An Assessment of SAPHIR Calibration Using Quality Tropical Soundings

Abstract

The Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie (SAPHIR) instrument on board the Megha-Tropiques (MT) platform is a cross-track, multichannel microwave humidity sounder with six channels near the 183.31-GHz water vapor absorption line, a maximum scan angle of 42.968 (resulting in a maximum incidence angle of 50.78), a 1700-km-wide swath, and a footprint resolution of 10 km at nadir. SAPHIR L1A2 brightness temperature (BT) observations have been compared to BTs simulated by the radiative transfer model (RTM) Radiative Transfer for the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (RTTOV-10), using in situ measurements from radiosondes as input. Selected radiosonde humidity observations from the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year (CINDY)-Dynamics of the Madden-Julian Oscillation (DYNAMO) campaign (September 2011-March 2012) were spatiotemporally collocated with MT overpasses. Although several sonde systems were used during the campaign, all of the sites selected for this study used the Vaisala RS92-SGPD system and were chosen in order to avoid discrepancies in data quality and biases.To interpret the results of the comparison between the sensor data and the RTM simulations, uncertainties associated with the data processing must be propagated throughout the evaluation. The magnitude of the bias was found to be dependent on the observing channel, increasing from 0.18 K for the 183.31 6 0.2-GHz channel to 2.3 K for the 183.31 6 11-GHz channel. Uncertainties and errors that could impact the BT biases were investigated. These can be linked to the RTM input and design, the radiosonde observations, the chosen methodology of comparison, and the SAPHIR instrument itself.

A review of sources of systematic errors and uncertainties in observations and simulations at 183 GHz

“…For instance John and Buehler (2004) found that absorption by O 3 leads to a decrease of up to 0.5 K in the 183 ± 1 GHz channel of MHS using ARTS, with even smaller values for the other channels. Additional sensitivity tests performed by Clain et al (2015) using MonoRTM showed that the impact is also marginal in the 183.31 ± 11 GHz channel of SAPHIR (0.05 K on average). The details of how the molecular absorption is modeled may vary between different models, but the overall absorption is most commonly calculated for both the contribution near the line centers and the smoothly varying continuum.…”

“…More recently, Chen et al (2010) have shown that, on a sample of diverse atmospheric profiles, fast and line-by-line RTMs agree within 0.1 K for MHS channels, and that this result was highly stable under humid conditions. In their evaluation of SAPHIR using RAOBs, Clain et al (2015) have shown that the three RTMs, RT-TOV.v10, ARTS and MonoRTM (Monochromatic Radiative Transfer Model; Clough et al, 2005;Payne et al, 2011), provide fairly consistent BTs on a common set of tropical profiles, the differences being in the range −1.50 K / 0.78 K, with the largest differences observed for the central channel (183 ± 0.2 GHz). These three RTMs rely on the currently most widely accepted model MT_CKD (MlawerTobin_CloughKneisysDavies;Mlawer et al, 2012) for the parametrization of the absorption due to the water vapor continuum.…”

“…However, comparing the measurements to radiative transfer model (RTM) calculations, using profiles of temperature and humidity either from radiosondes (radiosonde observerations, RAOBs) or from numerical weather prediction (NWP) models, shows a channeldependent bias increasing from the center to the wings of the line. It was only with the arrival of ATMS and SAPHIR, both launched in October 2011, that the spectral shape of the bias became clear (Clain et al, 2015;Moradi et al, 2015). Indeed, these two instruments sample the 183.31 GHz line five and six times respectively between the line center (providing humidity information for the upper troposphere, above 300 hPa) and line wings (up to 11 GHz from the line center, for lower tropospheric sounding) compared to only three times for SSMI/S, MHS and AMSU-B (Advanced Microwave Sounding Unit-B that preceded MHS onboard NOAA 15, 16 and 17).…”

Estimating Confidence Intervals around Relative Humidity Profiles from Satellite Observations: Application to the SAPHIR Sounder

“…(Ciesielski et al 2014). Here, we use the RS92 selected by Clain et al (2015) These two sets have been already explored for the evaluation of the upper-tropospheric humidity products derived from SAPHIR , and here we use the entire profiles. The cumulative distribution functions of the RH of these two sets are presented in Fig.…”

“…10%; « 1 5 60:03 3 RH for RH , 10%); and 2) a ground-check calibration uncertainty term « 2 different between day (« 2 5 60:05 3 RH 1 0:5) and night (« 2 5 60:04 3 RH 1 0:5). As discussed in Clain et al (2015) and following Immler et al (2010), these individual uncertainties can be reasonably considered as random and independent, and thus sum up for a total measurement uncertainty at a given point as « i 5 ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi « …”