Orographic Cirrus is a common phenomenon during the cool season in the lee of the Appalachian Mountains. Most common just to the west of the mountains of North Carolina, Virginia, and West Virginia, orographic cirrus has been tracked as far south as Georgia and as far north as the mountains of Pennsylvania. These features, while seemingly mundane, have been known to spoil high temperature forecasts by as much as 10 degrees downstream from their origins. These features are most common in deep northwesterly flow regimes during the cool season. While they occur quite frequently, literature is quite sparse and models often do not have a high enough resolution to resolve these features although we have seen some luck with the RUC and local WRF model runs. As far as I can tell the last observational literature on this topic in regards to the Appalachians was a U.S. Department of Commerce Satellite Applications Information Note dating back to 1983. This work gave clues as to the physical characteristics of these cirrus events but did not do much to quantify these events. Last year I began a project to do just that along with clarifying the physical mechanisms and develop of forecasting methodology for these situations.
Year one of the project analyzed more than 40 orographic events with the details of this project outlined on a poster which can be found here. A preliminary forecasting methodology was developed and is included below in a two page quick reference guide. Below that is an example of a case of orographic cirrus which occurred just a few days ago.
The project has continued into year two during which analysis of this cool seasons events will be added to the previous database. A closer look into what occurs in non-events when conditions look good will also help weed out which are the most important factors in realizing orographic cirrus. These results will be compiled and condensed into a paper which will be forthcoming.
Case Study from 1/4/2012
On January 4, 2012 the 00z sounding for Roanoke, VA showed signs of a possible orographic cirrus event. There was a strong inversion with the top of the inversion around 650 mb. This is a little high for a classic orographic cirrus sounding but is in the ballpark. The wind profile above this level is mainly north northwesterly with some slight backing aloft near the tropopause. Speed generally increases with height from the top of the inversion to the tropopause, another classic signal. Overall, this is a good sounding for orographic cirrus but not a slam dunk.
By 12z however, the inversion has now descended, with the top of the inversion around 850 mb, in the 850-750 mb range that usually manifests cirrus. In addition winds have backed around to northwesterly at the inversion top with a more westerly component at the troposphere. Wind speeds increase through the column and back slightly. This sounding is spot on for orographic cirrus. So how did it play out?
As this animation shows, orographic cirrus began to manifest itself in southern Virginia and northern North Carolina at 1015z. Cirrus flares up over the next several hours and is present through at least 15z across most of southern Virginia. In this case the orographic cirrus can be distinguished from other mid and high level clouds advancing over the mountains by the stationary western edge of the cloud cover along the lee side of the mountain range while the eastern edge continues to advance. The production of orographically enhanced cirrus ends when the western edge of the cloud cover no longer is stationary.
Questions, comments and suggestions on this work are always welcome. Feel free to contact me at Ryan.Ellis@noaa.gov.