如何在ajax中使用formdata发送String数据

<script type="text/javascript">
$(document).ready(function(){
    $("#btnUpdate").click(function(){
        alert($("#frm_data").serialize());
        var formData = new FormData($("#frm_data")[0]);
        var Desc= CKEDITOR.instances.editor1.getData();

        $("#btnUpdate").attr('value', 'Please Wait...');
        $.ajax({
            url: 'update_job.php',
            data: formData,
            cache: false,
            contentType:false,
            processData:false,
            type: 'post',
            success: function(response)
            {
                $("#btnUpdate").attr('value', 'Update');
            }
        });
        return false;
    });
})
</script>

i use ckeditor for textarea field. but its can update value with new value, so i want to use another way with send textarea value with form data.
so how to send Desc data with fromData. in ajax.