Compact Mid-Wave Infrared Instrument Design Evaluation for Temperature and Humidity Sounding of the Troposphere

Humidity and temperature profiles produced from infrared (IR) instruments provide essential inputs to our weather forecasting system. Current sounders typically utilize long-wave IR bands, specifically the 6.3-μm band for humidity sounding and the 15-μm CO₂ band for temperature profiles. However, observing these long wave IR (LWIR) bands requires the use of large optical systems and very cold detectors (40-60K). Recently, there has been increased interest in mid-wave IR (MWIR) sounders, which have the potential to be smaller and less complex than their LWIR counterparts.

We present a trade analysis of MWIR sounders to determine the optimal configuration and trade-offs in the sounder design. In our analysis, we model dispersive spectrometer configurations with multi-order split-band designs. This configuration efficiently packs multiple bands onto limited focal plane space resulting in a more compact spectrometer. Additionally, the split-band spectrometer design provides more design degrees of freedom with which to trade spectral coverage and spectral resolution.

For this analysis, bands are chosen from the 4.3-μm CO₂ band  for atmospheric temperature calculation and the 5-μm H₂O band for atmospheric humidity profiles. LBLRTM is used as a forward model to produce upwelling radiation and relevant Jacobians, and an end-to-end sensor model is used to calculate instrument noise. Full sounding simulations are performed with a variety of sounder configurations, testing the vertical resolution and retrieval accuracy as they relate to spectral resolution, spectral coverage, and instrument noise. While other scenarios will be considered, we will focus on designs that optimize sounding in the lower troposphere.