This question should be asked more often than it is.
I tend to agree with Henry's response -- but I'll add a few additional thoughts to consider.
First, we don't have enough information about the make-up of your projects, your organization or what is missing from senior management's perspective. That last piece might be the one to answer first -- but I'll get back to that after we revisit the difference between project, program and portfolio management.
Project: Keeping this short, as you know, 'A project is a temporary endeavor undertaken to create a unique product, service or result.'
Program: A Program is a group of related projects managed in a coordinated manner to obtain benefits not available from managing them individually. I'll come back to this one.
Portfolio: A Portfolio is the centralized management of one or more portfolios that enable executive management to meet organizational goals and objectives through efficient decision making on portfolios, projects, programs and operations.
So, back to 'what is missing from senior management's perspective.' If senior management is already managing return on investment and whatever other metrics (by way of tools and process that Henry mentions) to measure and assess the results of its endeavors, then you may already have the Portfolio piece intact.
Now on to Program Management. When you have multiple projects that must work together to achieve a common end-result (lets say a new car), you typically need a Program through which all project managers can coordinate their efforts to ensure 1) projects are communicating where their outputs and inputs must agree 2) manage shared resources 3) roll-up status and communicate with senior management on issues and progress 4) manage budgets 5) participate-in and communicate strategic decisions and changes.
In this car development example, a Program will also communicate and coordinate with non-project resources such as marketing, sales, HR, etc as needed whereas projects typically do not. There's more -- but the idea is Programs are used to coordinate inter-project activities so that projects can remain focused in intra-project activities.