The Limb Adjustment of Observations from the TROPICS Microwave Sounder Constellation

The NASA TROPICS (Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats) mission represents a significant advancement in the monitoring of tropical cyclones using a constellation of cubesats. These small satellites, equipped with scanning microwave radiometers, are designed to measure key atmospheric parameters such as temperature, humidity, precipitation, and cloud properties. The mission's primary objective is to enhance the temporal resolution of observations to support nowcasting applications, particularly for monitoring atmospheric rivers and tropical cyclones.

The TROPICS constellation operates in low Earth orbit with a low inclination angle, allowing it to observe the diurnal cycle across latitudes of approximately ±40 degrees. This capability provides valuable insights into the diurnal variations of temperature and water vapor in the tropics, contributing to improved weather nowcasting and a deeper understanding of tropical cyclone dynamics. This study focuses on the adjustment of TROPICS' cross-scan instruments to a common viewing geometry (nadir angle), which is crucial for accurate nowcasting.

The limb effect arises due to the increasing atmospheric path length as the instrument scans away from the nadir. This effect leads to variations in brightness temperatures across different viewing angles, which can complicate the use of the data in certain applications. To standardize these observations to a common viewing geometry, a limb adjustment algorithm originally developed for the NOAA Atmospheric Sounding Unit (AMSU) is applied. This two-step algorithm first calculates coefficients to adjust the off-nadir weighting functions to match the nadir weighting function. These coefficients are then refined using real observations to account for systematic biases not captured by the physical radiative transfer algorithm. Furthermore, the algorithm allows adjustments to different viewing angles, not just nadir, to enable imagery for additional layers in the atmosphere, which will also be demonstrated. Limb adjustments are particularly important for applications involving imagery, such as water vapor monitoring, where consistent viewing geometry is necessary for tracking the flow and transport of moisture in the atmosphere. Moreover, the limb-adjusted data facilitate the averaging of observations into level 3 grids, which can be used for a variety of other applications, including climate studies and model comparisons.

In addition to nowcasting, the study will include comparisons with the Advanced Technology Microwave Sounder (ATMS) and simulated brightness temperatures from the ECMWF ERA5 reanalysis. These comparisons are essential for assessing the accuracy and stability of TROPICS observations. By comparing TROPICS data with these established datasets and models, we aim to identify and understand any discrepancies. ATMS can serve as a in orbit calibrator for cubesat/smallsat MW constellations.

The limb adjustment coefficients are now being included as part of the official NASA TROPICS set of products. We are working with the National Weather Service (NWS) to demonstrate the practical use of limb-adjusted TROPICS data in operational nowcasting. Additionally, we plan to further explore the potential of TROPICS data for improving weather and climate models, especially in understanding the diurnal variations and discrepancies in water vapor measurements.

To summarize, at the conference, we will discuss the limb adjustment methodology, demonstrate the application of limb-adjusted TROPICS observations for nowcasting, and assess the accuracy and stability of TROPICS measurements through comparisons with ATMS data over a one-year period, as well as with ECMWF ERA5 simulated brightness temperatures.