Louisiana and Southeast Texas
Fog Research and Modeling (continued)

 

1) Radiation Fog

This is the type of sounding that occurs with a purely radiation fog event. This sounding is never found with advection or marine fog and rarely with frontal fog. If the height of the lowest inversion is 100 feet, then that will the maximum height this fog may reach. Although, radiation fog does not always follow this rule and it may be well beneath this height. It does not move much and develops from ground up. It is never very deep which means it burns off faster. The wind is calm to very light. This type of fog usually takes place as post-rain events up to 36 hrs or moisture increases during the day. When there is not sufficient moisture depth, this fog develops near the ground and stays within 2 feet of the surface. The radiation caused inversion starts almost immediately off the ground to 100 feet on average. Only when enough moisture is present will this fog get to 1/4 of a mile in visibility. Usually this requires some type of moisture advection process from the gulf or warm marsh and lake waters during daylight hours. Clear skies will always be needed as well. Radiation fog rarely envelopes the downtown areas of a city. The heat island effect is normally strong enough to keep radiation fog from forming.

Radiation Fog Needs:

1) Winds 3kt or less

2) Rainfall within 36 hrs or moisture advection during the day.

3) Neutral or negative omega

4) Weak to no boundary layer positive vorticity.

5) Clear skies

6) Outside downtown areas

This fog is extremely vulnerable to existing atmospheric conditions. If any parameter changes, it will disperse readily. It may also be the second hardest to forecast since it can be a localized phenomenon. Although when widespread, this fog usually stays for the duration until sunrise.

2) Frontal Fog

These two sounding types may be found with all fog events but they are most common with marine, advection and frontal fog. The moisture depth is most noticeable. The moisture depth is also necessary for any type of advection fog. The depth of the lowest inversion will tell how deep the fog potential will be. Environmental air from above the inversion should not mix with the air flowing below it. If this happens, a decoupled layer at the inversion will not be evident and drying of the moist layer will begin. Decoupling at this interface will not allow this to happen.

Frontal fog is caused by the dynamic effects of a frontal system. As a cold front moves within about 250 miles of LCH and begins to slow or stall, the subsident area ahead of the system begins to set up over a location. The fog formation is only enhanced if the front slows enough to cause a wave to form ahead of the cold front. This is a pre-frontal trough. The pre-frontal trough and the front itself will work in unison to intensify the subsident area between the two. This situation is heightened even more when the pre-frontal trough forms showers and thunderstorms. This has resulted in southerly winds for a day or so returning and pooling moisture from the gulf. Additionally, the ground will hold moisture from the convective rains. With plenty of moisture pooled in the boundary layer, a moderate to strong subsidence inversion, radiational cooling of night and winds decoupling at the inversion due to an increasingly stable boundary layer, fog will begin to form readily as the temp/dp reach the same value. This type of fog will also form ahead of warm fronts but rarely behind them for close to the same reasons. Frontal fog shows redundancy by happening night after night until one of the needed parameters is alleviated. This fog occurs mostly in the fall and spring. The winter doesn't see much of this type because most fronts are moving through at a fast pace. This would provide no time for moisture pooling, no pre-frontal trough and the subsident area over the location for only a short time.

Frontal Fog Needs

1) Moisture advection or rain within 36 hrs

2) Neutral or negative omega

3) Clear skies or very high ceiling

Frontal dynamics may cause other types of fog, such as radiation, marine or advection fog, to form. If a front will be responsible for enhancing fog associated with these types, then those conditions for forming each will also have to exist. Most times that fog forms, the front will be moving 10kt or slower and even more often it will be stalled within 250 miles. A pre-frontal trough doesn't have to pass but if it does it will add to the subsidence and moisture profiles.

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