# The Microwave Temperature and Humidity Profiler: Description and Preliminary Results
## 1 Introduction  
The recent National Academies of Sciences, Engineering, and Medicine 2017-2027 Decadal Survey (DS) for Earth Science and Applications from Space (NASEM, 2018) identified the Planetary Boundary Layer (PBL) as a Targeted Observable (TO) [1]. TOs are identified as high-priority observables for which cost-effective spaceborne observing strategies are not yet mature. PBL is the lower part of the troposphere that extends from theEarth's surface up to the capping inversion layer or temperature inversion layer, typically located at a height of 1-2 km [2]. This region defines the boundary between the surface-influenced atmosphere and the free atmosphere, thereby mediating fluxes of energy, water, carbon, and other constituents within the Earth system. Given the interdisciplinary nature of PBL science, it is not surprising that interest in PBL arose out of multiple DS panels including Weather and Air Quality, Climate, and Hydrology. The key PBL observables were identified as vertical profiles of temperature and moisture (water vapor), along with PBL height [3]. Four PBL science themes have been identified: Convective PBL, cloudy PBL, PBL and surface interaction, and PBL mixing and air quality [3]. To address these science themes, a mix of observation architectures is required, including ground-based, suborbital, and orbital components. To date, four different types of instruments can be used to directly or indirectly study the PBL: Differential Absorption Lidar (DIAL) [4], Differential Absorption Radar (DAR) [5; 6], hyperspectral infrared imagers [7], and hyperspectral microwave receivers. The first two types of instruments, DIAL and DAR, are active instruments, and they require sending high-energy pulses to receive their echo. Instruments that rely on high-power components tend to require large vehicles, limiting their suitability for scalable small satellite constellations. On the other hand, hyperspectral infrared is a technique proven for ground-based instruments [7], but not for top-of-the-atmosphere measurements, which prevents the technique from being implemented globally. Thermal infrared spectrometers have been also implemented in space through the Infrared Atmospheric Sounding Interferometer (IASI) [8], showing accuracies of 1 K, and spatial resolutions of 1 km. However, this type of system is limited only to cloud-free regions. A solution for retrievals in cloud-covered regions is the use of microwave receivers. Low-spectral resolution radiometers operating near the 50-60 GHz or 118 GHz oxygen absorption lines have been used to retrieve coarse-resolution vertical temperature profiles at \(\sim\)1-4 km vertical resolution [9; 10; 11; 12] from the ground, airborne, and spaceborne platforms. Similarly, systems operating near the 183 GHz water vapor absorption line reliably profile air humidity. However, the lack of a finer frequency resolution and multi-angular measurements limits the retrieval of high-resolution vertical profiles, therefore limiting the ability to precisely measure PBL position based on such vertical temperature profiles. In this manuscript, we present the Microwave Temperature and Humidity Profiler (MTHP) [13] developed at the Jet Propulsion Laboratory (JPL) and deployed in the Technological Innovation into Iodine and GV aircraft Environmental Research (TI3 GER) campaign [14] led by the University Corporation for Atmospheric Research/National Center for Atmospheric Research (UCAR/NCAR). MTHP is a multi-incidence angle spectroradiometer at 60 GHz and 183 GHz that aims at retrieving vertical temperature and humidity profiles to enable PBL studies from the top of the atmosphere.