Intra-surface radiative transfer limits the geographic extent of snow penitents on horizontal snowfields

Abstract

AbstractPenitents are broad snow spikes and ridges that range in height between centimeters and meters. Two key features of penitents remain unexplained: (1) they generally form at low latitudes and (2) their ridges and troughs have an east–west orientation. Here we show that surface-to-surface exchange of shortwave radiation and the local geometry of the sun’s daily arc across the sky are the key processes that, in the absence of other effects, determine the geographic extent of where surface roughness features can grow or dissipate on snowfields and ice surfaces. As an application of our analysis, we examine the question of the geographic extent of snow penitents on horizontal surfaces. The results show that surface-to-surface radiative transfer can explain why penitents do not form on horizontal surfaces more than ~55˚ off the equator. We further show on the basis of the sun’s path across the sky that penitents that are corrugated, i.e. occur in long ridges or rows, must align their ridge axis within 308 of the east–west transect.