Hoar crystal development and disappearance at Dome C, Antarctica: observation by near-infrared photography and passive microwave satellite

摘要

Hoar crystals episodically cover the snow surface inAntarctica and affect the roughness and reflective properties of the air–snowinterface. However, little is known about their evolution and the processesresponsible for their development and disappearance despite a probableinfluence on the surface mass balance and energy budget. To investigate hoarevolution, we use continuous observations of the surface by in situnear-infrared photography and by passive microwave remote sensing at Dome Cin Antarctica. From the photography data, we retrieved a daily indicator ofthe presence/absence of hoar crystals using a texture analysis algorithm. Theanalysis of this 2 yr long time series shows that Dome C surface is coveredalmost half of the time by hoar. The development of hoar crystals takes a fewdays and seems to occur whatever the meteorological conditions. In contrast,the disappearance of hoar is rapid (a few hours) and coincident with eitherstrong winds or with moderate winds associated with a change in winddirection from southwest (the prevailing direction) to southeast. From themicrowave satellite data, we computed the polarisation ratio (i.e. horizontalover vertical polarised brightness temperatures), an indicator known to besensitive to hoar in Greenland. Photography data and microwave polarisationratio are correlated, i.e. high values of polarisation ratio whichtheoretically correspond to low snow density values near the surface areassociated with the presence of hoar crystals in the photography data.Satellite data over nearly ten years (2002–2011) confirm that a strongdecrease of the polarisation ratio (i.e. signature of hoar disappearance) isassociated with an increase of wind speed or a change in wind direction fromthe prevailing direction. The photography data provides, in addition,evidence of interactions between hoar and snowfall. Further adding thecombined influence of wind speed and wind direction results in a complexpicture of the snow–atmosphere interactions in Antarctica which deservesfurther quantification and modelling.