roughness in CFD analysis

I study CFD analysis in SOFiSTiK. There is an example “high rise baines”. In dolfyn module there is parameter Zo -roughness. And when I change this parameter I get some curious results.

Zo=1.0 Positive pressure is 1.09

Zo=0.01 Positive pressure is 0.596

So pressure near the lake is lower than in city.


I didn’t change any other parameter. What did I do wrong?