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2021 How Does the Landfall of a Tropical Storm Affect its Landfall? Evan Thomason

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The State University

College of Earth, Ocean, and Atmospheric Sciences

How Does the Landfall of a Tropical Storm Affect its Intensity?

By:

Evan Thomason

A Thesis submitted to the

Department of Earth, Ocean, & Atmospheric Sciences

in partial fulfillment of the requirements for graduation with

Honors in the Major

Degree Awarded:

Spring, 2021 Thomason 2

The members of the Defense Committee approve the thesis of Evan

Thomason defended on April 9, 2021.

Mark Bourassa Thesis Director

Ingo Wiedenhoever Outside Committee Member

Zhaohua Wu Committee Member

Thomason 3

Introduction

Tropical storms are one of the most impressive weather phenomena. There is always this curiosity and wonder into such an intricate piece of mother nature. One moment, there is awe and amazement from this beauty. The next moment, lives are broken forever. I was five years when had opened up the gates to flood all of the inner city of New

Orleans. Although I was young and had just moved into the area a month before, I saw and felt the sadness and the grief. Landfalling Tropical cyclones take a toll on the economy, jobs, and most importantly, life itself. To reduce this destruction, landfall research is needed to mitigate the worst impacts and better prepare all those near the shorelines of landfall (Webb et. al 1995).

The meaning of the word landfall is vague to most people. Technically, it clearly describes the moment when the of a tropical storm reaches the land, but there are land effects to this storm before it even reaches this moment. For example, convective cloud cover from the storm usually reaches land before landfall. The storm intensity and size can change during this interval. As a result of a TC moving largely over land, the storm usually weakens and dies off quickly into the synoptic flow (Tuleya & Kurihara 1978). Certain characteristics of the storm could change at different rates in the different parts of the process of a landfall. When we look at variables of the storm such as the minimum surface pressure within it or the speeds around it, we do see that they decrease after landfall (Bhowmik et. al 2005). The timing of this transition is still a research question due to various environments that the tropical system surrounds itself with(Vickery et. al 2009).

One of the characteristics that I will be examining is the cloud cover related to the storm. This information will be used as a proxy for the extent of the storm impact.

One of the two data sets I am working with provides remotely sensed data about clouds. I use Thomason 4 satellite brightness temperature data to decipher clouds from clear skies and to identify locations of strong convection, as has been demonstrated by Strabala et. al (1994), Knapp et. al (2018), and

Alber et. al (2021).

The rest of the characteristics that I will be examining will be from the HURDAT data set

(Landsea et al. 2009). Since 1850, this data set has measured several parameters of each tropical storm in the North Atlantic Basin. Each parameter in every storm has been individually recorded and checked over by many different authors (Landsea et. al 2004). Maximum sustained 10 minute surface wind speeds and minimum pressure levels from three storms of the Hurricane season of 2005 will be used herein. It should be assumed in this paper that wind speeds are maximum sustained wind speeds and surface pressures are minimum surface pressures. This data base has also been tested by Hagan (Hagan et. Al 2012), Wang (Wang & Lee 2009), and

Hall (Hall & Hereid 2015).

Three storms in the hurricane season of 2005 were chosen to conduct this case study.

Two of those hurricanes, Rita and Katrina, are two of the most destructive and powerful hurricanes to have hit the mainland after being in the Atlantic Ocean basin. These are examples of storms that have maintained their strength while approaching landfall. Ideally the analysis based on these storms would be a good model guidance for how landfalls can change even the strongest of storms, however, confidence in that statement would benefit from a study of many more storms. Hurricane Dennis was also recorded among this group of hurricanes to gain a perspective of a common, weaker hurricane. Hurricane Dennis in 2005 was a Category 3 hurricane briefly but was mainly a weak Cat 1 or 2 hurricane for most of its lifespan. This storm can be used to compare the difference between how weaker hurricanes are affected by the landfall compared to stronger ones. Thomason 5

The methods section shows the full details of the process which included a slow, troubleshooting experience with coding, but it did produce adequate results once the data was computed. By collecting the three variables for three storms at all the different times chosen, the trends in each of those variables across time could then be evaluated for. Using that, it could finally be determined if landfall was hurting or helping the strength of the storm.

Data

Data about where tropical cyclones have been observed (latitude, longitude and position of landfall) including observations from before, during and after landfall were obtained from the

HURDAT data set (Landsea et al. 2009). This contains a huge archive of tropical storms throughout history since 1850. There were 1893 storms recorded through the past 169 years from 1850-2019. Since this includes storms back in the 19th century, accuracy is a problem. In order to get past this, I only recorded data during 2005 to record more modern storms. This is the data that is stored by computers rather than by human observation and analysis. Maximum sustained wind speeds and minimum surface pressures were recorded at mostly 6-hour intervals throughout the storm’s lifetime.

The GRIDSAT data set (Knapp et. Al 2011) records characteristic of cloud cover. This dataset records the positioning and extent of the highest layer of clouds at each location for the entire globe. It has 3-hour intervals since 1980 at 8 km resolution. The data set is comprised of measurements of outgoing long wave radiation from a large number of intercalibrated satellites, converted to the temperature of a black body that would emit at the observed intensity. This intercalibration is important because it greatly reduces satellite to satellite differences in the observed radiation. This intercalibrated outgoing radiation is largely a function of the Thomason 6 temperature and emissivity of the emitter; the variability related to aspects of the antennas has been removed. These black-body -based temperatures are known as brightness temperatures.

Brightness temperatures are used to find whether or not there are clouds present at the observed location. Temperatures decrease with height through the atmosphere. So, if much colder brightness temperatures in the data are set over a location that do not fit as a surface temperature, then we can assume that there are clouds there.

Methodology

The time of the landfall was determined first. This was indicated by the HURDAT data

(Landsea et al. 2009) a “L” a data record. Once this was found, I was then able to look at the intervals before and after that landfall time for every characteristic in Table #within the

HURDAT and GRIDSAT data sets to see whether they were increasing, decreasing, or staying consistent. These characters were taken about 24 hours before and after every landfall. There were some problems with trying to get these storms measured at the same 24 hours before and after landfall. For example, Hurricane Katrina made landfall at 1445Z. The landfalls were mostly recorded between 6-hour intervals starting at 00Z.This is an issue because beside the landfall, there was no data recorded between these 6-hour intervals of time. Whenever this occurred, data that was close to 24 hours before and after landfall respectively was recorded.

This was performed for every characteristic of Hurricane Katrina, , and Hurricane

Dennis. Most of the variables were recorded from the raw data at the different time intervals but the processing the cloud cover was not a straight-forward process. The GRIDSAT data (Knapp et. al2004) merely gave brightness temperatures without stating any direct link to clouds. So I determined that if the cloud top temperature observed was between 235K-210K, then it was Thomason 7 contoured on the map produced. Once the contours that were in the shape of the hurricane were found, the map was condensed just down to only include the dimensions of how long and wide the contours were. This needed to be done in order to see how big the hurricane was in terms of its latitude and longitude range. Once the hurricane filled up the entire produced map like in the map for Hurricane Katrina 24 hours before landfall in Figure 1, I knew that the area of cloud cover was the latitude difference multiplied by the longitude difference. Then, in Excel, formulas converted the degree differences from the globe to actual distances in kilometers. Once all the data was recorded, patterns of change for each characteristic could then be examined. Not all storms had the same results, but the overall effects to each characteristic before and after the moment of landfall was recorded.

Figure 1

Hurricane Katrina’s brightness temperature contours on GRIDSAT at 00Z on August 29, 2005. Thomason 8

Analysis

Across the time of landfall, it was discovered that the extent area of cloud cover mostly decreases. Over the 48-hour span of time intervals, every storm had its cloud cover extent decrease until there was no more cloud cover left captured by the GRIDSAT. In Hurricane

Katrina, four of the five time intervals had clouds decreasing over time until no more clouds were found in GRIDSAT. Hurricane Rita had four out of six intervals present with decreasing cloud cover and Hurricane Dennis had three of its five intervals also present with decreasing cloud cover. The intervals of increase that were found in the three storms were similar in the timing with respect to the landfall, but were different in their increased magnitude. Hurricane

Katrina increased its cloud cover area by almost 48,393km^2 from 12 to 6 hours before landfall while Hurricane Rita increased its cloud cover area by around 15,659km^2 from 6 hours before landfall to the exact moment of landfall. As seen in Figure 2, Hurricane Dennis also had increasing cloud

Hurricane Dennis Cloud Cover 70000 60000 50000 40000 30000

Area Area (km^2) 20000 10000 0 18Z July 9, 00Z July 06Z July 12Z July 18Z July 00Z July 06Z July 12Z July 18Z July 2005 10, 2005 10, 2005 10, 2005 10, 2005 11, 2005 11, 2005 11, 2005 11, 2005 Time

Figure 2 Thomason 9

The cloud cover of Hurricane Dennis over it’s time before, during, and after landfall as measured by GRIDSAT. cover between 6 hours before landfall and right at landfall. This increase is much greater in magnitude in this time interval than the very first three that were measured. So, this marks that not one, but two of the three hurricanes had increasing cloud cover during the final 6 hours before landfall. Once landfall did occur though, all three storms did not have any instance of increasing cloud cover. In fact, GRIDSAT was only able to measure cloud cover for one more time for Hurricane Katrina and Hurricane Rita. It didn’t even measure cloud cover one more time for Hurricane Dennis at all. So, all three storms rapidly decreased their cloud coverage after landfall. The maximum cloud decrease was still unique though in regards to both the timing and the magnitude. All three storms had the most amount of cloud cover area decrease in different intervals. Hurricane Katrina lost 79,415 km^2 of clouds between 6 hours and 12 hours after landfall, Hurricane Rita lost 49,962 km^2 of clouds between 12 hours and 6 hours before landfall, and Hurricane Dennis lost 64,021 km^2 of clouds between landfall and 6 hours after landfall.

Hurricane Katrina Maximum Wind Speed 200 150 100 50 0 WindSpeed (kt) 18Z 00Z 06Z 12Z 18Z 00Z 06Z 12Z 18Z August August August August August August August August August 28, 200529, 200529, 200529, 200529, 200530, 200530, 200530, 200530, 2005 Time

Figure 3

The maximum sustained wind speeds of Hurricane Katrina over it’s time before, during, and after landfall as determined by HURDAT. Thomason 10

Hurricane Rita Maximum Wind Speed 140 120 100 80 60 40 20

0 Wind Speeds (m/s)

Time

Figure 4

The maximum sustained wind speeds of Hurricane Rita over it’s time before, during, and after landfall as determined by HURDAT.

Hurricane Dennis Maximum Wind Speed 140 120 100 80 60 40 20 0

Wind Speeds (kt) 18Z July 00Z July 06Z July 12Z July 18Z July 00Z July 06Z July 12Z July 18Z July 9, 2005 10, 10, 10, 10, 11, 11, 11, 11, 2005 2005 2005 2005 2005 2005 2005 2005 Time

Figure 5

The maximum sustained wind speeds of Hurricane Dennis over it’s time before, during, and after landfall as determined by HURDAT.

Although it did contain some increasing intervals, the maximum sustained wind speeds of the hurricanes were mostly decreasing. Hurricane Rita and Katrina had much faster wind speeds than Hurricane Dennis, but they still had mostly common trends. For example, once each of the hurricanes made landfall, their maximum wind speeds all decreased continuously through time Thomason 11 as seen in Figures 3, 4, and 5. This is where the greatest decrease of the winds was for all three storms. Between landfall and six hours after landfall, Hurricane Katrina had it’s wind speeds decrease by 35 kts and Hurricane Dennis had it’s wind speeds decrease by 65 kts. In the next 6- hour time interval, , Hurricane Rita had it’s wind speeds decrease by 30 kts and Hurricane

Katrina decreased it’s wind speeds by another 35 kts. The only increases in the wind speed were for Hurricane Dennis at least 12 hours before the storm had made landfall. Both Hurricane

Katrina and Rita decreased their maximum wind speeds through time.

Hurricane Katrina Minimum Surface Pressure 1000 980 960 940 920 900

880 Pressure Pressure (mb) 860 840 18Z 00Z 06Z 12Z 18Z 00Z 06Z 12Z 18Z August August August August August August August August August 28, 200529, 200529, 200529, 200529, 200530, 200530, 200530, 200530, 2005 Time

Figure 6

These are the minimum surface pressures of Hurricane Katrina over time before, during, and after landfall as measured by HURDAT. Thomason 12

Hurricane Rita Minimum Surface Pressure 1000 980 960 940 920

900 Pressure Pressure (mb) 880

Time

Figure 7

These are the minimum surface pressures of Hurricane Rita over time before, during, and after landfall as measured by HURDAT.

The minimum surface pressure of the three hurricanes generally increased through time.

The only decreases that were found in the surface pressure were for Hurricane Dennis which had a decreasing pressure trend from 24 hours before landfall to 6 hours before landfall. Hurricanes

Katrina and Rita both were consistently increasing through time as shown by Figures 6 and 7.

In particular, the biggest increase of pressure happened some time between landfall and 12 hours afterward for all three storms. Hurricane Katrina had an increase of 25 mb between landfall and

6 hours after landfall. Hurricane Rita also had an increase of 25 mb between 6 hours after landfall and 12 hours after landfall. Hurricane Dennis had an increase of 28 mb between landfall and 6 hours after landfall.

There were several correlations that were found between the cloud cover, maximum wind speed, and minimum surface pressure. For example, there is an inverse relationship between the wind speed and the surface pressure. This means that when the wind speed is decreasing, the surface pressure of the storm is increasing. This is a perfect statement for both Hurricane Thomason 13

Katrina and Hurricane Rita. Over the course of the 24 hours measured before and after landfall, the wind speed was decreasing and the surface pressure was increasing the entire time as seen in

Figures 3, 4, 6, and 7. For Hurricane Dennis, this expected behavior occurred for seven of the eight time intervals recorded. The only time this inverse relationship did not hold true in this storm was from 12 hours before landfall to 6 hours before landfall. During this time, the wind speed decreased by 5 kts and the surface pressure decreased by 5 mb. These changes are both at the limit of resolution for the data set, and hence are the change in behavior could easily be an associate with noise and rounding errors, rather than unexpected physics. It was very hard to determine a finite relationship between cloud cover with the wind speed and the surface pressure.

When the cloud cover decreases in an interval, there were both examples of when the wind speed and surface pressure would increase and decrease. The same problem also stands for when the cloud cover is increasing during an interval.

Conclusion

I can now conclude that a storm does weaken over the course of time before and after landfall. The degree to which the storm decreases though depends on what how far out in time the landfall is either in the future or in the past. Right before the landfall, most storms were steadily weakening from decreasing wind speeds and increasing surface pressures. There were some contrasts during this time though to where the cloud cover would increase and the wind speed would also increase a few times. Following the storm’s landfall, there would be a drop-off quickly in the wind speeds and an up-tic in the surface pressure. Then, the graph’s as seen in

Figures 3, 4, 5, and 6 would then return back to the same slow declining slope. By mainly increasing the minimum surface pressure, decreasing the maximum sustained wind speeds, and Thomason 14

decreasing the cloud cover area over time, it was then determined that the cyclone’s power

decreased through all three characteristics that were measured.

There are some keen differences between how stronger hurricanes are affected by their

landfalls compared to smaller ones. Hurricanes Katrina and Rita had more straight forward

trends than Hurricane Dennis did. To put that more into context, Hurricane Katrina and

Hurricane Rita both had their wind speeds consistently decreasing and their surface pressures

consistently increasing throughout the entire time period. Hurricane Dennis had it’s wind speeds

increasing and it’s surface pressures decreasing before the landfall occurred. From 12 hours

before landfall to 6 hours before landfall, Hurricane Dennis even had interval with wind speeds

decreasing with corresponding decreasing surface pressures. Hurricanes Katrina and Rita had no

such intervals within their data at all. In the end though, the main trends over the entire time

remained the same between two different strengths of hurricanes. All three hurricanes overall

had their cloud cover decreasing, their wind speeds decreasing, and their surface pressures

decreasing through time.

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