[logback-user] How to close log files?

I am using the sifting appender to write a new log file for each pass of my program. The problem is that each log file appears to be still open even after the next log file is created. It is not possible to delete the log file. Windows reports that "the file is still open in Java(TM) platform SE binary". Only after shutting down the program I can delete the log files. Please advise how to close the log file after I am done with it so that I can delete it later.