A novel self-organizing hierarchical architecture is proposed for improving the performance and scalability properties of ad hoc wireless networks. This paper presents the results of a simulation study of performance and throughput capacity of a specific three-tier hierarchical ad hoc network with 802.11 radios, forwarding nodes and access points. The performance of the proposed hierarchical network is evaluated for two well-known classes of ad hoc routing protocols: Dynamic Source Routing (DSR) and Ad hoc On-demand Distance Vector (AODV), and compared with that of a conventional "flat" ad hoc network. The results for an example sensor network scenario show significant capacity increases with the hierarchical architecture for both DSR and AODV cases. Modifications to ad hoc routing metrics for energy efficiency are also considered. The scalability properties of the three-tier hierarchy are studied further in terms of the achievable system capacity as a function of the relative densities of sensor nodes, forwarding nodes and access points. It is shown that the capacity of the three-tier hierarchical network scales well when the number of forwarding nodes and access points are increased in the right proportions.