Transport by Suspension

Field measurements of blowing snow show that the average snow particle becomes smaller with increasing height above the snow surface (Budd, 1966). The region above the saltation layer is characterized by smaller particles that are suspended in the air stream. These particles are small compared with the scale of turbulence and will at high wind speeds be suspended in the airflow. It is generally accepted that this phenomenon is explained by turbulent diffusion (Mellor and Radok, 1960).

Particles that go into suspension will depart from the snow surface as particles in saltation, but they will not return to the surface in the same manner. Vertical fluctuations of the turbulence are conquering the falling velocity and accordingly the particles will go into suspension. Paradoxically, even if the number of suspended particles is substantial, the total portion of transport of these small particles may be smaller than expected.

Snow transport by suspension in Narvik, Norway.

Average snow drift conditions indicate that transport by saltation is the most important drift factor, but at higher velocities transport by suspension becomes more significant. The table below gives the total distribution of mass flux up to a given height, for three different wind speeds. It can be seen from the table that 86% of the total mass flux is within the first 30 cm, at a wind speed at 12 m/s measured at the reference height of 10 m. At a wind speed of 22 m/s the portion of snow in diffusion will increase, and 86% of the total mass flux is now within the first 4.6 m. The measurements in the table below are based on measurements up to 10 m, which is sufficient for most practical purposes. It should be noted that suspended snow may rise hundreds of metres above the surface (Radok, 1977).

Height above the surface [m]	Percentage of total mass flux to the given height
	u10 =12 m/ s	u10 =17 m/ s	u10 =22 m/ s
10	100%	100%	100%
6.1	97	94	91
4.6	95	91	86
4.3	95	90	85
4.0	95	90	84
3.7	94	89	83
3.4	94	88	81
3.0	94	87	80
2.7	93	86	79
2.4	93	85	77
2.1	92	84	75
1.8	92	83	73
1.5	91	81	70
1.2	90	79	68
0.9	89	76	64
0.6	88	73	59
0.3	86	66	51
0.15	84	60	46

Field measurements of the distribution of total snow transport as a function of height and the velocity measured 10 m above the snow surface, u10. Data from Budd, Dingle and Radok, (Tabler, 1988).

Skriv ut

Sideoversikt