Enhanced Identification of Snow Melt and Refreeze Events From Passive Microwave Brightness Temperature Using Air Temperature

Abstract

Snow melt and refreeze events are important determinants of spring runoff timing, and snowpack stratigraphy and metamorphism. Previous studies have established the utility of differences between twice‐daily passive microwave brightness temperature (Tb) observations, called the diurnal amplitude variation (DAV), for identifying snow melt and refreeze. Liquid water in snow leads to a large increase in microwave emissivity compared to a completely frozen snowpack, so phase changes from nighttime freezing and daytime melting result in high DAV values. However, the physical temperature of the land surface also contributes to brightness temperature, independent of the phase of water. Thus, it is important to account for physical temperature change when using Tb differences to detect snow melt and refreeze. Here, we use near‐surface air temperature (Ta) to approximate the physical temperature of the land surface and compare diurnal Tb changes (ΔTb) from the Advanced Microwave Scanning Radiometer for the Earth Observing System satellite instrument to coincident Ta changes. We find that an approximately linear relationship exists between ΔTb and ΔTa for frozen snow and fit this relationship using modal linear regression. Melt and refreeze events are identified as large positive and negative excursions from the regression line, respectively. We demonstrate the method in the Northern Great Plains, USA, and evaluate it using ground‐based data from Senator Beck Basin Study Area, Colorado, USA. Melt and refreeze events identified from satellite observations mostly occur after the annual peak snow accumulation and are consistent with snow temperature and snowpack energy balance observations at Senator Beck Basin.

Snowmelt Detection with Calibrated, Enhanced‐Resolution Brightness Temperatures (CETB) in Colorado Watersheds

“…This comparison demonstrates the feasibility of using this definition of an observed melt event. Furthermore, other studies have used air temperature as a proxy for melt detection or shown that melt events correspond with air temperature above 0°C (Sturdivant et al, 2019;Tuttle & Jacobs, 2019).…”

“…Changes in climate, land cover, and population growth in the western United States create a need for more information related to mountain hydrology (Bales et al, 2006). In addition to the main melt onset timing crucial for spring runoff determination, it is also important to have an understanding of winter melting events (Semmens et al, 2013), especially in mid-latitude environments prone to snowpack variations that influence the snow stratigraphy and underlying soil layers (Tuttle & Jacobs, 2019). Satellite-based snowmelt products may decrease the reliance on ground measurements, which cannot capture the spatial and temporal variability of snowpack properties in high-relief and heterogeneous terrain.…”

“…Seasonal snowmelt, defined here as the period in which snowmelt leads to the disappearance of the snowpack, begins with diurnal cycles of daytime melting and overnight refreeze. Isolated early melt events also occur during winter at mid-latitudes that precede seasonal snowmelt (Tuttle & Jacobs, 2019). Passive microwave radiometers have been used to detect a melting snowpack due to the difference in physical properties of wet and dry snow (Burke et al, 1984).…”

“…Other studies have used passive microwave DAV methods for snowmelt detection in Arctic and Antarctic settings (Tedesco, 2007;Tedesco et al, 2007Tedesco et al, , 2009, and there has been some work with passive microwaves and snowmelt detection in mid-latitude environments with seasonal snow cover, such as the Sierra Nevada Mountains, CA (Li et al, 2012). Semmens et al (2014) showed a high DAV signal corresponding with the melt season at Senator Beck Basin, CO, and Tuttle and Jacobs (2019) recently used AMSR-E data to further examine snowmelt in that basin. Additionally, previous work has successfully used the DAV method for detection of early melt events (EMEs) in the Yukon River Basin, defined as shorter duration melt events that precede the seasonal MOD (Semmens et al, 2013).…”