Simulations of the night-time stable boundary layer over snow
Hannu Savijärvi, U.Helsinki
(manuscript submitted to QJRMS)
 
The nocturnal stable boundary layer (SBL) over typical snow was studied via 
high resolution 1-D simulations in late winter clear-sky weather at 67N, 
varying the wind speed. A windy nocturnal SBL was relatively high and nearly 
isothermal. Turbulent cooling rates were moderate but LW cooling rates weak 
as the temperature and moisture gradients remained small. In a calm SBL the 
surface temperature Ts dropped rapidly in the evening, leading to a steep 
surface inversion in T and q. The resulting strong LW cooling dominated the 
SBL evolution in this case. 
 
During weak wind (Vg = 1.5 - 4 m/s) Ts decreased fairly rapidly in the 
evening, with T2m closely coupled. The nocturnal inversion was moderately 
strong with strong turbulent cooling, especially in the surface layer. 
The T,q-profiles induced moderate LW cooling at mid-inversion levels but 
LW heating instead in the lowest metres, thereby resisting the strong 
turbulent cooling of the surface layer. 
 
The snow surface was coldest after calm nights and warmer at windy sunrises, 
whereas the coldest air temperatures were obtained during weak wind (as 
observed e.g. in SHEBA), provided that the model's stability functions were 
of the 'short tails' type.  A maximum in the downward sensible heat flux was 
associated with this. 
 
When the model's multi-layer snow scheme was replaced by a coarse scheme, 
the nocturnal Ts and T2m stayed too warm. The use of old snow properties 
instead of typical snow produced warmer nocturnal temperatures, whereas 
fresh snow led to quite cold temperatures.