Systematic underestimation of irradicance coming from the west for vertical bifacial east/west panels

[olipfeifferthkoeln]

Hello,
I am simulating the irradiance of vertical bifacial panels that are east/west oriented with bifacial_radiance. When I compare the results with yields from measuring cells, there is a systematic underestimation of the irradiacne values for the second half of the day, when the irradiance from the sun comes from the west. In the first half of the day however the results fit quite good. This is especially true for days with high shares of direct irradicance.

Do you have an idea how to fix this problem?

kind regards
Oliver Pfeiffer