  In financial markets, liquidity is not constant over time but exhibits strong
seasonal patterns. In this article we consider a limit order book model that
allows for time-dependent, deterministic depth and resilience of the book and
determine optimal portfolio liquidation strategies. In a first model variant,
we propose a trading dependent spread that increases when market orders are
matched against the order book. In this model no price manipulation occurs and
the optimal strategy is of the wait region - buy region type often encountered
in singular control problems. In a second model, we assume that there is no
spread in the order book. Under this assumption we find that price manipulation
can occur, depending on the model parameters. Even in the absence of classical
price manipulation there may be transaction triggered price manipulation. In
specific cases, we can state the optimal strategy in closed form.
