The Cyclone Global Navigation Satellite System (CYGNSS) is a spaceborne mission concept focused on tropical cyclone (TC) inner core process studies. CYGNSS attempts to resolve the principle deficiencies with current TC intensity forecasts, which lies in inadequate observations and modeling of the inner core. CYGNSS consists of 8 GPS bistatic radar receivers deployed on separate nanosatellites. The primary science driver is rapid sampling of ocean surface winds in the inner core of tropical cyclones.

The Advanced Scatterometer (ASCAT) on the MetOp-A satellite is a radar instrument designed specifically to retrieve the ocean surface wind speed and direction. The ASCAT wind vector products are produced and utilized operationally in support of the National Oceanic and Atmospheric Administration (NOAA)'s weather forecasting and warning mission. The standard ASCAT winds at NOAA are produced using the ASCAT wind data processor developed at the Royal Netherlands Meteorological Institute (KNMI) utilizing the CMOD5.n geophysical model function (GMF). Recent validation of the ASCAT wind retrievals revealed a low bias at high wind speeds when compared to both the QuikSCAT winds and the National Centers for Environmental Prediction numerical weather prediction (NWP) model winds. The goal of this paper is to investigate the ASCAT high-wind-speed performance and to modify, as appropriate, the high-wind-speed portion of CMOD5.n GMF. This effort would potentially improve the utility of ASCAT wind retrievals in supporting wind warning and analysis and thus better mitigate the loss of QuikSCAT data products. Traditionally, the GMF is developed empirically by collocating scatterometer measurements and other truth data such as buoy and NWP model winds. However, NWP models are known to underestimate the intensity of higher wind speeds, and data sources such as ship-based or buoy-based observations provide an inadequate quantity of measurements for empirical GMF development. In this paper, a method utilizing aircraft-based scatterometer measurements in the high-wind-speed regimes is used in conjunction with satellite scatterometer measurements to refine the satellite GMF. As a result of this paper, a high wind C-band satellite GMF, CMOD5.h, was developed and implemented in NOAA's ASCAT processor. The validation comparison of the high wind and standard ASCAT wind products revealed 0.6-m/s reduction in the wind speed bias for winds greater than 15 m/s with respect to QuikSCAT, WindSat, and Step Frequency Microwave Radiometer high wind measurements.

With the operational deployment of the Stepped Frequency Microwave Radiometer (SFMR), hurricane reconnaissance and research aircraft provide near real-time observations of the 10 $\mathrm{m}$ ocean-surface wind-speed both within and around tropical cyclones. Hurricane specialists use these data to assist in determining wind radii and maximum sustained winds—critical parameters for determining and issuing watches and warnings. These observations are also used for post-storm analysis, model validation, and ground truth for aircraft- and satellite-based wind sensors. We present observations on the current operational wind-speed and rain-rate SFMR retrieval procedures in the tropical cyclone environment and propose suggestions to improve them based on observed wind-speed biases. Using these new models in the SFMR retrieval process, we correct an approximate 10% low bias in the wind-speed retrievals from 15 to 45 $\mathrm{m}$ $\mathrm{s}$ ⁻¹ with respect to GPS dropwindsondes. In doing so, we eliminate the rain-contaminated wind-speed retrievals below 45 mm $\mathrm{h}$ ⁻¹ at tropical storm- and hurricane-force speeds present in the current operational model. We also update the SFMR radiative transfer model to include recent updates to smooth-ocean emissivity and atmospheric opacity models. All corrections were designed such that no changes to the current SFMR calibration procedures are required.

Measurements have been made on the electron excitation coefficients of four Ar levels: 2${\mathit{p}}_1$, 2${\mathit{p}}_5$, 2${\mathit{p}}_7$, and 2${\mathit{p}}_9$. Radiative decay of these levels to 1s levels is responsible for the strongest line features in argon spectra, at 750 nm due to radiation from 2${\mathit{p}}_1$ and 2${\mathit{p}}_5$ levels, and at 811 nm due to transitions from 2${\mathit{p}}_7$ and 2${\mathit{p}}_9$ levels. We have measured excitation coefficients of these levels in the range of E/N from 50 Td to 9 kTd (1 Td=${10}^{\mathrm{-}21}$ V ${\mathrm{m}}^2$). Here E is the electric field and N is the gas density. The optical emission intensities normalized to the local current density were placed on the absolute scale after normalizing the data for the 2${\mathit{p}}_{1\mathrm{-}}$1${\mathit{s}}_2$ transition (750.4 nm) to the previous experimental and theoretical results at 50 Td. We have also made measurements of the excitation coefficients of several 4d and 6s vs E/N, in order to estimate the effect of cascading from upper levels to 2p levels. We have found that the excitations of 2${\mathit{p}}_9$ and 2${\mathit{p}}_7$ are influenced by collisional population transfer between 2p levels at low E/N (E/N2.7×${10}^{23}$ ${\mathrm{m}}^{\mathrm{-}3}$), by cascading from 3d and 4s levels above 100 Td, and by cascading from 4d and 6s levels at E/N>3 kTd.

A cyclone global navigation satellite system (CYGNSS)calibration analysis is presented using version 2.0 of the Level 1 dataset available on PO.DAAC. Three separate analyzes are conducted, namely, an examination of the specular bin location (in delay) andrelationship, an investigation of the impact of recently improved characterizations of the GPS effective isotropically radiated power on CYGNSS, and an intersatellitecalibration analysis. We first noted a correlation between the specular delay bin location and, where an increase in the specular delay bin resulted in an increase inregardless of the wind speed level; a specular delay bin location ranging from 4.75 to 5.00 and from 7.00 to 7.25 resulted in a 14.74 and 17.72 dB median, respectively. Noticeable improvements in the medianwere present in the version 2.0 dataset, when separating the data by GPS block type: for blocks IIR, IIF, and IIR-M, medianwere 15.39, 15.42, and 15.10 dB, respectively (compared to 19.38, 20.53, and 21.38 dB in version 1.1). Finally, an unexpected correlation between the instrument noise floor andwas observed for all eight observatories while conducting the intersatellitecalibration analysis. Approximately 0–1 dB absolutedifference biases (with up to1 dB standard deviation) between spacecrafts were observed. A report of this analysis was presented to CYGNSS scientists and engineers, who eventually found an issue with the D/A DDM scaling algorithm. We expect better statistical performances in future releases of the data.

The NASA EV-2 Cyclone Global Navigation Satellite System (CYGNSS) is a spaceborne mission focused on tropical cyclone (TC) inner core process studies. CYGNSS attempts to resolve the principle deficiencies with current TC intensity forecasts, which lies in inadequate observations and modeling of the inner core. The inadequacy in observations results from two causes: 1) Much of the inner core ocean surface is obscured from conventional remote sensing instruments by intense precipitation in the eye wall and inner rain bands. 2) The rapidly evolving (genesis and intensification) stages of the TC life cycle are poorly sampled in time by conventional polar-orbiting, wide-swath surface wind imagers. CYGNSS is specifically designed to address these two limitations by combining the all-weather performance of GNSS bistatic ocean surface scatterometry with the sampling properties of a constellation of satellites.