Winds up to 20 km are NOT quite calm. They can max out to 60 m/s at height of 10 km, which is more than 3 times than enough to blow a solar aircraft far-far away. You have to choose proper meteo conditions for climbing and descending and plan the trajectory taking into account those winds to be able to land at a given place.
I've been analyzing GFS data a couple of years ago for similar project. The problem is that lack of energy demands to design a really low speed aircraft for low densities that you can find in stratosphere. At 10 km winds are stronger than in stratosphere and density is higer.
You can find the results in https://journals.sagepub.com/eprint/GVXWXDABPE6A8PNRGTBU/ful....
The whole aircraft model probably is not like yours and is subject to many conservative estimates, but I am pretty much sure that wind model is accurate in this publication and generalizable to different regions.
