Horizontal Gradient Information Research Articles

Independent temporal and spatial variation analysis of ionospheric TEC over Asia-Pacific area based on BDS GEO satellites

GPS has been widely used to monitor ionospheric total electron content (TEC) variations. However, GPS-derived TEC contains ionospheric horizontal gradient information due to the motion of GPS satellites. As a result, the temporal and spatial variation information is mixed. With the advent of the geostationary earth orbit (GEO) of the BeiDou navigation satellite system (BDS), observations from BDS GEO satellites provide a new opportunity for independent research on the temporal and spatial variations of the ionospheric TEC. Furthermore, the GEO satellite also enables continuous and long-term TEC monitoring. Due to its higher orbit, the BDS GEO-derived TEC also contains more plasmaspheric electron content. In this study, the temporal and spatial variation characteristics of the ionospheric TEC derived from the BDS GEO satellites over the Asia-Pacific area are investigated. The achievable TEC theoretical accuracy from the BDS GEO measurements is first deduced. The TEC time series at 26 BDS stations over the Asia-Pacific area is then analyzed. Results acquired from the vertical TEC (VTEC) observations in 2017 indicate obvious periodic variations and spatial variations with a semi-annual anomaly. Through an analysis of the VTEC series over a span of five years, it is revealed that the long-term TEC variations are quite dependent on the solar activity intensity in terms of the F10.7 index. Besides, the BDS GEO-derived VTEC variation trend is consistent with those acquired from the global ionospheric map (GIM) and the NeQuick model. However, significant biases exist at some stations due to uneven accuracy of the two global ionospheric models. The maximum RMS discrepancies reach 4.3 TECU and 9.9 TECU for the GIM and NeQuick, respectively.

Electron density retrieval from occulting GNSS signals using a gradient-aided inversion technique

In the coming years, opportunities for remote sensing of electron density in the Earth’s ionosphere will expand with the advent of Galileo, which will become part of the global navigation satellite system (GNSS). Methods for accurate electron density retrieval from radio occultation data continue to improve. We describe a new method of electron density retrieval using total electron content measurements obtained in low Earth orbit. This method can be applied to data from dual-frequency receivers tracking the GPS or Galileo transmitters. This simulation study demonstrates that the method significantly improves retrieval accuracy compared to the standard Abel inversion approach that assumes a spherically symmetric ionosphere. Our method incorporates horizontal gradient information available from global maps of Total Electron Content (TEC), which are available from the International GNSS Service (IGS) on a routine basis. The combination of ground and space measurements allows us to improve the accuracy of electron density profiles near the occultation tangent point in the E and F regions of the ionosphere.

Effects of horizontal gradients on GPS radio occultation observation operators. I: Ray tracing

AbstractGlobal Positioning System (GPS) radio occultation (RO) bending angle and refractivity data provide information about the vertical structure of temperature and humidity in the earth's atmosphere. Assuming spherical symmetry in a GPS RO observation operator creates errors when calculating GPS RO observables from a background. These errors find their source in the presence of horizontal gradients of temperature and humidity. Tangent point drift (TPD) causes occulting rays to scan the atmosphere in various occultation planes. Horizontal gradients along the ray (HGR) cause the bending of each ray to be different from calculations that assume spherical symmetry.Using simulations, we show that HGR‐induced r.m.s. errors are larger than TPD‐induced r.m.s. errors in the lower troposphere only. TPD‐induced errors are more important in the stratosphere. We account for TPD and HGR using a multi‐plane two‐dimensional (2D) ray‐tracing observation operator. We show that bending angle and refractivity can be calculated more accurately with this 2D operator than an operator that assumes spherical symmetry. We use 6335 CHAMP and SAC‐C occultations as a validation. These occultations were processed by the Jet Propulsion Laboratory using geometrical optics and canonical transform methods. Our results indicate that analyses and 6‐hour forecasts can provide sufficiently accurate horizontal gradient information to the 2D operator. We show that the effects due to HGR can be satisfactorily simulated using ray tracing in one plane only. The effects due to TPD can be accounted for by considering a slanted profile which follows the tangent point trajectory. These results have important implications for the development of fast observation operators for data assimilation in numerical weather prediction. Copyright © 2004 Royal Meteorological Society

Assimilation of Scalar Versus Horizontal Gradient Information from the VAS into a Mesoscale Model

Comparisons are made between analyses and forecasts which incorporate VAS geopotential data as either scalar or horizontal gradient information for a case study on the AVE/VAS day of Mar. 6, 1982. On this day, incorporating the VAS information in analysis as a variational constraint on horizontal geopotential gradients significantly mitigated the effects of large data biases which made VAS assimilation by standard scalar methods very difficult. A subsequent forecast made from the gradient assimilation was superior to one made from the standard analysis and of comparable quality in geopotentials to a control forecast from synoptic data. Most impact was noted in the forecasts of vertical motion and precipitation in the gradient vs this control simulation.

A Bernoulli Inverse Method for Determining the Ocean Circulation

Abstract A new inverse method for finding the large-scale ocean circulation is described. Unlike most previous methods it uses no horizontal gradient information, and is designed for widely spaced data. The method assumes that density, (linear) potential vorticity and Bernoulli function are all approximately conserved on a streamline, so that the Bernoulli function depends solely on density and potential vorticity, both of which are known from data. The requirement that Bernoulli functions should match at points where density and potential vorticity match then leads to a heavily overdetermined problem for the surface pressure field, solved by a singular-value decomposition. The method is tested on an analytical solution due to Welander, and on the results of a numerical circulation model of Cox and Bryan, before use on climatological data, both over the main North Atlantic and over the beta triangle area. Analysis shows that for closely spaced data, the Bernoulli method reduces to beta-spiral dynamics, bu...