LookML order month name by chronologically

[yanwu1205-16427]

For the dimension time frame, I want to sort the month name by chronologically. how to edit the code below? 

dimension: timeframe{

  type: string
  sql:
  CASE
  WHEN{% parameter timeframe_picker %} = 'Month' THEN CAST(${order_month_name} as string)
  WHEN{% parameter timeframe_picker %} = 'Year' THEN CAST(${order_year} as string)
  END;;
}