrieved from the mean bending angles. The averaging of a large number of profiles suppresses noise in the data, enabling observed bending angle data to be used up to 80 km without the need for a priori information. Some background information for the Abel integral is still necessary above 80 km. This work is a follow-up study, having the focus on the comparison of the average profile inversion climatologies (API) from the two processing centers WEGC and DMI, which study monthly COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) data from January to March 2011. The impact of different backgrounds above 80 km is tested, and different implementations of the Abel integral are investigated. Results are compared for the climatological products with ECMWF analyses, MIPAS, and SABER data. It is shown that different implementations of the Abel integral have little impact on the API climatologies. On the other hand, different extrapolations of the bending angle profile above 80 km play a key role in the resulting monthly mean refractivities above 35 km in altitude. Below that respective altitude the API climatologies show a good agreement between the two processing centers WEGC and DMI. Due to the downward propagation within the retrieval, effects of the high-altitude initialization lead to differences in dry-temperature climatologies down to 20 km in altitude. When applying an exponential extrapolation to the bending angles above 80 km at both centers, the dry-temperature climatologies agree among WEGC, DMI, ECMWF analysis, and MIPAS up to 35 km in altitude within ±0.5 K and up to 40 km in altitude within ±1 K. We conclude that the API retrieval is a valid approach up to the lower stratosphere. It is a computationally efficient alternative method for producing dry atmospheric RO climatologies.

An optimal tropospheric tomography approach with the support of an auxiliary area

Abstract. Among most current tropospheric tomography studies, only the signals crossing out from the top boundary of the tomographic area are used for reconstructing the three-dimensional water vapour field, while signals penetrating from the side faces of the tomographic body are ignored as invalid information. Such a method wastes the valuable Global Navigation Satellite System (GNSS) observations and decreases the utilisation efficiency of GNSS rays. This is the focus of this paper, which tries to effectively use signals penetrating from the side faces of the tomographic body for water vapour reconstruction. An optimised tropospheric tomography method is proposed using an auxiliary area. The top height of the tomography body is determined based on the average water vapour distribution derived from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) products. In addition, the coefficients of a negative exponential function between the adjacent layers for vertical constraints are fitted using the COSMIC RO profiles. Thirteen GPS stations are selected in the CORS Network of Texas to perform the tomographic experiment and validate the performance of the proposed method at 00:00 and 12:00 UTC daily using the radiosonde data for a period of 15 days. Compared to the conventional method, the accuracy of the reconstructed water vapour information derived from the proposed method is increased by 14.37 % and 16.13 %, respectively, in terms of mean root mean square (rms) and mean absolute error (MAE). The tomographic results obtained from the proposed method are further validated with the slant water vapour (SWV) data derived using the GAMIT (GNSS processing software package). Results show that the rms and MAE accuracy of SWV values has been improved by 18.18 % and 27.62 %, respectively, when compared to the conventional method. Keywords. History of geophysics (atmospheric sciences)

Low‐Latitude Ionospheric Density Irregularities and Associated Scintillations Investigated by Combining COSMIC RO and Ground‐Based Global Positioning System Observations Over a Solar Active Period

AbstractThis study for the first time presents a locally integrated analysis of occurrences of ionospheric E and F region irregularities/scintillations in southeast China, by employing radio occultation (RO) profile data retrieved from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites and observations from a ground‐based Global Navigation Satellite System receiver over a solar active period from 2014 to 2015. Their occurrences in both nighttime and daytime were examined by using both amplitude scintillation index (S4) and the rate of change of total electron content index. It is found that (1) F region irregularities occurred predominantly during 20–03 local time (LT) and exhibited maximum (minimum) during equinoxes (solstices) and equinoctial (solstice) asymmetry. Their geographic mapping reveals the maximum occurrence in the westward tilted structure of equatorial plasma bubbles. In addition, the altitude‐time variations indicate that their occurrences at higher altitudes were prevailing at 20–22 LT. (2) The E region irregularities were found prominently during 15–00 LT at altitudes of 90–110 km with an even geographic distribution. Their occurrences with maximum in summer (May–August) were distinctly detected by RO observations but insignificantly by ground‐based observations. (3) By examining simultaneous observations of E and F region irregularities, it is found that they appeared absent during 21–00 LT and predominant after midnight. This could be related to the weakening/disruption of sporadic E (Es) layers during the development of equatorial plasma bubbles. A sign of coupling of E and F regions during nighttime is likely revealed from RO profile data.

Assessment of Atmospheric Wet Profiles Obtained from COSMIC Radio Occultation Observations over China

Atmosperic profiles derived from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements make up for the lack of operational radiosonde soundings with a high spatiotemporal distribution, and their performance over China is assessed in this paper. COSMIC-retrieved atmospheric wet profiles from 2014 to 2015 are compared to the contemporaneous radiosonde profiles from 120 stations, and the vertical mean differences are used. The results show that the vertical mean biases of temperature, pressure and vapor pressure are −0.10 K, 0.69 hPa and −0.01 hPa, respectively, and that for refractivity is 0.17 N. Moreover, the temperature differences are positively correlated with station altitude, yet both the pressure and vapor pressure differences are negatively correlated with station latitude, as is the refractivity difference. The large temperature difference arising from the Qinghai-Tibet Plateau (QTP) region may be associated with the complex topography of the area and the limitations in the background model used in the COSMIC profile retrieval. Furthermore, negative refractivity bias between COSMIC and radiosonde data occurs below 5 km and is large in wet southern China, with a value of less than 1%. This result may be related to more humid conditions and super-refraction.

Electron number density profiles derived from radio occultation on the CASSIOPE spacecraft

AbstractThis paper presents electron number density profiles derived from high‐resolution Global Positioning System (GPS) radio occultation (RO) observations performed using the Enhanced Polar Outflow Probe payload on the high inclination CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) spacecraft. We have developed and applied a novel inverse Abel transform algorithm on high rate RO total electron content measurements performed along GPS to CASSIOPE radio links to recover electron density profiles. The high‐resolution density profiles inferred from the CASSIOPE RO are (1) in very good agreement with density profiles estimated from ionosonde data, measured over stations nearby to the latitude and longitude of the RO tangent points; (2) in good agreement with density profiles inferred from GPS RO measured by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC); and (3) in general agreement with density profiles estimated using the International Reference Ionosphere climatological model. Using both CASSIOPE and COSMIC RO observations, we identify, for the first time, that there exist differences in the characteristics of the electron number density profiles retrieved over landmasses and oceans. The density profiles over oceans exhibit widespread values and scale heights compared to density profiles over landmasses. We provide an explanation for the ocean‐landmass discrepancy in terms of the unique wave coupling mechanisms operating over oceans and landmasses.

Assessment of COSMIC radio occultation and AIRS hyperspectral IR sounder temperature products in the stratosphere using observed radiances

AbstractUpper air temperature is defined as an essential climate variable by the World Meteorological Organization. Two remote sensing technologies being promoted for monitoring stratospheric temperatures are GPS radio occultation (RO) and spectrally resolved IR radiances. This study assesses RO and hyperspectral IR sounder derived temperature products within the stratosphere by comparing IR spectra calculated from GPS RO and IR sounder products to coincident IR observed radiances, which are used as a reference standard. RO dry temperatures from the University Corporation for Atmospheric Research (UCAR) Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission are compared to NASA Atmospheric Infrared Sounder (AIRS) retrievals using a previously developed profile‐to‐profile collocation method and vertical temperature averaging kernels. Brightness temperatures (BTs) are calculated for both COSMIC and AIRS temperature products and are then compared to coincident AIRS measurements. The COSMIC calculated minus AIRS measured BTs exceed the estimated 0.5 K measurement uncertainty for the winter time extratropics around 35 hPa. These differences are attributed to seasonal UCAR COSMIC biases. Unphysical vertical oscillations are seen in the AIRS L2 temperature product in austral winter Antarctic regions, and results imply a small AIRS tropical warm bias around ~35 hPa in the middle stratosphere.

Estimation and evaluation of COSMIC radio occultation excess phase using undifferenced measurements

Abstract. In the GPS radio occultation technique, the atmospheric excess phase (AEP) can be used to derive the refractivity, which is an important quantity in numerical weather prediction. The AEP is conventionally estimated based on GPS double-difference or single-difference techniques. These two techniques, however, rely on the reference data in the data processing, increasing the complexity of computation. In this study, an undifferenced (ND) processing strategy is proposed to estimate the AEP. To begin with, we use PANDA (Positioning and Navigation Data Analyst) software to perform the precise orbit determination (POD) for the purpose of acquiring the position and velocity of the mass centre of the COSMIC (The Constellation Observing System for Meteorology, Ionosphere and Climate) satellites and the corresponding receiver clock offset. The bending angles, refractivity and dry temperature profiles are derived from the estimated AEP using Radio Occultation Processing Package (ROPP) software. The ND method is validated by the COSMIC products in typical rising and setting occultation events. Results indicate that rms (root mean square) errors of relative refractivity differences between undifferenced and atmospheric profiles (atmPrf) provided by UCAR/CDAAC (University Corporation for Atmospheric Research/COSMIC Data Analysis and Archive Centre) are better than 4 and 3 % in rising and setting occultation events respectively. In addition, we also compare the relative refractivity bias between ND-derived methods and atmPrf profiles of globally distributed 200 COSMIC occultation events on 12 December 2013. The statistical results indicate that the average rms relative refractivity deviation between ND-derived and COSMIC profiles is better than 2 % in the rising occultation event and better than 1.7 % in the setting occultation event. Moreover, the observed COSMIC refractivity profiles from ND processing strategy are further validated using European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data, and the results indicate that the undifferenced method reduces the noise level on the excess phase paths in the lower troposphere compared to the single-difference processing strategy.