Observing System Simulation of Snow Microwave Emissions Over Data Sparse Regions—Part II: Multilayer Physics

Abstract

A multilayer formulation of snow hydrological processes implemented in an existing snow hydrology-emission model (MLSHM-ML) was applied in observing system simulation mode (OSS) to two very different climatic and physiographic regions (Valdai, Russia and Colorado, USA) for both wet and dry snow regimes, and over multiple years. The results were evaluated against ground-based observations of snowpack physical properties and microwave radiometric observations from the Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave/Imager (SSM/I), and Advanced Microwave Scanning Radiometer-EOS (AMSR-E) observations at 18-19-, 22-23-, and 36-37-GHz vertical and horizontal polarizations (V-pol and H-pol, respectively). Whereas snow water equivalent (SWE) results are similar to the results obtained with single-layer physics when the snow is dry, the multilayer physics have a better skill at capturing the overall temporal evolution of bulk density, snow temperature, and snow depth during the accumulation season, and at the onset and throughout the melting season. However, snow density profiles overestimate density at the bottom of the snowpack, consistent with the lack of an explicit representation of depth hoar in the rearrangement of mass and grain size distribution in the snowpack. Regarding the radiometric behavior, the multilayer SMMR OSS for Valdai shows improved results for nighttime simulations (descending SMMR paths, 11 P.M. LST) and H-pol (~ 3-5 K decrease in error statistics), particularly at 37 GHz. For daytime simulations (ascending SMMR paths, 11 A.M. LST), there are modest improvements at 18 (~ 1 K) and 37 GHz (~ 2-3 K) for H-pol, and generally loss of skill for V-pol at all frequencies. Systematic improvements at nighttime but not during daytime suggest that surface heterogeneities, including subgrid scale variability of transient melting, play an important role on surface emissivity. This is the case for cold land process experiment in Colorado, where spatial variability in fractional forest cover, geology, and complex topography explains the modest differences between the single and multilayer SSM/I OSS for H-pol, whereas significant gains (~ 4-8 K decrease in error statistics) were attained for V-pol at 37 GHz only. For AMSR-E, the multilayer OSS looses skill for H-pol, and only bias and mean absolute error improve for V-pol at all frequencies. These somewhat mixed results suggest that representation of snow stratigraphy alone is not sufficient to improve the OSS ability to describe the nonlinear interactions among hydrologic and electromagnetic processes. Chief among these are the temporal evolution of snow correlation length with depth and the representation of subgrid scale variability constrained by the spatial resolution and inherent uncertainty of the meteorological forcing. Nevertheless, the multilayer OSS improved performance at 37 GHz is an important finding toward reducing ambiguity in the sensitivity of 37-GHz H-pol brightness temperature to SWE in retrieval models.