Stephen Hodanish*, Dave Sharp*, Waylon Collins**, Charlie Paxton**
National Weather Service
*Melbourne, Florida, and
**Tampa Bay, Florida
It should be stated that a short-term climatology such as this can be easily biased by significant individual lightning events. For example, the "Storm of the Century" (March 1993) likely affected the density of detected ground flashes for March. This winter storm produced sixty thousand CG flashes with a peak flash rate near five thousand per hour over the state of Florida (Orville, 1993). Other "single event" biases can be debated for nearly every month but seem to play a larger role during months when thunderstorm frequency is lower (i.e., the Cool season). For instance, the local maximum southeast of Lake Okeechobee during December was likely associated with a flood event which occurred over eastern Palm Beach County on 4-5 December, 1994. Consequently, generalities are presented when interpretive confidence may be low for a given area. In addition, extremely small areas of local maxima/minima were also considered artifacts of either scale resolution or "single event" bias and were not addressed.
The sequence of monthly flash density plots do not reflect diurnal variabilities or the influence of daily changes to the surface wind flow regime which might be of interest. These plots simply convey the CG flash density, by month, over a six year period which adds perspective to other parameter climatologies. Monthly trends of deep convection that produced CG flashes were examined with emphasis placed on areas with the highest (lowest) flash density. It is hoped that this study will continue as additional years of lightning data are accumulated.
Colloquially, Florida has been labeled the "Lightning Capital" of the United States, if not the world. This title is well-deserved as mesoscale meteorological influences often provide an environment conducive to thunderstorm production. This fact was recognized by early pioneers of thunderstorm research such as Byers and Braham (1949). Surrounded by water on three sides, Florida has a coastline over 1 930 kilometers long with obvious maritime influences. Within its border are 87 105 square kilometers of land and 7 120 square kilometers of land-locked water (Fernald, 1981). Differential heating, related to land/water contrasts, provides for an abundance of mesoscale thermal boundaries serving as lifting/focusing mechanisms for deep convection. Irregular geographic features include coastline juts at Apalachicola and Cape Canaveral, the large bay at Tampa, the St. Johns River, Lake Okeechobee, the Everglades swamp, and the island chain of the Florida Keys. The Atlantic Ocean borders the east coast with the embedded Gulf Stream Current near shore at Ft. Lauderdale and about 145 kilometers offshore at Jacksonville. To the west, the more shallow waters of the Gulf of Mexico border the panhandle and west peninsula coasts. Each of these features has a local impact on the distribution of lightning density. Compared to most other states, topographical relief is minimal. The extent of the entire coastal zone, as well as the southern one-third of the peninsula, is less than 15 meters in elevation. The highest elevations, only 60 - 90 meters, can be found along the northern reaches of the panhandle and along the central Florida ridge which runs north-south down the center of the northern two- thirds of the peninsula. Orography is not considered a major lifting/focusing mechanism.
Climatologically, Florida is situated in a near-tropical environment. This becomes more evident with decreasing latitude. During the cool and transitional seasons, the state experiences the passage of mid-latitude synoptic scale cold fronts which are electrically active. This is especially true for the panhandle region. South Florida, on many occasions, serves as the dying ground for these fronts. Quite often, the state experiences a secondary frontal passage as the boundary moves back northward as a warm front. Conversely, during the warm and transitional seasons, differential heating generates mesoscale fronts in the form of sea (lake) breezes. These boundaries often become electrically active. Additionally, from June through November, Florida is a prime landfall target for tropical cyclones. While this has great impact to precipitation climatologies, it has very little impact on lightning climatologies. Lightning activity is usually minimal with tropical cyclones due to their thermodynamic structure and associated low buoyancy. For lightning to develop, strong updrafts are necessary to promote charge separation between supercooled water and mixed ice particles. (Samsury et. al., 1994).
Other Florida lightning and convective climatologies have been previously constructed. However, most were limited to warm season convection over the peninsula. Blanchard and Lopez (1985) examined the spatial pattern of convection over south Florida. They subjectively argued that there are primarily three basic convective pattern types observed over south Florida during the summer months. Lopez and Holle (1987) investigated the distribution of summer time lightning as a function of low-level wind flow over central Florida during the summer. They defined certain low-level wind flow regimes which tended to increase flash densities for certain parts of the peninsula. Holle et al. (1992) further described this climatology using an eight-year (1983-1990) data set. Reap (1994) analyzed lightning strike distributions associated with synoptic map types over Florida. His purpose was to produce statistical thunderstorm forecasts based on "map type". In addition, various "thunderstorm-day" climatologies have been drafted in the past. These were often based on the occurrence of "thunder heard" at a given location while indifferent to the type of lightning or its distance from the observing station. The following study may offer a more representative picture of how local geography effects CG lightning distributions per square area during each month of the year.
During the Cool season, defined as November through February, lightning flash densities over Florida are at a minimum statewide (figs. 11, 12, 1, 2). Flash densities across a majority of the state are below 0.1 f/km**2, with many areas below 0.04 f/km**2 Low flash rates during this time of the year are a consequence of dry, stable air typically over the region. Lightning which occurs during this time of the year is associated with the passage of mid-latitude frontal systems.
Excluding December, the Florida panhandle region shows a relative flash density maximum for the state. This increased density for the panhandle is due to warm moist, unstable air advecting northward in advance of eastward moving synoptic cyclones traversing the continental United States. As these cyclones move east, cold fronts associated with the cyclone cross the panhandle, lifting the warm, moist unstable air from the Gulf of Mexico. Although low-level moisture is available across the peninsula, the main core of synoptic scale energy often passes to the north, affecting mainly the panhandle.
As discussed in the introduction, the limited data set and the infrequent amount of lightning during the Cool season increases the potential for event bias within the distributions. However, it is believed that the higher flash densities observed over the southeast part of the peninsula during the Cool season are valid and are a consequence of stalled frontal systems. On the other hand, the "bulls eye" near Lake Okeechobee in December, and other local maxima present over the northern one half of the peninsula, are likely a consequence of single events which were extremely electrical.
Typically, cold fronts during the Cool season stall over south Florida or the Florida Straits as the anticyclone to the north moves off the nation's mid-Atlantic coast. These stalled frontal systems will usually erode, but may become active once again if the pressure gradient to the north of the front increases. Synoptic scale lift can be enhanced if the subtropical jet interacts with the stalled frontal system. This type of frontal system often becomes electrically active due to increasing low-level convergence. They usually remain quasi-stationary and produce significant convective rainfall over southeast Florida. (Gonzales and Moore 1990). It is believed that this type of weather system causes the general flash density maximum over southeast Florida during the Cool season.
2.2. Spring Transitional
The Spring Transitional season (March through May) shows an increase of lightning activity over the entire state of Florida (figs. 3, 4, 5). It is during this time of year when Florida experiences most of its dynamically driven severe convective weather. Both solar insolation and low-level moisture have increased while temperatures aloft remain relatively cool and the polar jet is often located over the continental United States. (Schmocker et. al., 1990). As the season progresses, convective forcing shifts from synoptic scale fronts to mesoscale boundaries as the polar jet lifts northward and the sea/lake breeze convergence zones begin to intensify. During March, lightning densities over the state are generally uniform and governed synoptically. A flash density maximum is seen in the extreme western panhandle, while a minimum is noted over the southern peninsula. As an extension of the Cool season, the maximum in the panhandle is a consequence of previously described affects. Conversely, the minimum over the south Florida can be explained by two factors. The first factor involves the less frequent passage of frontal systems as compared to the rest of the state. The second, and more impacting factor, is the geography of the region. Local effects to convection within this region are influenced by the "Sea of Grass" - the Florida Everglades. This region of grass and water locally reduces the extent of differential heating, which in turn leads to a minimum in mesoscale forcing and subsequent thunderstorm development (Gannon 1978, McCumber, 1980). This density minimum is noticeable during every month of the year.
Before continuing, a brief discussion regarding sea breeze movement over the peninsula is necessary. Numerous observational and numerical studies (Blanchard et al 1985, Pielke 1974 and others - see Reap 1993 for review) have documented that the movement of the sea breeze(s) over Florida peninsula is controlled by the low-level synoptic flow. As these sea breeze boundaries develop, showers and thunderstorms may form along them and propagate inland. Low-level winds will either assist or impede their inland progress. For example, embedded in easterly low-level wind flow, the east coast sea breeze (ECSB) would initiate just inland during the late morning and uniformly increase in lightning discharge as it moves westward. In contrast, inland penetration of the west coast sea breeze (WCSB) would be impeded. Lightning discharge would remain concentrated along the west coast, and later be enhanced as the two sea breeze boundaries collide in the late afternoon. The reverse concept would be valid for westerly low-level flow, or neutral for weak low-level flow. As a result, the relative impact on flash densities and their location can give insight to the influences of the low-level wind. In general, given sufficient thermodynamic instability for thunderstorms to form along sea breeze boundaries, the strength and direction of the low-level wind will determine the movement of these boundaries and where they will collide. Importantly, it is the collision of the boundaries which greatly enhance the density of CG flashes for the peninsula. The panhandle sea breeze does not experience this collision process.
April is truly a transition month given the density distribution of lightning over parts of Florida. Over the peninsula, flash densities increase, especially along the upper west coast, central sections and east of lake Okeechobee, while over the panhandle the flash values remain similar, or decrease slightly, from the previous month. Although flash densities increase over the peninsula, the cause is not readily clear. Climate data indicates that the average low-level wind flow over the peninsula is from the east- southeast at approximately 4 m/s (NOAA 1992). This flow regime brings increasing low- level moisture from the Atlantic over the peninsula, resulting in an increase in the average dewpoints from minimal values in February (mid 40s north peninsula to mid 50s south peninsula - [degrees F]) to mid 50s to lower 60s (degrees F) across the peninsula in April (Fernald 1981). As discussed above, the combination of easterly flow and increasing low-level moisture would favor the a west coast collision of the ECSB and WCSB. This would help explain the flash density maximum along the west coast. However, a weak point in this reasoning is that distributions of rainfall (NOAA 1989) indicate April to be the driest month of the year statewide. If the maximum along the west coast was caused solely by a collision of the two sea breezes, it would also be expected that an area of enhanced rainfall accumulations would be noted along the west coast. Unfortunately, this is not the case. Perhaps another reason for this density maxima is simply a reflection of a few intense synoptically driven convective events which occurred during the study years. Often the air aloft is still cool and dry in contrast to the air at the low-levels which is warm and moist. This contrast is perhaps the greatest during April and early May. Overall, this reduces stability and the potential for vigorous thunderstorm updrafts, leading to an increase in electrical discharge. Also, thunderstorms within a westward moving system will experience some degree of frictional convergence, improving lift, as they make landfall on the west coast.
The Lake Okeechobee flash density maximum during April is caused by collision of the lake breeze and sea breeze. This collision occurs when either the southern part of the state is under weak zonal flow, or when the strength of the synoptic flow is negligible. When the wind flow is weak from an easterly component, the ECSB will develop and move slowly inland. As it approaches the east side of Lake Okeechobee, it encounters a weak lake breeze. As these boundaries collide, convection is enhanced. If the wind flow is from a weak westerly component, the lake breeze and the ECSB will both form. In this situation, the lake breeze will move east and encounter the ECSB to enhance convection near the east coast. When the synoptic flow is negligible, both breezes will form and collide mid-way between the lake and the east coast. It is important to note that the occurrence of CG lightning increases only during weak flow patterns for this area. If the wind flow is relatively strong, lightning activity will be at a minimum as either the east side of the lake breeze will be washed out under strong easterly flow or the ECSB will be washed out under strong westerly flow. Also, a northerly or southerly wind flow does not favor collision.
Over the panhandle, flash densities remained the same (or decreased slightly) from March to April. Although the availability of low-level moisture increases over the panhandle along with increased solar insolation during this time of the year, densities did not significantly increase compared to the peninsula. The transition to stronger mesoscale signals, which first begins at the southern part of the state, has not yet reached north Florida.
During the month of May, flash densities increase statewide, with the lightning coverage appearing rather uniform across the entire region. Maximum flash densities are located over the central part of the peninsula while minimums are located along the immediate coast of the panhandle and over south Florida.
The maximum over the central peninsula during May (and eventually during the months of June through August) is believed to be a result of the climatological positioning of the subtropical ridge axis over the state. Monthly surface wind data for Florida shows that a surface ridge axis initially forms during the month of March over north Florida and remains over the northern one half of the peninsula until the end of August. The wind regime north of the axis is southwesterly, while south of the axis the wind regime is east- southeasterly. The wind flow around the ridge axis influences the lightning distribution over the peninsula by controlling the location, motion, and ultimate collision of the ECSB and WCSB. In general, along the east coast, the sea breeze front south of the axis will move steadily towards the west, while that part of the sea breeze near the ridge axis will only move slowly inland. To the north of the axis, the east coast sea breeze, if it forms at all, will either remain along the coast, or will be pushed off the coast by the westerly component wind. Along the west coast, the west coast sea breeze south of the ridge axis, if it forms at all, will remain along the immediate west coast, or will be pushed off the coast by the easterly wind. That part of the west coast sea breeze near the axis will only move slowly to the east, while that part of the west coast sea breeze north of the axis will move steadily inland. As the day progresses, the sea breeze fronts, due to the differential motion, will orient themselves northeast/southwest across the central part of the state. These boundaries will eventually collide, enhancing the convection over the north central part of the peninsula.
Climatologically, the surface wind data shows a ridge axis over the north central part of the state for this time of year, but it does not represent the wind flow over the state on a daily basis. Naturally, there will be days when the low-level wind will have a stronger westerly (easterly) component which will favor the west (east) coast sea breeze to move across the state and collide with the east (west) coast sea breeze along the east (west) coast.
The minimum over south Florida during May is due to the weaker low-level forcing over the Everglades and subsidence south of the subtropical ridge axis.
The minimum flash density along the immediate coast of the panhandle is due to the shallow nature of the sea breeze when it initially develops. As the panhandle sea breeze moves inland, it then becomes deeper and increases in electrical character. The same can be said of the peninsula sea breezes except that collisions near the coast mask this feature. It is likely that if monthly lightning density plots where separated diurnally (to show morning and afternoon distributions), that a minimum would also be seen around the entire perimeter of the Florida peninsula.
Also during May, the local maxima east of Lake Okeechobee as seen during the previous months is no longer apparent. Scale resolution has likely masked this relative
flash density maximum.
The scale resolution, presented by the color-shading of the data over different parts of the state, can be misleading at times. Although the green shading (representing flash rates between 0.37 and 1.11 f/km**2 covers nearly all of the state, the flash values over the peninsula are likely in the high green range (~1 f/km**2 while the values in the panhandle are likely in the low green range (~0.4 f/km**2.
2.3. Warm Season
Without mistake, the Warm season in Florida brings forth an abundance of lightning. During this time of the year (figs. 6, 7, 8, 9), CG lightning flash densities increase dramatically statewide, particularly over the peninsula. During three of the four months (June, July, August), the statewide flash density distribution is nearly identical. Most of the state experiences densities >1 f/km**2 with values exceeding 3.3 f/km**2over the central peninsula. It is during the the Warm season that Florida earns its nickname as the "Lightning Capital".
June portraits a similar pattern to May in that minimum flash densities occur near to the coast in the panhandle and in extreme south Florida, while the greatest flash densities continue over central Florida. However, the most obvious difference between May and June is that flash densities increase a remarkable three to nine times statewide. This increase is due to the deeper, more intense updrafts associated with thunderstorms that now occur on nearly a daily basis. The distribution of flash density maxima over the central peninsula is governed by the location of the subtropical ridge axis. The west coast shows a slightly greater areal coverage which is caused from by dominance of prevailing easterly wind flow. Once again, a maximum east of Lake Okeechobee is not discernable, but likely exists.
July is the most active period for lightning over the state. The bi-modal distribution of maximum in flash density that was apparent over the central peninsula in June is also evident during July. The west coast maxima is now much larger than the east coast maxima. During July, the flash density maximum "reappears" east of Lake Okeechobee. Interestingly, the relative minimum previously noted along the panhandle coast and over south Florida disappear. This may be an artifact of scale resolution. However, free convection is achieved lower in the atmosphere (earlier in the day) allowing thunderstorms to form closer to the coast.
August begins to point to a decline in flash density statewide. Land/water thermal differences are not as acute with water sources now well warmed. In addition, the airmass over Florida has become characteristically "tropical" in nature (with a stronger subsidence inversion south of the ridge axis) which has been documented to decrease widespread thunderstorm activity (Burpee 1979, Blanchard and Lopez 1985). During this month, the flash density pattern over the state is similar to that of June, except for the area east of Lake Okeechobee. At this location, areal flash density coverage increases. The reason for this increase is due to the weak wind flow which favors the development of both the lake and sea breeze fronts and resulting collision east of lake Okeechobee. Climatologically, the weakest wind flow over south Florida is during the months of July and August (NOAA 1992).
Lightning flash densities continue to decline statewide during the month of September. The greatest decline in lightning coverage is over the panhandle, north Florida and extreme south Florida. The decrease in lightning over north Florida is a result of decreasing solar insolation and cooler, drier air occasionally advecting into the state from the north.
Over the peninsula, maximum flash densities are still great, although the immediate east coast shows a minimum in lightning activity during September. Lower values along the east coast are due to the changing climatological low-level wind flow over the state. During September, wind flow changes from east-southeast to east-northeast (except over south Florida where east southeast flow dominates). This change is caused by the westerlies migrating south which forces the Bermuda high eastward into the Atlantic. In addition, the westerlies bring anticyclones into the southeastern United States which causes the low-level flow over Florida to shift into the east-northeast. Although trajectories bring air off the warm Atlantic, this airmass has had its origins over the cooler continent which forces cooler, more stable air over the eastern side of the peninsula. As the sea breeze boundary initially develops, thunderstorm development is inhibited due to the cooler air. As this boundary moves inland, it encounters more unstable air as the interior warms. This, in turn, causes more vigorous convection to form along the westward advancing sea breeze and higher CG flash densities over the interior and western Florida.
2.4. Autumn Transitional
During Autumn Transitional, defined here as the month of October, the overall lightning activity decreases sharply statewide (fig. 10). Flash density values of less than ~0.1 f/km**2are located over the panhandle while values over the peninsula are less than 0.4 f/km**2 except along the east coast south of Cape Canaveral, where values reach as high as ~1.0 f/km**2 The overall decrease in lightning activity is caused by stable air advecting into the state as anticyclones over the southeastern United States advect cool, dry air over the region. Lightning which does occur over the state is primarily supported by synoptic scale fronts, although thunderstorms can still occasionally form along the sea breeze boundaries in the peninsula. The maximum over the east coast is believed to be caused by a combination of sea breeze convergence zones and frontal systems which stall over southern Florida.
