Here is the example script include that can help you to get the Top Parent department name from the user department.

var DepartmentUtil = Class.create();
DepartmentUtil.prototype = Object.extendsObject(AbstractAjaxProcessor, {
	getmemberDep : function (){
		var gr = new GlideRecord('sys_user');
		if(gr.get(this.getParameter('sysparm_user'))){
			var topDep = this.getTopParent(gr.department.sys_id);
			return topDep;
		}
	},
	getTopParent: function (dep){
		var cdep=this.getParentDepart(dep);
		while(cdep.parent != ''){
			cdep=this.getParentDepart(cdep.parent.sys_id);
		}return cdep.name;
		/*if(cdep.parent != '') cdep=this.getParentDepart(cdep);
		else return cdep.name;*/
	},
	getParentDepart: function (currentDep){
		var gr = new GlideRecord('cmn_department');
		if(gr.get(currentDep)){
			return gr;
		}
	},
    type: 'DepartmentUtil'
});