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The National Oceanic and Atmospheric Administration (NOAA)-20 satellite, designated Joint Polar Satellite System-1 before launch, was successfully launched into a sun-synchronous orbit on 18 November 2017. The equator crossing time (ECT) of NOAA-20 is around 1330 local time for the ascending node, which covers the majority of the Earth twice daily. NOAA-20 carries the second Advanced Technology Microwave Sounder (ATMS). The first ATMS was onboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite, launched on 28 October 2011, also with an ECT of 1330 local time. While the ECTs of NOAA-20 and S-NPP are the same, the reference points on the equator of 1330 local time are different. ATMS is a cross-track microwave radiometer that observes radiances at a total of 22 channels for atmospheric temperature and moisture profiling with a spatial resolution of about 32 km for temperature sounding channels at nadir. The detailed channel features for the S-NPP ATMS, such as center frequencies, specifications, on-orbit noise equivalent delta temperature (NEDT), and so on, are listed in Table 1 (ATBD, 2013; Kim et al., 2014; Zou et al., 2014; Zou and Tian, 2018). The ATMS onboard NOAA-20 has exactly the same channel settings as the S-NPP ATMS but with substantially updated hardware, the impact of which is quantified in this study and discussed in later sections. The first set of NOAA-20 ATMS observational data was transmitted back to Earth on 29 November 2017.
Channel no. Frequency (GHz) Specification (K) NEDT (K) Beam width (°) Peak Weighting Function (hPa) 1 23.8 0.5 0.25 5.2 Surface 2 31.4 0.6 0.3 5.2 Surface 3 50.3 0.7 0.35 2.2 Surface 4 51.76 0.5 0.28 2.2 950 5 52.8 0.5 0.25 2.2 850 6 53.596 ± 0.115 0.5 0.27 2.2 700 7 54.4 0.5 0.25 2.2 400 8 54.94 0.5 0.25 2.2 250 9 55.5 0.5 0.28 2.2 200 10 57.29 0.75 0.4 2.2 100 11 57.29 ± 0.217 1 0.53 2.2 50 12 57.29 ± 0.322 ± 0.048 1 0.55 2.2 25 13 57.29 ± 0.322 ± 0.022 1.25 0.82 2.2 10 14 57.29 ± 0.322 ± 0.010 2.2 1.13 2.2 5 15 57.29 ± 0.322 ± 0.0045 3.6 1.8 2.2 2 16 88.2 0.3 0.27 2.2 Surface 17 165.5 0.6 0.39 1.1 Surface 18 183.31 ± 7.0 0.8 0.35 1.1 800 19 183.31 ± 4.5 0.8 0.41 1.1 700 20 183.31 ± 3.0 0.8 0.48 1.1 500 21 183.31 ± 1.8 0.8 0.53 1.1 400 22 183.31 ± 1.0 0.9 0.68 1.1 300 Table 1. ATMS channel characteristics.
Spaceborne microwave remote sensing observations, such as ATMS, are a key data type for numerical weather prediction (NWP). Zou et al. (2013) demonstrated that the assimilation of ATMS radiances into the Hurricane Weather Research and Forecasting model helps to improve both the hurricane track and intensity forecast performance. Previous studies have also shown positive impacts on global weather forecast skill brought by the Advanced Microwave Sounding Unit-A (AMSU-A), which is the predecessor of ATMS (Eyre et al., 1993; Andersson et al., 1994; Derber and Wu, 1998; Qin et al., 2012). Besides being a part of observational inputs for NWP models, their applications also include retrieving surface temperatures, atmospheric temperatures, total precipitable water, liquid water paths, and ice water paths under almost all weather conditions except for heavy precipitation. Tian and Zou (2016) showed that the measurements from both AMSU-A and ATMS can be used to analyze the three-dimensional hurricane warm-core structures with a temperature profile retrieval algorithm they proposed. Tian and Zou (2018) combined the microwave temperature sounder instruments on multiple satellites to retrieve the three-dimensional warm-core structure temporal evolutions in Hurricanes Harvey, Irma, and Maria. Zou and Tian (2018) further refined the temperature retrieval algorithm by training the retrieval coefficients with Global Positions System (GPS) radio occultation (RO) temperature profiles for achieving better accuracies of the temperature retrieval products.
However, before any of these applications, the bias features of each channel have to be characterized. Any bias has to be properly quantified and then removed. Zou et al. (2014) characterized the noise and bias characteristics of the ATMS onboard S-NPP using NWP analysis/forecast fields. ATMS brightness temperatures (TBs) were simulated with atmospheric temperature and water vapor profiles from GPS RO observations as an input to the Community Radiative Transfer Model (CRTM). CRTM is known to be able to rapidly simulate radiances with an accuracy of less than 0.1 K for microwave sensors (Liu et al., 2013). It was shown that S-NPP ATMS biases for the temperature sounding channels 5–15 could be well characterized by GPS RO data. In this study, the in-orbit accuracy of the ATMS onboard both the recently launched NOAA-20 satellite and the S-NPP satellite were estimated using GPS RO level-2 retrieval profiles from the two Global Navigation Satellite System (GNSS) Receivers for Atmospheric Sounding (GRAS) onboard the Meteorological Operational (MetOp)-A and MetOp-B satellites (Gorbunov et al., 2011).
Since NOAA-20 operates in the same orbit as S-NPP but about 50 min ahead of it, NOAA-20 allows an important overlap in ATMS observational coverage. This gives meteorologists a new opportunity to obtain ATMS information at a half-hour interval, twice daily, for fast-evolving weather systems such as hurricanes. An example is shown in this regard for Hurricane Florence (2018).