Sigrid Kopfermann (June 30, 1923-June 25, 2011) was a German painter, graphic artist, and art teacher.
Biography on German Wikipedia: From Google Translate

Landschaft (Landscape) by Sigrid Kopfermann
1993 / Mixed media on canvas / 39-2/5″x27-2/5″ / Found on Van Ham auction site

Sigrid Kopfermann on Artnet: http://www.artnet.com/artists/sigrid-kopfermann/

Further reading: (all in Google Translate from German)
Sigrid Kopfermann
Sigrid Kopfermann was born in 1923…
Sigrid Kopfermann – An Obituary