Pressure drag and effective roughness length with neutral stratification

A two-dimensional numerical mesoscale model is used to determine the pressure drag of sinoidal
mountains and valleys in a neutral atmosphere. In the first part, pressure distributions and flow patterns
for isolated obstacles are considered. For large aspect ratios, the pressure drag exerted by valleys becomes
small compared to that of mountains. In the second part, interactions between several obstacles are
investigated. For mountains, the drag on downstream obstacles is reduced considerably by the first obstacle
when the obstacles are close together. For valleys there is a slight increase of the average drag exerted by
each obstacle. In the limit for a large number of obstacles, average drag exerted by one mountain is equal
to average drag for one valley. For smaller aspect ratios, this average drag can be entered into the resistance
law from the Rossby number similarity theory to yield an ‘effective roughness length’.