The seemingly endless 2020 Atlantic Hurricane season continues as the latest named storm, Hurricane Eta, has just become a Category 4 storm. While Eta continues to churn in the Caribbean Sea, the focus of this blog post will be on Hurricane Zeta, which made landfall in Louisiana last Wednesday, October 28th. As seen with many other tropical cyclones this year, Zeta rapidly intensified into a Category 2 hurricane with sustained winds of 110 mph just before it made landfall. After making landfall, Zeta moved northeastward to the mid-Atlantic region at speeds upwards of 30 mph (Figure 1). Due to the fast movement of this storm, the risk for inland flooding was lower compared to slower moving storms such as Hurricane Sally, but the threat for tropical storm force winds were prevalent all the way from Louisiana to Delaware. As Zeta raced inland, it entered a less favorable synoptic environment for tropical cyclone strengthening which caused the storm to weaken. In addition to this, Zeta merged with a mid-latitude system that was present over northern Texas. Due to this, Zeta began to exhibit more extratropical characteristics, such as frontal boundaries (Figure 2).
Figure 1: This loop displays a portion of the 5 day outlook for Hurricane Zeta as it moves across the United States. (Source is under “Figure 1”)
Figure 2: This image displays surface fronts and sea level pressure over the United States at 18z Thursday, Oct 29th. (Source is under “Figure 2”)
Shortly after landfall, Zeta was downgraded from a category 2 hurricane to a tropical storm. A quick glance at the synoptic level can explain why Zeta weakened as it moved over land. The upper level wind shear produced by the jet stream over the southeastern United States contributed to an unfavorable environment for tropical cyclone development (Figure 3). In addition to this, Zeta moved from the warm, moist Gulf air to more cool, dry air over land. Tropical cyclones are warm core systems that need warm, moist air as fuel, but as dry air is entrained in the cyclone, it begins to fall apart. Due to these factors, Zeta lost its classical symmetrical tropical cyclone look as it became more asymmetrical due to frontal boundaries being associated with the system. (Figure 4) This is typical of cyclones as they transition from tropical to extratropical over mid-latitudes.
Figure 3: This image displays winds at the 250 mb level. The darker, more red and purple colors represent faster wind while lighter blue shading represents slow wind. The image is from 18z on Thursday, Oct 29th. (Source is under “Figure 3”)
Figure 4: The image on the left of figure 4 depicts a tropical cyclone with a classic symmetrical shape and a warm core. The image on the right depicts an extratropical cyclone with frontal boundaries and a cold core. (Source is under “Figure 4”)
The mesoscale environment becomes much more interesting regarding Zeta as it weakens and merges with the extratropical cyclone over Texas as it now begins to exhibit frontal characteristics. Frontal boundaries are associated with features such as large horizontal contrasts in temperature or moisture and maximums in cyclonic vorticity. The following figures in the panel below display each characteristic respectively. There is a stark contrast in temperature, as you move eastward from Louisiana to Georgia. The temperatures are colder west of Alabama and warmer east of the state, and this is indicative of a cold front. Looking at the mid-Atlantic to northeastern United States, the temperature is warmer from Kentucky/West Virginia southward and cooler to the north of these states, indicative of a warm front. (Figure 5). This frontal setup involving a warm sector is a classic characteristic of an extratropical cyclone. In addition, regions of maximum cyclonic vorticity at the 850 mb level are displayed along the cold and warm frontal boundaries. Regions of maximum cyclonic vorticity are also observed around the center of Tropical Storm Zeta due to its counterclockwise rotation (Figure 5). Now that frontal boundaries have been identified, it can be determined if frontogenesis is occurring along them.
Figure 5: In this panel of images, the chart on the left displays 850 mb level temperature at 18z Thursday Oct 29th. The blue shading is cooler temperatures whereas red is warmer. The image on the left is 850 mb cyclonic vorticity at 18z Oct 29th. The darker reds/oranges indicate where cyclonic vorticity is at a maximum. (Source is under “Figure 5”)
The frontogenesis equation involves 4 unique terms that can be analyzed to determine if frontogenesis or frontolysis is occurring at the frontal boundary with respect to that term. (Figure 6 Source: Dr. Zachary Handlos EAS 4813). If the term being analyzed has an overall positive sign, that means the front is strengthening, therefore the term is frontogenetic. If the term has an overall negative sign, that means the front is weakening, therefore the term is frontolytic. I analyzed 2 terms, shearing and diabatic, with respect to the warm front region and area of frontogenesis displayed by the purple contours (Figure 7). To analyze the shearing term, wind direction and temperature must be investigated. Since the winds go from a more westerly component to a more easterly component in the y prime direction, that portion of the term is negative, and since the temperature seems to increase as you move eastward, parallel to the front in the x prime direction, that portion of the term is positive (Figure 8). Overall, the sign of the shearing term would be negative, making it frontolytic. The diabatic term is more ambiguous when preforming sign analysis. Due to the fact there is cloud cover and precipitation on both sides of the front, indicating diabatic cooling, the sign of the term is negative, but due to the negative on the outside of the term, overall it becomes positive (Figure 9). Since the overall sign of the diabatic term is positive, it is frontogenetic. Due to the shearing term being negative and the diabatic term being positive, it is difficult to say whether the front is experiencing frontogenesis or frontolysis. Due to the strong cyclonic vorticity and temperature gradient associated with the front along with the positive diabatic term, the front can be said to be experiencing mild frontogenesis. Zeta’s characteristics changed drastically from tropical to extratropical as it moved from the Gulf as a hurricane and merged with an already existing mid-latitude cyclone.
Figure 6:: This equation displays the frontogenesis equation. Term A is the shearing term, Term B is the confluence term, Term C is the tilting term, and Term D is the diabatic term. (Source: Dr. Zachary Handlos EAS 4813
Figure 7: This map displays temperature advection (blue and red shading), temperature contours (blue and red contours), and frontogenesis contours (purple contours) at 12z Thursday Oct 29th. The area of strongest frontogenesis is near the warm front associated with the extratropical cyclone. Areas of frontogenesis are also seen around Zeta as it transitions to extratropical. (Source is under “Figure 7”)
Figure 8 & 9: Figure 8 (left) is a map displaying temperature contours and wind barbs at 12z Thursday Oct 29th. These contours and wind barbs were used in the sign analysis of the shearing term. Figure 9 (right) is a map displaying 6-hour averaged precipitation rate (green/yellow/red fill), mean sea level pressure (black contours), and geopotential height (red and blue contours). This map was used in the sign analysis of the diabatic term as areas where precipitation was occurring lead to cloud cover and cooling. (Source is under “Figure 8” and “Figure 9”)