Model Calculations of Solar Spectral Irradiance in the 3.7-μm Band for Earth Remote Sensing Applications

Abstract

Since the launch of the first Advanced Very High Resolution Radiometer (AVHRR) instrument aboard the Television and Infrared Observational Satellite (TIROS-N), measurements in the 3.7-m atmospheric window have been exploited for use in cloud detection and screening, cloud thermodynamic phase and surface snow/ice discrimination, and quantitative cloud particle size retrievals. The utility of the band has led to the incorporation of similar channels on a number of existing satellite imagers and future operational imagers. Daytime observations in the band include both reflected solar and thermal emission energy. Since 3.7-m channels are calibrated to a radiance scale (via onboard blackbodies), knowledge of the top-ofatmosphere solar irradiance in the spectral region is required to infer reflectance. Despite the ubiquity of 3.7-m channels, absolute solar spectral irradiance data come from either a single measurement campaign (Thekaekara et al.) or synthetic spectra. In the current study, the historical 3.7-m band spectral irradiance datasets are compared with the recent semiempirical solar model of the quiet sun by Fontenla et al. The model has expected uncertainties of about 2% in the 3.7-m spectral region. The channel-averaged spectral irradiances using the observations reported by Thekaekara et al. are found to be 3.2%-4.1% greater than those derived from the Fontenla et al. model for Moderate Resolution Imaging Spectroradiometer (MO-DIS) and AVHRR instrument bandpasses; the Kurucz spectrum, as included in the Moderate Spectral Resolution Atmospheric Transmittance (MODTRAN4) distribution, gives channel-averaged irradiances 1.2%-1.5% smaller than the Fontenla model. For the MODIS instrument, these solar irradiance uncertainties result in cloud microphysical retrieval uncertainties that are comparable to other fundamental reflectance error sources.

Evaluating MODIS cloud retrievals with in situ observations from VOCALS-REx

“…Given the slightly different nature of the 3.7 µm channel in that it includes a thermal component and has no on board measurement of downwelling solar irradiance it is plausible that these factors result in a difference in retrieval bias. Platnick and Fontenla (2008) highlighted the uncertainties involved in estimating the solar irradiance in the 3.7 µm channel given the lack of available observations and showed that using different estimations of this quantity can result in around a 5 % shift in r e retrievals. A shift of this order for instance could significantly change the relationship between r 2.1 and r 3.7 examined in this study.…”

“…Using an optimal estimation technique the thermal and solar components do not need to be separated, instead a surface temperature and temperature profile is used by the forward model to include thermal contributions to simulated measurements. In order to model the solar component of the radiance a reference value of the top of atmosphere solar irradiance is calculated from the parametrisation of Platnick and Fontenla (2008) and used as input to the forward model.…”

Solar Surface Magnetism and Irradiance on Time Scales from Days to the 11-Year Cycle

“…Reconstructions of solar UV irradiance based on NLTE spectral synthesis of radiance spectra have also being progressing. Besides, first encouraging tests of the models have been carried out in the IR (Fontenla et al 2006;Platnick and Fontenla 2008;Unruh et al 2008). More work is still needed in these directions.…”

Bathymetry From Worldview-3 Satellite Data Using Radiometric Band Ratio

“…Subsequently, in order to reduce a local variation and remove noise, a low pass filter (LPF) was applied to three different maps [31]. LPF, implemented in ArcMap software, calculates the average (mean) value for each 3 × 3 neighbourhood.…”

“…The values of band-averaged solar spectral irradiance used for the Atmospheric Correction (AC) are reported in Table 3 [31].…”