Snow Wetness Retrieved from L-Band Radiometry

Abstract

Abstract:The present study demonstrates the successful use of the high sensitivity of L-band brightness temperatures to snow liquid water in the retrieval of snow liquid water from multi-angular L-band brightness temperatures. The emission model employed was developed from parts of the "microwave emission model of layered snowpacks" (MEMLS), coupled with components adopted from the "L-band microwave emission of the biosphere" (L-MEB) model. Two types of snow liquid water retrievals were performed based on L-band brightness temperatures measured over (i) areas with a metal reflector placed on the ground ("reflector area"-T B,R ), and (ii) natural snow-covered ground ("natural area"-T B,N ). The reliable representation of temporal variations of snow liquid water is demonstrated for both types of the aforementioned quasi-simultaneous retrievals. This is verified by the fact that both types of snow liquid water retrievals indicate a dry snowpack throughout the "cold winter period" with frozen ground and air temperatures well below freezing, and synchronously respond to snowpack moisture variations during the "early spring period". The robust and reliable performance of snow liquid water retrieved from T B,R , together with their level of detail, suggest the use of these retrievals as "references" to assess the meaningfulness of the snow liquid water retrievals based on T B,N . It is noteworthy that the latter retrievals are achieved in a two-step retrieval procedure using exclusively L-band brightness temperatures, without the need for in-situ measurements, such as ground permittivity ε G and snow mass-density ρ S . The latter two are estimated in the first retrieval-step employing the well-established two-parameter (ρ S , ε G ) retrieval scheme designed for dry snow conditions and explored in the companion paper that is included in this special issue in terms of its sensitivity with respect to disturbative melting effects. The two-step retrieval approach proposed and investigated here, opens up the possibility of using airborne or spaceborne L-band radiometry to estimate (ρ S , ε G ) and additionally snow liquid water as a new passive L-band data product.

Snow Density and Ground Permittivity Retrieved from L-Band Radiometry: Melting Effects

“…Another advantage of using correlations between V-mode retrievals as "quality flags" is the distinct drop in the population of un-flagged retrievals with the beginning of the "early spring period". This observation based on our measurements is seen as an additional method for detecting the time of the onset of moist snow and the beginning of the snow-melt-down season as a complement to the snow water content retrieval approach, as presented in the companion paper [40].…”

“…Figure 7a,c are for retrievals based on early "morning" (02:00-08:00) measurements, the time window in which the aforementioned retrieval assumptions (homogeneously frozen ground covered with dry snow) are best met. Figure 7b,d are for corresponding retrievals based on "afternoon" (12:00-18:00) measurements, the time window in which departures from assumptions made for the retrieval (due to "melting effects" that cause moist snow and varying ground permittivities among footprints as demonstrated in the companion paper [40]), as well as attendant retrieval distortions, are expected to be most prominent.…”

“…During the 2016/2017 winter campaign, L-band radiometry was performed over areas with a metal reflector placed on the ground. These measurements and the resulting snow liquid water content retrievals are thoroughly described in the companion paper [40]. Figure 1), respectively.…”

“…The presented period from 15 December 2016 to 15 March 2017 encompasses the "snow-free period", "cold winter period", and "early spring period". The vertical dashed lines on 3 and 31, January indicate the onset of the dry snow cover present throughout the entire "cold winter period" and the beginning of the "early spring period" defined by the first occurrence of measurable amounts of liquid snow water (see Section 5.3 in [41] and Section 4.1 in the companion paper [40]), respectively. Concurrent in-situ measured ground permittivity ε G (averaged over the two in-situ transects-the data as in Figure 2) and bottom-layer snow density ρ S .…”

“…Beyond the possibility of estimating snow liquid water using the sensitivity of L-band brightness temperatures to the snow moisture investigated in [41]. Therefore, the reader is strongly advised to consult the companion paper [40], which is closely linked to the analyses presented here.…”