jQuery Grid Plugin - jqGrid

I have a grid is created dynamically based on a select box. It works great - but now I want to get fancy.

The select box looks like this
<select id="search_field" size="2">
   <option value="filename" selected="selected">Filename</option>
   <option value="filetype">Type</option>
</select>
<input type="text" id="search_name" onkeydown="doSearch(arguments[0]||event)" />

this then creates the grid on the fly via

function gridReload(){
      var search_field = jQuery("#search_field").val();
      var nm_mask = jQuery("#search_name").val();
      jQuery("#masterGrid").setGridParam({            url:"server.php?q=1&search_field="+search_field+"&nm_mask="+nm_mask
          , page:1
          })
        .trigger("reloadGrid");
    }

What I'd now like to add is an onchange so when a user changes there selection of the selectbox searchfield, the input box is cleared of their last search

Great product - thanks