1. Introduction
Factors affecting the variability of storm initiation, organization, and lifetime with boundary layer convergence lines (boundaries) are the subject of this paper. There is a large variability in the ability of boundaries to initiate storms. Some never do, others only occasionally initiate short-lived storms, and others produce a long-lived well-organized squall line. A strong motivation for this work is to help the forecaster anticipate which convergence lines are likely to produce the more extensive and long-lived convection. This is particularly important for forecasting aviation en route and terminal weather. The data used for this study were collected during the summer of 1991 in Florida in association with a large multiagency field program called the Convection and Precipitation/Electrification (CaPE) project.
Intense lines of single-cell andmulticell thunderstorms with heavy rain and frequent lightning that last 1–2 h are common in Florida during the summer. Using the definition of Bluestein and Jain (1985) many of these can be classified as squall lines; that is, the line is greater than 50 km long, has a length to width ratio of at least 5:1, and lasts at least 15 min. However, long-lived (>2 h) squall lines or supercell-type storms are rare during this period in Florida. While this paper is concerned with Florida thunderstorms, results are likely applicable to other regions.
Byers and Rodebush (1948) long ago recognized that the initiation of thunderstorms in Florida was regulated by low-level convergence associated with the sea breezes entering the peninsula from the east and west coasts. Three-dimensional modeling simulations by Pielke (1974) for synoptically undisturbed days also showed that the location of thunderstorms was strongly controlled by the location and movement of the sea breezes. Simpson et al. (1980) have postulated that the merger and intensification of Florida storms result from the collisions and interactions of sea breezes and thunderstorm outflows. Later Purdom (1982) and Wilson and Schreiber (1986) showed that boundary layer convergence lines could be identified in satellite and radar data and that thunderstorm initiation was closely related to the movement and interaction of these convergence lines. A variety of observational (e.g., Wilson et al. 1992; Mueller et al. 1993; Fankhauser et al. 1995; Atkins et al. 1995; Kingsmill 1995) and modeling studies (e.g., Thorpe et al. 1982; Rotunno et al. 1988; Lee et al. 1991) have been conducted to examine factors affecting storm initiation by convergence lines. Weisman and Klemp (1986) discussed how wind shear profiles and buoyancy considerations can be used to estimate storm type (single cell, multicell, and supercell) and storm longevity. Mueller et al. (1993) and Crook (1996) showed that storm initiation was very sensitive to small variations in stability and moisture depth.
The following modeling studies have a direct bearing on the problem being studied here. Thorpe et al. (1982) and Rotunno et al. (1988) have shown that the low-level vertical wind shear profile relative to the gust front is related to the extent and longevity of thunderstorms. Rotunno et al. (1988) stressed the importance of creating a deep updraft to initiate storms. Their simulations showed that the optimum condition for producing deep uplift at the gust front occurs when horizontal vorticity associated with low-level environmental shear balances the circulation induced by the cold thunderstorm outflow. Weisman and Klemp (1986) showed that in low wind shear situations the gust front tends to move away from the storms resulting in the demise of the storm updraft as it moves away from the near surface convergence, resulting in the storm’s rapid decay. Numerical simulations by Moncrieff and Miller (1976) found that a steady long-lived squall line required the propagation speeds of the density current and the cumulonimbus to be equal.
Considering the findings from these earlier modeling studies, an extensive examination is made in this paper of the initiation, organization, and lifetime of storms associated with convergence lines occurring in the generalvicinity of the Kennedy Space Center in east-central Florida. This study primarily uses Doppler radar, mesonet, radiosonde, and satellite data.
The data and analysis techniques are described in section 2. Section 3 describes the difference and reasons for differences in the amount of convection between the east coast sea-breeze front (ECSBF) and the west coast front (WCF). The west coast front is actually a gust front that propagates across the Florida peninsula. It is believed to originate as the west coast sea-breeze front that is then strongly modified by outflow from storms that frequently form along the sea breeze. Section 4 examines stability and shear parameters in relationship to the amount of storminess with a boundary. Section 5 introduces the concept of boundary relative cloud motion and its relationship to storm organization and longevity. Section 6 discusses factors affecting storm organization and section 7 discusses the forecasting of storm organization and lifetime.
2. Data and analysis procedures
The CaPE data collection network is described by Wakimoto and Lew (1993). Figure 1 shows the location of the collection facilities used in this study: NCAR’s (National Center for Atmospheric Research) C-band Doppler radars [CP-3 and CP-4 described by Keeler et al. (1991)], NCAR’s PAM II (Portable Automated Mesonet) mesonetwork (Brock et al. 1986), NCAR’s stationary and mobile CLASS (Cross-chain Loran Atmospheric Sounding System; Lauritsen et al. 1987) upper-air sounding systems, and the Cape Canaveral Air Force radiosonde.
The C-band radars are able to detect and measure winds in the optically clear-air boundary layer (Wilson et al. 1994). Convergence lines are typically evident as thin lines of enhanced reflectivity and convergent radial velocity signatures. The thin line is typically 1–3 km wide. Christian and Wakimoto (1989) and Wilson et al. (1994) have shown that thin lines in the clear-air boundary layer indicate regions of convergence and updraft. This is the primary method used to track the boundaries for this study. The PAM winds were used to determine the location of boundaries in regions of weak radar signal.
Dual-Doppler radar analyses were conducted within regions where the angle between the two radars was greater than 30° (see Fig. 1). The dual-Doppler synthesis procedure is described by Wilson et al. (1994). Time–height histories of dual-Doppler-derived horizontal divergence fields were obtained for 13 cases. The average divergence within the reflectivity thin line was obtained for each case at 300-m height intervals between 150 and 1950 m above radar height. The area for the average was defined by manually outlining the enhanced reflectivity region associated with the thin line. The thin line was generally so well defined that there was little subjectivity in defining the area. The area of maximum convergence was observed to be contained in this region. These divergence values are used in Table 2, and Figs. 3 and 4.
The storminess associated with a boundary was defined as the percentage of a specified area behind the boundary that had precipitation reflectivities greater than 30 dBZ at a height of 4 km. The specified area enclosed a region defined by the location of the boundary and a region extending beyond it which represented a distance anyclouds occurring along the boundary would have moved in the past 30 min. The method involved using actual average cloud motions to specify the area. Typically this is a region extending from the boundary to roughly 20 km behind the boundary. Behind is defined as the direction from which the boundary is moving. Cumulus cloud and/or storm motions were determined by tracking radar echoes within the vicinity of the boundaries from radar volume scans collected at roughly 3-min intervals. In section 3b, storminess is quantitatively determined using this definition. In section 3a, storminess is subjectively determined by examining the area from the boundary to 20 km behind it for 30-dBZ echo. The subjective definition is used since all convergence lines are being rated relative to one another over the entire time they are within 60 km of the radar.
Following the analysis of Knight and Miller (1993) and Mueller and Wilson (1994), sensitive radars like CP-3, CP-4, and the WSR-88D can be used to track even small cumulus clouds not containing hydrometers. In this paper we define
radar reflectivities greater than 30 dBZ as storms,
individual radar reflectivity cells between 10 and 30 dBZe as large cumulus clouds, and
individual cells between −10 and 10 dBZe as small cumulus clouds.
3. Storminess and divergence comparison between boundaries
The CP-4 radar and PAM mesonet data were examined every day between 0930 and 1700 EDT (1330–2100 UTC) from 15 July to 15 August 1991 to identify convergence lines within 60 km of CP-4. Table 1 shows qualitatively the degree of storminess associated with each boundary during the time it was located within 60 km of the radar. Storminess associated with each boundary was subjectively rated as the amount of convective precipitation echo greater than 30 dBZ within 20 km to the rear of the boundary. The relative ratings were none (N), below average (B), average (A), and above average (+). Average is relative to all the boundaries in Table 1. Examples of each can be seen by examining Figs. 2 and 6. The rating for boundary B2 in Figs. 2 and 6 are above average. Boundary B1 in Fig. 2 is classified as below average, although at 1800 the amount of storminess is about average; the other time periods are below average, resulting in a classification of belowaverage. Boundary B1 in Fig. 6 is ranked as average. Although at 1800 and 1830 it is above average, the sparsity of storms at later time periods results in the overall average rating. Boundaries were classified as ECSBF, WCF, gust front, or collision. The WCF was defined in section 1. Gust fronts are thunderstorm outflows moving from the north or south or locally produced by storms near the radar. There is no reason to believe that gust fronts are physically different than the WCF. Collisions are the coming together of two boundaries, frequently this was the ECSBF and the WCF. Each case that was classified as a WCF was not actually carefully tracked across the peninsula to assure its origin was on the west coast. Rather extensive gust fronts moving in from the west during the afternoon were assumed to have their origin on the west coast. Table 1 also shows the 1600 UTC wind from PAM station 25 (see Fig. 1 for location) and the 700-mb wind from the 1500 UTC sounding from the Cape Canaveral Air Force Station (CCAFS).
Table 1 shows that there are only four boundaries that did not produce storms and these were ECSBFs. It is also evident that relative to the other boundaries the ECSBF typically was associated with a below-average number of storms. The reason for this is examined below. WCFs occurred on 10 of 14 times when the 700-mb wind contained any westerly component. There was only one WCF with an easterly wind component (170°). It is also evident that the surface winds were light (≤6 m s−1), as well as the 700-mb winds (≤9 m s−1 except for one case). The 1600 UTC surface wind represents the environmental flow before storm outflows or the sea breeze reached the station. These light winds have implications for both the intensity of the convergence associated with boundaries and the rate of storm movement.
a. 15 July case
Figure 2 shows a 3-h time history for the case of 15 July as first the westward-moving ECSBF (B1) moves through the study area; it disappears after about 1905 when it collides with the eastward-moving WCF (B2). The WCF first became visible as a radar fine line and radial velocity discontinuity shortly after 1800 near the 60-km range mark west of its position at 1900 in Fig. 2d. Following the collision with the ECSBF, the WCF continues to move east through the study area. The initiation of storms by these two boundaries, their collision, and the following storm initiation for this same day has been discussed in detail by Kingsmill (1995). He reported that the cool air behind the WCF was deeper (1.7 km) than that with the ECSBF (0.7 km). The storms produced by the ECSBF were few and originated at vertices of scallops along the ECSBF where there was enhanced convergence and upward motion. The storms with the WCF were far more numerous and appeared to initiate when the WCF intercepted clouds along horizontal convective rolls. Dual-Doppler analysis by Kingsmill (1995) showed that the updrafts with the ECSBF, at a height of 900 m, were generally about 1 m s−1 with a single maximum of 4 m s−1 at one of the vertices. The 1 m s−1 magnitude with the ECSBF was also observed (not shown) by two research aircraft that flew a vertically stacked pattern through the ECSBF, not at one of the vertices, from 1800 to 1822 at heights between 150 and 1200 m. These flights showed a narrow, weak updraft with a maximum speed of only 1.3 m s−1 at a height of 500 m. Kingsmill’s dual-Doppler analysis showed that the updrafts with the WCF, at a height of 1.3 km, were generally at least 4 m s−1 with numerouscenters of 10 m s−1.
Dual-Doppler syntheses of divergence were prepared for 10 time periods between 1741 and 1923 as the boundaries moved through the southern dual-Doppler lobe. Two examples of these analyses at a height of 150 m are shown in Fig. 3. Figure 3a shows the ECSBF at1756. The leading edge of the sea-breeze front is evident by the thin line of enhanced reflectivities (shaded area). A band of convergence between 1 and 5 × 10−3 is associated with the ECSBF. No storms were associated with the boundary at this time in the south lobe; only small cumulus clouds were observed. Note that there are bands of convergence of about equal strength to the west of the ECSBF: these are horizontal convective rolls. It was rather common that the magnitude of the convergence associated with horizontal convective rolls was similar to that of the ECSBF. In fact, storms occasionally form on the rolls (Weckwerth 1995), as well as on the sea-breeze front. As previously shown by Kessinger and Mueller (1991), Wilson et al. (1992), Wakimoto and Atkins (1994), and Kingsmill (1995), preferred areas of storm initiation were at the intersection of cloud bearing rolls with the sea-breeze front or other type of convergence line.
Figure 3b shows both the ECSBF and WCF in the south lobe at 1901, just prior to their collision. As reported by Kingsmill (1995), the convergence with the WCF is clearly stronger than with the ECSBF. Storms are present with the WCF (dark shading). Figure 4 shows a time history of the average convergence with both boundaries at heights of 150, 450, 750, and 1050 m. The WCF does not enter the 30° lobe until just prior to 1901; thus, there are no divergence values available prior to this time. The ECSBF divergence changes little during the 80-min period prior to collision. At a height of 150 m the convergence ranges between 0.8 and 1.8 × 10−3 s−1 and the depth is less than 700 m. Similar to the results of the Kingsmill (1995) analyses, the WCF is clearly more intense and deeper.
b. Additional cases
To determine if the divergence difference between the ECSBF and WCF observed with the 15 July case is representative, 11 additional ECSBF and WCF cases were selected for study. These cases are highlighted by bold type and italicized in Table 1. For a case to be chosen, good dual-Doppler data were required, as well as a sounding that sampled the warm air being lifted by the boundary. Table 2 lists the dual-Doppler determined, average low-level convergence (150 m), and depth of the convergence for each boundary (many other parameters are listed that will be discussed later). The convergence is for a representative time when the boundary was in the dual-Doppler lobe. The storminess associated with each boundary, at the same time as the convergence computations, is determined by computing the percentage of area covered by 30-dBZ echo. The definition of how this is determined is given in section 2. It should be noted that the degree of storminess in Table 2 refers to that at a single observation time, whereas storminess in Table 1 was an average condition over the time period the boundary was within 60 km of the radar. As discussed below, there was a major difference in storminess between the ECSBF and the WCF for either the average conditions in Table 1 or the “snapshot” conditions in Table 2.
The most obvious finding from Table 2 is that the percentage of echo coverage (storminess) was much higher with the WCFs (31%–60%) than with the ECSBFs (0%–10%). This quantitatively demonstrates what was qualitatively indicated in Table 1. It is also apparent that the low-level convergence for the WCF cases is larger than for the ECSBF cases. Figure 5 shows the divergence profiles for all the boundaries in Table 2. The ECSBF cases are solid lines and the WCF dashed lines. It is clear that the convergence with the WCF is more intense and generally deeper than the ECSBF. Thus it would appear that the reason for more storms with the WCFs is simply that the convergence is deeper and stronger. Possible reasons for this deeper stronger convergence are investigated below.
4. Comparison of stability and shear parameters
a. Stability
Table 2 lists stability parameters for the 13 boundaries. The soundings were all taken less than 100 min prior to the analysis time given in Table 2 and were taken on the warm side of the boundary. Careful inspection of the stability and shear parameters (LI, CIN, and CAPE) in Table 2 indicates that there is no obvious difference in these parameters between the ECSBF and WCFs or the amount of convection on a given day.
The lack of a correlation between the amount of storminess and the stability parameters is consistent with the results of Mueller et al. (1993) and Weckwerth et al. (1996). Mueller et al. (1993) obtained numerous boundary-proximity soundings in Colorado that showed that there was no apparent correlation of negative LI values with the likelihood of storms. However, with both positive LIs and values of CIN > 120 J kg−1, storms were very unlikely. Weckwerth et al. (1996) demonstrated that in the well-mixed boundary layer when horizontal convective rolls were present the LI and CAPE could vary by 2°C and 1200 J kg−1, respectively. The variability was dependent on whether the sounding was taken in the updraft or downdraft portion of a horizontal convective roll. This natural variability over distances of only a few kilometers is a considerable portion of the variability from case to case in Table 2; thus it is not surprising that a significant correlation was not found with thunderstorm likelihood.
Examination of storm initiation along a convergence line shows that there are often regions on the scale of tens of kilometers where storms are absent. These gaps are likely caused by regions of greater stability being intercepted by the boundary. This is the case in Fig. 2 where the southern end of the WCF at 1930 and 2000 is mostly absent of storms (see Figs. 2b and 2c). Satellite imagery showed cirrus clouds left over from the earlier convection along the coast drifted westerly over this region stabilizing the air. Mueller et al. (1993) suggest that these regions of increased stability can often be identified by the absence of cumulus in advance of the boundaries. This issue is addressed further in section 5.
b. Shear
It is useful to compare the CAPE, lowest 6-km shear, and bulk Richardsonvalues in Table 2 with those used by Weisman and Klemp (1986) in their numerical modeling experiments to estimate convective storm type and lifetime. The CAPE and shear values in Table 2 are all less than those used in their experiments. The minimum values they used were a CAPE of 2200 J kg−1 and a mean vertical shear ū of 7 m s−1 per 6 km, which gave a bulk Richardson number of 89. This minimum condition produced multicell short-lived multicellular storms. The distinctive feature of these simulated low-shear cases was that the gust front moved away from the storms, resulting in the loss of updraft into the cell and its rapid decay. Even if storms continued to form at the leading edge of the gust front, these cells were soon left behind to quickly decay. This is similar to much of the convection observed during CaPE.
It is also useful to compare the surface to 6-km shear, CAPE, and CIN in Table 2 with those of Bluestein and Jain (1985) in their Table 3 for Oklahoma severe squall lines and isolated supercells. The average CAPE is less and average CIN is greater for the Florida cases in Table 2; however, the most significant difference is the lowest 6-km vertical wind shear. The average value for Florida is 0.6 × 10−3 s−1, compared to 3.9 × 10−3 s−1 for the Oklahoma severe squall lines.
Rotunno et al. (1988) and Thorpe et al. (1982) conclude that the low-level shear directed perpendicular to a squall line is the important kinematic feature that promotes a long-lived system. Rotunno et al. (1988) further conclude the optimum condition for deep updrafts occurs when the import of positive vorticity associated with the low-level shear just balances the net buoyant generation of negative vorticity by the cold pool. Table 2 provides the surface to 2.5-km shear directed normal to the boundary (Δu). The ECSBF cases all have Δu values less than the WCF cases. In fact, most of the ECSBF cases have negative Δu values, which means the wind directed normal to the front increases with height, resulting in vorticity of the same sign as generated within the cold pool; just the opposite of that required to develop deep updrafts. Figure 20 of Rotunno et al. (1988) helps explain the shallow nature of the ECSBF relative to the WCF. In their simulations it is shown that the nose of the cold pool and depth of the updrafts increases considerably as Δu increases from 0 to 20 m s−1.
Beyond discriminating between the very low amount of storminess between the ECSBF and the relatively large amount of storminess with the WCF, Δu does not discriminate between the amount of storminess within the WCF cases. In fact, the 26 July case that had the most storminess and long-lived storms had the smallest shear value of the group. This is probably because when the winds are very low at the height of the storm steering level (<10 m s−1), the gust front will tend to move away from the storms. Thus, the importance of the low-level shear is less. The storm steering level flow during the Florida summer is generally quite slow. Table 1 showed the 700-mb winds, which very roughly correspond to echo motion (discussed later), were generally between 3 and 7 m s−1. Since gust fronts typically have speeds greater than this, it would be expected that the gust fronts would typicallymove away from the storms that produce them, resulting in short-lived storms, and indeed this is commonly observed during summer in Florida.
The vertical wind shear and relative motion between the storms and boundary are investigated further to see if they can help explain the observed variability in storm initiation, organization, and longevity.
5. Boundary relative cell speed
a. 9 August case
The 9 August case is useful for demonstrating that (a) storm initiation is less likely when the clouds and convergence lines are moving in opposite directions, (b) storms rapidly decay when the convergence line moves away from them, and (c) storms tend to merge and become more long lived when their movement is similar to the boundary motion. Figure 6 depicts a 3.5-h period on 9 August, showing the evolution of boundaries and storms. First, note the line of thunderstorms developing along the ECSBF (B1) from 1728 to 1831 (see Figs. 6a–c). Detailed inspection (not shown) of the cell evolution during this period shows that individual cells form near the ECSBF, intensify, and merge just east of the ECSBF. It appeared that many of the cells initially formed where horizontal convective rolls from the west side intersected the sea-breeze front in a manner similar to that previously described by Atkins et al. (1995). During this period the ECSBF is stationary and the individual cells drift slowly toward the southeast. After 1815 the ECSBF begins to move westward, partially in response to outflow from the thunderstorms. Note the increased easterly wind at 1831 at two of the PAM stations just west of the storms. As the ECSBF, partially modified by these outflows, moves away from the storms, the storms decay (see Figs. 6c–e). The temperature difference across the ECSBF is about 4°–5°C in the area enhanced by the storm outflow and about 3°C otherwise.
Close inspection of the CP-3 and CP-4 radar data shows that once the ECSBF begins to move, it passes under cumulus clouds moving from the northwest without significantly affecting their intensity. This is demonstrated in Fig. 7, where echo locations and maximum echo intensity observed by CP-3 are plotted relative to the ECSBF at 1848. The area covered by Fig. 7 is shown in Fig. 6c. Figure 7 shows the history of labeled echoes present at 1848. Note that there is little or no intensification of the echoes as they move over the boundary. Wilson and Mueller (1993) and Kingsmill (1995) have shown that small cumulus may rapidly intensify when a convergence line passes under them. Cloud echoes in the vicinity of the ECSBF, on the average, were moving from 312° at 5.2 m s−1, which is in a direction opposed to the motion of the ECSBF.
Figure 8a is a vertical cross section of the boundary-relative, dual-Doppler winds and reflectivity field at 1834 through the ECSBF. The location of the cross section is shown in Fig. 6c; the orientation of the cross section corresponds to the boundary-relative cell motion. It is apparent that the low-level flow is only weakly convergent at the boundary (X = −15) and the updrafts are weak, shallow, and sharply tilted from front to rear.
Figures 6e–h show that the storms tend to remain with the WCF (B2) as it moves toward the east. Figure 9 shows the evolution of storms along a portion (see Fig.6f) of the WCF between 1954 and 2018. Small cumulus cloud echoes (Figs. 6c and 6d) in advance of the boundary are seen to intensify and merge as the boundary passes under them. Both the boundary and the cloud echoes were moving from a westerly direction, but the boundary was moving faster. In this case the clouds were still moving away from the boundary but at a slower rate than observed earlier with the moving ECSBF. The absence of storms along the southern portion of the WCF (Figs. 6g and 6h) is discussed later.
A vertical cross section at 2059 through the WCF is shown in Fig. 8b. Similar to the cross section for the ECSBF, the winds are boundary relative and the cross section is oriented parallel to the boundary-relative cell motion. In this case there is strong low-level convergence and deep relatively vertical updrafts. The deep circulation centered near X = −5 is apparently the gust front head whose updraft portion merges with the updraft forced by the leading edge of the gust front (X = 0). A substantial outflow from the storm helps generate the strong low-level convergence near X = 0. The deeper head with the WCF (Fig. 8b) compared to the ECSBF (Fig. 8a) is as expected from the simulations of Rotunno et al. (1988) discussed above since Δu is 7.2 m s−1 and −4.4 m s−1 for the WCF and ECSBF, respectively.
Figure 10 illustrates this definition for the 9 August case. While the sea-breeze front was stationary, Ub was 2.4 m s−1 (Fig. 10a). After 1815 the ECSBF in the area of the storms accelerated to 7.5 m s−1 and Ub increased from 2.4 m s−1 to 9.9 m s−1 (Fig. 10b). As seen in Figs. 6b–d the storms rapidly dissipated from 15 to 45 min after this increase in Ub.
In contrast to the ECSBF, the WCF is moving from the west and northwest, which is a direction similar to the cell motion. On the average, the developing storms within the vicinity of the WCF were observed to be moving from 305° at 4.3 m s−1, similar to cells with the ECSBF. Depending on time and location along the WCF, the speed of the boundary motion varies between 6 and 12 m s−1 and the orientation between 0° and 75°. Variations in boundaryspeed and orientation are typically the result of the boundary bowing outward in response to fresh outflows from storms along the boundaries. Figure 10c is an example of the boundary-relative cell speed for a boundary orientation of 0° and movement of 7.5 m s−1 from the west, just the opposite motion from the example for the ECSBF in Fig. 10b. This motion translates into Ub = 3.1 m s−1, compared to 9.9 m s−1 for the moving ECSBF example in Fig. 10b. The WCF in Figs. 6e–h has roughly the orientation and speed of the example in Fig. 10c, which is associated with an extensive area of storms. Typically, Ub varies along the boundary. This is demonstrated in Fig. 10d, which shows Ub values along the WCF at 2059; they vary between 1.4 and 7.0 m s−1 depending on boundary motion and orientation.
The above suggests that storminess is favored with low Ub values. While it will be shown later that this is generally the case, there are complicating factors. An example can be seen from Fig. 10d and Fig. 6h, which are for the same boundary and same time. Note that a large storm complex can be seen along the northern part of the boundary (Fig. 6h) that is associated with Ub values in Fig. 10d of 7.0 and 3.2 m s−1, whereas along the southern part of the boundary almost no storms are associated with Ub values less than 2 m s−1. This seeming contradiction can be explained as follows. First, the 7 m s−1 Ub value along the northern part of the boundary has just increased to this value in response to outflow from the storm complex; note the bowing of the boundary. Second, the stability along the southern part of the boundary with the low Ub is relatively high. Satellite imagery shows that there is cirrus cloud covering this area left over from anvil cloud associated with the earlier storms along the stationary ECSBF. This is very similar to the situation described earlier for 15 July (Fig. 2). Radar data show that there are no cumulus cloud echoes underneath the cirrus clouds and the PAM data show temperatures are 2°–3°C cooler under the cirrus clouds. North of the cirrus cloud in advance of the WCF, where the stability is apparently lower, both satellite and radar data showed that there were small cumulus clouds. When the WCF intercepted these clouds, they intensified into the storms observed along the northern part of the WCF, as seen in Figs. 6g and 6h.
It would appear that the ECSBF remains shallow and weak relative to the WCF when the storm-steering level flow is westerly. One reason for this, as discussed in section 4b, is that the low-level shear directed normal to the boundary is very weakly positive to negative. Numerical simulations show this results in shallow updrafts and a shallow nose to the cold pool. Second, the relative absence of storms means there is not a continual reinforcement of the cold pool by thunderstorm outflows. In the cases studied, once the ECSBF began to move westward it was unable to initiate but a few new storms because of (a) the very tilted nature of the updrafts and (b) the short residence time of a cloud near the updraft source. Outflows reaching the ECSBF from the east only exacerbate thissituation since they accelerate the boundary movement westward. In contrast the updrafts with the WCF are more vertical and deeper and the growing clouds remain near the boundary and updraft source. Outflows from the storms then maintain a relatively intense deep convergence line. This is as expected from the earlier modeling simulations of Moncrieff and Miller (1976), Thorpe et al. (1982), Weisman and Klemp (1986), and Rotunno et al. (1988).
b. 26 July case
The 26 July case is examined in greater detail to gain insight about storm evolution along the boundary. Figure 11 shows the position of the WCF and cells along with Ub values (circled numbers) at separate locations along the boundary at three time periods. As was previously noted, there is often a wide range in the Ub values in time and space, primarily related to the onset and dissipation of individual storm outflows. Along the southern end of the boundary the Ub values range between 8 and 12 m s−1 and a gradual decrease and weakening of the echoes in that area is observed as the boundary moves away from the storms. Along the center portion the Ub values are between 3 and 5 m s−1 and storms gradually increase in coverage and intensify. The extreme northern part has a value of 3 m s−1 and the storms remain intense and without breaks until 1958 when they suddenly dissipate. The dissipation is believed to be related to the storms moving over the ocean (note shoreline in Fig. 11). It was typical for storms to dissipate when they moved over the ocean due to the increase in stability.
Detailed examination of the increase in activity with the central part of the boundary shows that small cumulus observed by radar and satellite in advance of the boundary intensify and grow as the boundary intercepts them and they gradually merge with one another, forming the solid line depicted in Fig. 11c at 1958. As was also observed with the 9 August case, while the ECSBF was stationary, merger of echoes took place along the convergence lines when the Ub values were small (Fig. 6b and 6c). This is reminiscent of the discussions by Simpson et al. (1980), who found that the merging process was a major factor in producing large organized shower systems. While they stated the merging process is not fully understood, they proposed that it was related to low-level convergence associated with the sea breeze and the interaction of downdrafts from storms that formed along the sea breeze. The results here suggest that the merging process requires that the storms and associated convergence line move at similar speeds.
6. Storm organization
In an attempt to quantify storm organization, two quantities termed “echo coverage” and “echo continuity” were computed along convergence lines. For the purposes here, the greater the percent coverage and the greater the percent continuity, the greater the storm organization. The parameters are computed for an area extending 20 km rearward of the boundary. The procedure for computing these parameters is illustrated in Fig. 12. The percent coverage was simply the area of echo greater than 30 dBZ divided by the total area extending 20 km behind the boundary; this is very similar to the term storminess defined in section 3. A measure of “solidness” or continuity of storms along the boundary is obtained by drawingperpendicular lines to the boundary at 2-km intervals and determining if an echo greater than 30 dBZ is intercepted within 20 km of the boundary. Continuity is then defined by the percent of perpendiculars that intercept the 30-dBZ echo. In addition, the average distance to the leading edge of the 30-dBZ echo is obtained.
Measurements of coverage, continuity, distance from the boundary, and Ub were made for all the WCF cases and the 9 August ECSBF case in Table 2. The analyses in Table 2 were for a single time period; the following analyses are for 1.5–2.5-h time periods. Average conditions were obtained for uniform sections of the boundaries 40–70 km in length at about 15-min intervals. The Ub values are averages for 30-min intervals. A single-cell motion was used for all times on a given day, which was an average determined from the analysis of a number of cells. Thus, changes in Ub represent changes in boundary speed and orientation. All the WCF cases, except for 26 July, begin shortly after the collision of the ECSBF and WCF. Figure 13 shows the time trends for continuity, coverage, distance from boundary, and Ub. The 9 and 15 August and 15 July cases show a significant decrease in coverage and continuity within an hour after collision of the ECSBF and WCF. In these cases Ub is generally increasing and greater than 5 m s−1. In addition, the distance of the cells from the boundary is observed to gradually increase. The decrease in continuity and coverage tends to lag the increase in distance of the echoes from the boundary by 30 min or less. For the 27 July case the continuity and coverage increase for 1.5 h following the collision and then decrease. For this case the Ub values are less than 5 m s−1, which is less than for the previous three cases. The 26 July case has the lowest Ub; typically less than 3 m s−1. In this case the continuity and coverage generally increase and remain high and the distance of the cells from the boundary remains relatively constant and is less than for all other cases. The 9 August ECSBF (Fig. 13f) case shows quantitatively what was earlier visible in Figs. 6a–d. The continuity and coverage rapidly decrease following the acceleration of the boundary away from the storms. Again there is about a 30-min lag in the decrease in continuity and coverage following the acceleration of the boundary away from the storms.
These analyses tend to support the earlier findings in Fig. 11 and sections 5a and 5b that Ub is a good indicator of cell organization and lifetime. For values below about 3–5 m s−1, the storms tend to be well organized and long lived. Above 5 m s−1, the distance between the boundary and storms increases with a resulting decrease in organization and lifetime. If Ub is small and then increases above 3–5 m s−1, there is roughly a 0.5-h lag before cell dissipation begins. The above discussion concerning storm organization and lifetime only considers Ub. Other factors such as magnitude of the convergence with the boundary, stability, and the presence of cumulus clouds in advance of the boundary can greatly modulate organization and lifetime.
7. Forecasting storm organization and lifetime
The forecaster desiring to anticipate storm organization and lifetime associated with aconvergence line needs to estimate Δu and Ub for possible boundary orientations and motions. The latter requires estimating cell speed and boundary speed.
a. Boundary speed
Observations of gust front propagation speeds show similar numbers. Goff (1975) examined 20 gust fronts from Oklahoma and obtained propagation speeds that varied between 5 and 25 m s−1, with an average of 11.5 m s−1. Mahoney (1988) studied 30 gust fronts from Colorado and obtained an average propagation speed of 8.6 m s−1. Propagation speeds ranged from 4 to 15 m s−1 based on movements over 60 min. Wakimoto (1982) examined three gust fronts from Illinois that had propagation speeds varying between 7 and 20 m s−1. The above calculations and observations all suggest that gust fronts propagate at speeds between 5 and 25 m s−1, with an average near 10 m s−1.
It is informative to compute Ub values for the boundaries in Table 2 using first the actual boundary motion and second assuming that the boundary is moving at the average gust front speed of 10 m s−1 (defined as U10b). These values are shown in Table 3. The computation of Ub is based on the average boundary orientation (direction of boundary motion is assumed normal to direction of orientation) and speed of motion in the dual-Doppler lobe at the time of the analysis in Table 2. The procedure for determining cell motion was discussed in section 2. Equation (2) is then used to compute Ub and U10b. There is a significant difference in U10b values between the ECSBF and WCF. The U10b values with the ECSBF are all in excess of 11 m s−1, which means that the cells would move at least 20 km away from the boundary in 30 min, thus rapidly moving away from the updraft source. The U10b values for the WCF vary between 4.7 and 10.0 m s−1. While these are smaller than for the ECSBF cases, they still suggest that the gust fronts will move away from the storms. Only the 15 and 26 July WCF cases have cell motions approaching the speed that an average gust front would move (9.3 and 7.2 m s−1, respectively). Table 2 shows that these cases have the highest percent echo coverage. As discussed earlier, 26 July has the lowest Ub value and largest percentcoverage and continuity.
b. Cell motion
The cell motion can generally be estimated by the average winds within the layer containing the cell. Weisman and Klemp (1986) use the average wind in the lowest 5–7 km of the atmosphere. Fankhauser (1964) has used the average vector wind from the winds at 850, 700, 500, and 300 mb. The observed cell motions for the cases in Table 3 were compared with the layer-average winds for a wide variety of intervals between 1 and 7 km. The 2–4-km average layer wind gave the best result and is shown in Fig. 14. The coefficient of correlation r for direction and speed is 0.96 and 0.94, respectively. Only slightly lower coefficients of correlation were obtained for layers between 1 and 5 km, 1 and 4 km, and 2 and 5 km. The inclusion of winds at heights of 6 and 7 km led to a significant deterioration in the correlation. Thus, at least for Florida, the forecaster can estimate with a high degree of reliability the cell motion by the 2–4-km mean layer wind. This is true for individual smaller cells and not necessarily for the motion of storm complexes or large storms in strongly sheared environments similar to those considered by the above authors.
c. Guidelines
By estimating the cell motion for a given day, the forecaster can anticipate the amount of activity with a sea-breeze front or which directions of boundary motion are most likely to maintain storms.
Knowing the cell motion the forecaster can easily estimate Ub values for the sea-breeze front while stationary and then when it accelerates inland. Using a simple schematic like Fig. 15, the forecaster can estimate U10b values for gust fronts with different orientations relative to the cell motion. Figure 15 assumes a cell speed of 10 m s−1; for cell speeds less than 10 m s−1, similar diagrams can easily be obtained by using Eq. (2). If the cells are moving faster than 10 m s−1, the gust front will also move faster than 10 m s−1 since the downdrafts from the same storms drive the gust front. In this case Ub is likely to be near zero and the low-level vertical wind shear (Δu) will likely be of moderate value, resulting in erect updrafts (Rotunno et al. 1988), thus contributing organized and long-lived convection.
8. Conclusions
During the CaPE project the sea-breeze front along the east coast of Florida (ECSBF) was found to initiate relatively few storms in comparison to gust fronts moving from the west (WCF). The WCF was observed to be a frequently occurring phenomenon when the winds at 700 mb contained a westerly component. Storms associated with the ECSBF were typically small and short lived. The following three reasons were identified for causing this relative lack of storminess with the ECSBF: 1) the convergence was weaker and shallower, 2) the updrafts were more shallow and tilted, and 3) the residence time of clouds in the convergence zone was shorter.
The winds, while light and exhibiting weak vertical wind shear, were generally westerly; thus for the westerly moving ECSBF the ambient vertical shear directed normal to the front resulted in negative horizontal vorticity. As shown by the modeling simulations of Rotunno et al. (1988), this produced a relatively shallow head to a density current with shallow, tilted updrafts much as observed here. In addition, because of the westerly flow at steering level, the clouds moved in a directionopposite to the boundary motion, resulting in a short residence time of the clouds in the convergence zone. When thunderstorms did develop along the ECSBF, their outflows would accelerate the westward motion of the ECSBF resulting in even more tilted updrafts and shorter residence of clouds in the convergence zone. This resulted in the rapid decay of existing storms and little to no storm initiation. It would be expected that the ECSBF would produce more storms when the flow at storm-steering level was easterly. There were several days during CaPE with easterly flow at the steering level (see 700-mb winds in Table 1). On these days there was very little convection. For these cases the surface winds also tended to be easterly, resulting in a very weak ECSBF that rapidly moved inland. It is hypothesized that if there are westerly surface winds to retard the movement of the ECSBF and the storm-steering flow is from the east that storminess with the ECSBF would be much greater. It is suspected that the large number of cases with westerly flow during the summer of 1991 was atypical. This is partially based on a study by Blanchard and Lopez (1985), who classified into four categories convective patterns from two summers in south Florida. While they did not give specific number of occurrences for each category, there was a strong suggestion that easterly wind cases were common.
This observational study showed results similar to the modeling studies by Moncrieff and Miller (1976) and Weisman and Klemp (1986) that the lifetime of storms and/or squall lines depends on the relationship of the storm motion to the convergence line motion. That is, a well-organized, long-lived squall line requires the storms to propagate at nearly the same speed as the convergence line. If the convergence line accelerates away from the storms, they will soon decay, and if new storms are initiated they are scattered and short lived. Because of the very light storm-level, steering winds observed during CaPE, gusts fronts nearly always moved faster than the storms, resulting in short-lived storms and squall lines.
A quantity called the boundary-relative cell speed Ub has been defined and examined as a means for estimating cell initiation, organization, and lifetime with a convergence line. For Ub values less than 3–5 m s−1 there was considerable storm initiation and relatively well organized, long-lived squall lines. When Ub values were less than 5 m s−1 and were observed to increase above 5 m s−1, the cells tended to decrease in number after about 15–30 min. For Ub values greater than 10 m s−1 storm initiation is likely relatively rare and storms will tend to be scattered and short lived.
The Florida forecaster, when attempting to anticipate the degree of storm initiation, organization, and lifetime with the sea-breeze fronts and gust fronts, can use the information in the preceding paragraph as a guideline. To estimate Ub, the forecaster needs to estimate cell motion and boundary motion. As shown here, the 2–4-km average layer wind gives an excellent estimate of cell motion. The forecaster then only needs to assume a variety of convergence line orientations and movements to determine those that will give Ub values that most or least favor long-lived organized squall lines.
While these results were obtained in Florida, there is no reason to believe they would not be applicable to other regions, particularly under low vertical wind shear conditions. There are plans to test these procedures in Colorado, Georgia, and Tennessee using an automated thunderstorm nowcasting system described by Henry and Wilson(1995).
Acknowledgments
We are grateful to the NCAR technicians and engineers responsible for operating the radars, mesonet, and sounding systems. We are appreciative of scientific discussions with Cindy Mueller of NCAR, who inspired some of the original ideas for the paper. We are also appreciative of Tammy Weckwerth, Morris Weisman, and Rita Roberts, all of NCAR, and David Kingsmill, of the Desert Research Institute, who provided excellent indepth reviews of an earlier version of this paper.
This research is partially sponsored by the National Science Foundation through an interagency agreement in response to requirements and funding by the Federal Aviation Administration’s Aviation Weather Development Program. The views expressed are those of the authors and do not necessarily represent the official policy or position of the U.S. government. The CaPE field program was jointly funded by NASA, FAA, U.S. Air Force, and NSF.
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Map showing the area and data collection facilities for this study. All analyses were done within the 60-km radius of CP-4, and the dual-Doppler analyses were within the 30° dual-Doppler lobes. This figure was modified from Fig. 1 of Wakimoto and Lew (1993). CCAFS (Cape Canaveral Air Force Station) indicates the radiosonde location for the 1500 UTC 700-mb winds given in Table 1. PAM station 25 is the location of the 1700 UTC surface wind observations in Table 1.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Location of convergence lines and convective precipitation echoes on 15 July. The times shown in the upper-left corner of each image are in UTC. The thick black lines labeled B1 and B2 are the leading edge of the east coast sea-breeze front and west coast front, respectively. The shaded area represents CP-4 radar echoes greater than 35 dBZ. The vectors are the PAM winds. Range marks are at 30-km spacing centered at CP-4. The coast line, lakes, and rivers are outlined in light gray. The dashed rectangles in (c) and (d) show the location of the analyses in Fig 3.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Dual-Doppler-derived divergence at 150 m (a) 1756 and (b) 1901 UTC. The location of each divergence analysis is shown in Figs. 2c and 2d, respectively. Divergence contours of −1 × 10−3 and −10 × 10−3 are shown by the solid and dashed lines, respectively. Radar reflectivity values greater than or equal to 10 dBZe are lightly shaded and greater than or equal to 30 dBZe are heavily shaded. Distances are in kilometers from CP-4.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Time history on 15 July of the dual-Doppler divergence at four heights above radar level for the ECSBF and WCF as they move through the south dual-Doppler lobe.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Height profiles of average divergence for the eight ECSBF (solid lines) and five WCF (dashed lines) cases in Table 2.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Location of convergence lines and convective precipitation echoes on 9 August. The thick black lines labeled B1 and B2 are the leading edge of the ECSBF and WCF, respectively. The dashed rectangles in (c) and (h) indicate the locations of Figs. 7 and 9, respectively. The dashed lines in (c) and (h) show the locations of the vertical cross sections in Figs. 8a and 8b, respectively. The rest is as in Fig. 2.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
(Continued)
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
History of cell intensity and location relative to the ECSBF for cells (A, B, C, D, and F) present at 1848 UTC 9 August. Relative cell positions are shown at 7-min time intervals prior to 1848 UTC. The maximum dBZ value at each time period is shown. The location of the area is shown in Fig. 6c by the dashed rectangle.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Vertical cross section of dual-Doppler-derived winds and radar reflectivity factor through (a) the ECSBF at 1834 UTC and (b) the WCF at 2102 UTC. The location of the ECSBF is at X = −15 km in (a). The location of the WCF is at X = 0 in (b). The winds are boundary relative. The vertical dimension is exaggerated by a factor of 3 compared to the horizontal thus the updrafts appear more erect than actual. The location of each cross section is respectively indicated in Figs. 6c and 6h.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Evolution of storms along the WCF between 1954 and 2018 UTC 9 August. The location of the area is shown by the dashed rectangle in Fig. 6f. Contours of −5 and 50 dBZ are shown. The maximum reflectivity at selected locations is shown. Cells are lettered to assist in following from time to time. The position of the WCF is based on the location of the reflectivity thin line and maximum radial convergence.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Examples from 9 August of the “boundary-relative cell speed” Ub, defined as the normal to the boundary of the vector difference between the cell motion and boundary motion. (a) Stationary ECSBF, (b) ECSBF moving west at 7.5 m s−1, (c) WCF moving east at 7.5 m s−1, and (d) WCF at 2059.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Merging of cells along the WCF on 26 July at three time periods: (a) 1928, (b) 1944, and (c) 1958 UTC. The Ub values (m s−1) along the WCF are shown within the circles. The light shading corresponds to reflectivities greater than or equal to 30 dBZ and dark shading to greater than or equal to 50 dBZ. Range marks are at 20-km intervals.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Schematic illustrating the definition and computation of coverage, continuity, and distance of cells from the boundary. The total area used in the coverage computation is the summation of the light and dark shading.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Time histories of storm coverage, continuity, average distance from boundary, and boundary-relative cell speed Ub for six cases: (a) 9 August, (b) 15 August, (c) 15 July, (d) 27 July, (e) 26 July, and (f) 9 August. The storm coverage and continuity are given by the ordinate on theleft side and average distance and Ub by the right side ordinate. The time of the analyses provided in Table 2 is shown by the arrow along the abscissa.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Comparison of (a) cell direction and (b) cell speed with the layer mean wind direction and speed, respectively, between 2 and 4 km for the 13 cases in Table 2. The coefficient of correlation (r) is given in the upper left.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Hypothesized circular outflow that is spreading outward at 10 m s−1, which shows, at 30° increments, the boundary-relative cell speed U10b. A 10 m s−1 cell motion is assumed in the direction indicated.
Citation: Monthly Weather Review 125, 7; 10.1175/1520-0493(1997)125<1507:TIOALA>2.0.CO;2
Degree of storminess (N: none, B: below average, A: average, +: above average) associated with each convergence line observed within 60 km of CP-4 during CaPE. Also shown are the surface winds at 1700 UTC from PAM station 25 and the 700-mb wind at 1500 UTC from the CCAFS sounding (see Fig. 1 for location). Cases italicized and in bold type are used in subsequent analyses. Msg indicates missing data.
Degree of storminess (percent echo greater than or equal to 30 dBZ) associated with 13 convergence lines with respect to stability and kinematic variable. The convergence is an average along the boundary at a height of 150 m. The Δu is the surface to 2.5-km boundary-relative shear directed normal to the boundary orientation. The column labeled “time” indicates the time of the dual-Doppler, percent echo coverage, and ΔT analyses. The ΔT is the average temperature difference across the boundary. Other values are obtained from a sounding on the warm side of the boundary within 100 min prior to the indicated time. Msg signifies missing data.
Boundary-relative cell speeds Ub for the 13 convergence lines in Table 2. The U10b values assume the boundary is moving at 10 m s−1. Theta (θ) is the angle between the boundary and cell direction of motion. Parameters Sb, Sc, and θ are used in (2) to compute Ub.
