https://xiaolan.in/research/Recent content in Research on Xiaolan XuHugoenhttps://xiaolan.in/research/freeze-thaw/Mon, 01 Jan 0001 00:00:00 +0000https://xiaolan.in/research/freeze-thaw/I work on detecting whether the land surface is frozen or thawed — the signal that marks the onset of spring and the timing of frost across the landscape. Frozen ground scatters microwaves very differently from thawed ground, which makes the transition legible from orbit. This thread began with the Aquarius mission and grew into an operational SMAP freeze–thaw science product — one of the clearest cases where a piece of wave physics becomes a number people rely on, season after season.https://xiaolan.in/research/wave-physics/Mon, 01 Jan 0001 00:00:00 +0000https://xiaolan.in/research/wave-physics/Underneath every retrieval are first-principles models of how microwaves scatter inside snow and off rough, vegetated ground — solving the wave physics of terrain from the ground up rather than fitting curves to data. This is the core of my expertise, and the work recognized with the 2020 URSI Santimay Basu Prize. It’s also the backbone that makes the rest possible: soil moisture, snow, and freeze–thaw are all only as trustworthy as the scattering physics that connects the measured signal to the ground.https://xiaolan.in/research/snow/Mon, 01 Jan 0001 00:00:00 +0000https://xiaolan.in/research/snow/For more than a decade I’ve worked on estimating how much water a snowpack holds — its snow water equivalent — from radar. More recent work uses the phase of radar signals across repeat passes (InSAR) to track seasonal snow. What ties the old and new threads together is the physics of how a microwave behaves once it enters a snowpack: scattering among the grains, bouncing between layers, and emerging carrying a record of the snow it passed through.https://xiaolan.in/research/soil-moisture/Mon, 01 Jan 0001 00:00:00 +0000https://xiaolan.in/research/soil-moisture/I develop the algorithms that read soil moisture from radar and radiometer measurements at L-band — the approach behind NASA’s SMAP mission, carried forward into the joint SMAP–NISAR algorithm, and extended to reflected navigation-satellite signals through CYGNSS, where I built JPL’s soil-moisture product. Across these missions the through-line is the same: a soil-moisture value is only as trustworthy as the wave physics that links the measured signal to the water in the ground.