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  • View in gallery

    Composite reflectivity data from (a) first storm, (b) initiation, (c) mature, and (d) dissipation MCS life cycle stages from an event on 12–13 Jun 2002 over OK and TX. Squares indicate the location of mesonet stations selected to sample the cold pool(s) at the four life cycle stages, although they may not correspond to the time of the radar image. Mesonet stations at Foraker (F) and Norman (N), OK, are shown in (b) and (c), respectively. Reflectivity values above 5 dBZ are shaded and change shade every 5 dBZ. Values above 40 dBZ are black, while values above 50 dBZ are a lighter gray surrounded by black. (Courtesy of the image archive at NCAR)

  • View in gallery

    Time series from the Foraker mesonet site of wind direction (°), surface pressure (hPa), and equivalent potential temperature (K) over a 480-min period (8 h) centered approximately at the time of cold pool arrival at the site on 13 Jun 2002. Vertical gray line indicates analyzed time of cold pool arrival. Dark shaded areas indicate the 30-min period prior to the arrival of the cold pool, while the light gray areas indicate the 2-h period after the arrival of the cold pool. Maximum and minimum values of pressure and equivalent potential temperature are indicated.

  • View in gallery

    As in Fig. 2, but for the Norman mesonet site.

  • View in gallery

    Box and whisker plots of (a) pressure rise (hPa) and (b) potential temperature (K), (c) temperature (K), and (d) equivalent potential temperature (K) deficits associated with the analyzed cold pools for first storms, MCS initiation, mature MCS, and MCS dissipation life cycle stages, as well as for the stations that had the convective line pass overhead between 1800 and 0200 UTC. Boxes denote the 50% of values between the 25th and 75th percentiles (the interquartile range), with the thin vertical line indicating the maximum and minimum values and the horizontal line within each box indicating the mean value.

  • View in gallery

    Scatterplots of potential temperature deficits (K) vs surface pressure rise (hPa) associated with (a) the cold pools of all the mature MCSs as sampled using mesonet data and (b) the cold pools of mature MCSs sampled between 1800 and 0200 UTC.

  • View in gallery

    As in Fig. 4, but for maximum wind gusts (m s−1) within the cold pool.

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Surface Characteristics of Observed Cold Pools

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  • 1 NOAA/National Severe Storms Laboratory, Norman, Oklahoma
  • | 2 Cooperative Institute for Mesoscale Meteorological Studies, and NOAA/National Severe Storms Laboratory, Norman, Oklahoma
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Abstract

Cold pools are a key element in the organization of precipitating convective systems, yet knowledge of their typical surface characteristics is largely anecdotal. To help to alleviate this situation, cold pools from 39 mesoscale convective system (MCS) events are sampled using Oklahoma Mesonet surface observations. In total, 1389 time series of surface observations are used to determine typical rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool. The data are separated into one of four convective system life cycle stages: first storms, MCS initiation, mature MCS, and MCS dissipation. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms’ life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits associated with cold pools decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to the dissipation stage, with a decrease of approximately 1 K associated with each advance in the life cycle stage. However, the daytime and early evening observations show mean temperature deficits over 11 K. A comparison of these observed cold pool characteristics with results from idealized numerical simulations of MCSs suggests that observed cold pools likely are stronger than those found in model simulations, particularly when ice processes are neglected in the microphysics parameterization. The mean deficits in equivalent potential temperature also decrease with the MCS life cycle stage, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Mean wind gusts are above 15 m s−1 for all life cycle stages. These results should help numerical modelers to determine whether the cold pools in high-resolution models are in reasonable agreement with the observed characteristics found herein. Thunderstorm simulations and forecasts with thin model layers near the surface are also needed to obtain better representations of cold pool surface characteristics that can be compared with observations.

Corresponding author address: Dr. David J. Stensrud, National Severe Storms Laboratory, National Weather Center, Room 4368, 120 David L. Boren Blvd., Norman, OK 73072. Email: david.stensrud@noaa.gov

Abstract

Cold pools are a key element in the organization of precipitating convective systems, yet knowledge of their typical surface characteristics is largely anecdotal. To help to alleviate this situation, cold pools from 39 mesoscale convective system (MCS) events are sampled using Oklahoma Mesonet surface observations. In total, 1389 time series of surface observations are used to determine typical rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool. The data are separated into one of four convective system life cycle stages: first storms, MCS initiation, mature MCS, and MCS dissipation. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms’ life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits associated with cold pools decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to the dissipation stage, with a decrease of approximately 1 K associated with each advance in the life cycle stage. However, the daytime and early evening observations show mean temperature deficits over 11 K. A comparison of these observed cold pool characteristics with results from idealized numerical simulations of MCSs suggests that observed cold pools likely are stronger than those found in model simulations, particularly when ice processes are neglected in the microphysics parameterization. The mean deficits in equivalent potential temperature also decrease with the MCS life cycle stage, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Mean wind gusts are above 15 m s−1 for all life cycle stages. These results should help numerical modelers to determine whether the cold pools in high-resolution models are in reasonable agreement with the observed characteristics found herein. Thunderstorm simulations and forecasts with thin model layers near the surface are also needed to obtain better representations of cold pool surface characteristics that can be compared with observations.

Corresponding author address: Dr. David J. Stensrud, National Severe Storms Laboratory, National Weather Center, Room 4368, 120 David L. Boren Blvd., Norman, OK 73072. Email: david.stensrud@noaa.gov

1. Introduction

Cold pools are evaporatively cooled areas of downdraft air that spread out horizontally underneath a precipitating cloud. They can be an important focus for the development of new convective cells, since some of the environmental air that approaches a cold pool is lifted up and over it (Byers and Braham 1949; Purdom 1976; Wilson and Schreiber 1986). The development of new cells along the boundary between the cold pool and the environment, commonly called the outflow boundary, is known to be the primary mechanism for the sustenance of multicell thunderstorms (Weisman and Klemp 1986). If the cold pool is too strong, however, then it may quickly move ahead of the thunderstorm and cut off the supply of warm air to the updraft, leading to thunderstorm demise. In addition, the movement of supercell thunderstorms over preexisting cold pools is often observed to lead to the development of significant tornadoes within the cold pool region (Maddox et al. 1980; Markowski et al. 1998a).

The cold pools from a number of precipitating thunderstorms can combine into a coherent mesoscale-sized cold pool (100–400 km in horizontal extent) as is commonly observed in association with mesoscale convective systems (MCSs; Fujita 1959; Johnson and Hamilton 1988). Numerous modeling studies indicate that the cold pool is a key feature in the organization of deep convection (Thorpe et al. 1982; Rotunno et al. 1988; Szeto and Cho 1994), although the environmental wind shear also plays a role (Szeto and Cho 1994; Coniglio et al. 2006). The lifting of environmental air along mesoscale-sized cold pool boundaries can produce moist absolutely unstable layers of more than 100 hPa in depth, which may influence the internal structure of the convective system (Bryan and Fritsch 2000). Soundings taken within the cold pools of MCSs indicate that cold pool depths in excess of 3 km may be common (Bryan et al. 2005).

As the colder air within a cold pool accumulates at and near the ground surface, the surface pressure is increased hydrostatically as a function of the temperature and depth of the cold pool (Wakimoto 1982). This zone of higher pressure associated with the cold pool is called a mesohigh. In addition, winds within the cold pool are often gusty as momentum from higher levels is transported downward to the surface and as a response to the evolution of the surface pressure field (Vescio and Johnson 1992).

With the obvious importance of cold pools to convective development and evolution, it is rather surprising that no studies have systematically investigated a large number of cold pools to determine their typical surface characteristics for different stages in the life cycle of convective systems. Thus, this study investigates the surface characteristics of cold pools at different life cycle stages for MCSs that pass over Oklahoma. The surface characteristics examined are the changes in surface pressure, temperature, potential temperature, and equivalent potential temperature associated with the cold pool and the maximum wind speeds in the cold pool. The data and methodology used to identify cold pools are outlined in section 2, followed by results in section 3. A final discussion on the implications of the results is found in section 4.

2. Data and methodology

The surface data used to define the observed characteristics of cold pools are from the Oklahoma Mesonet, a network of over 110 surface observing stations that span the state of Oklahoma (Brock et al. 1995; McPherson et al. 2007). The data are available as 5-min average values for above-ground variables and must pass a robust quality control system (Shafer et al. 2000; Fiebrich and Crawford 2001). For this study, surface pressure (hPa), 1.5-m temperature (K), 1.5-m relative humidity (%), and 10-m wind speeds (m s−1) are used, with expected field accuracies of 0.4 hPa, 0.35 K, 3%, and 0.3 m s−1, respectively (McPherson et al. 2007). Values of potential temperature and equivalent potential temperature are calculated from these observations following Bolton (1980).

National composite radar imagery are examined from April to August of 2000–07 for MCSs that develop or propagate over Oklahoma. Only events that result in MCSs with leading convective lines of ∼200 km or more in length sometime during their life cycle are studied. This length requirement filters out the smaller and shorter-lived MCSs and, with an average mesonet station spacing of just under 30 km, typically allows for at least 5 mesonet stations to sample each cold pool. No requirements are placed on MCS longevity. Cold pool characteristics at four MCS life cycle stages are extracted: first storms, initiation, mature, and dissipation. The first storms life cycle stage is defined as when the first convective storms are observed, with reflectivities in excess of 50 dBZ, which eventually lead to the development of the MCS. The initiation stage is when these individual convective cells merge into a convective line. The mature stage is when the MCS has a region of nearly contiguous convective echoes usually in a line or arc along its leading edge. The dissipation stage is when the reflectivities associated with the leading convective line decrease and the line becomes less cohesive.

For each of the MCS life cycle stages, mesonet stations are identified that pass underneath the more intense regions of convection, either the individual convective cells or the convective line (Fig. 1). Once a convective line is formed, all available stations are chosen that sample the preconvective conditions before passing underneath the convective line. The mesonet station observations are examined over a period that extends from 4 h prior to 4 h after the time convection is observed over the station location. Time series of surface pressure, 1.5-m potential temperature, and 10-m wind speed and direction are used to determine the time of the outflow boundary passage and the characteristics of the cold pool. Owing to the MCS propagation speeds and locations of convective initiation (often to the west or north of Oklahoma), not all life cycle stages are sampled for each MCS.

The time of the outflow boundary passage at a given station is determined primarily by finding a rapid change in wind direction, or wind shift (Figs. 2 and 3). Strong shears in horizontal winds are typically associated with the leading edge of cold pools (Fujita 1963; Charba 1974; Goff 1976; Wakimoto 1982; Mueller and Carbone 1987), and on nearly all days examined the time of outflow boundary passage is determined from the observed change in wind direction. Surface pressure typically begins to rise prior to the outflow boundary passage (Fujita 1963; Charba 1974; Goff 1976), so the surface pressure prior to the arrival of the cold pool is determined by finding the minimum surface pressure during the 30-min period prior to the wind shift (Figs. 2 and 3). In an examination of 20 cold pools, Goff (1976) finds an average of 17-min separation between the beginning of the pressure rise and the arrival of the wind shift, although time separations of up to 45 min are reported. Thus, a 30-min window appears to be reasonable. This same 30-min window is used to determine the maximum temperature, maximum potential temperature, and maximum equivalent potential temperature of the environment ahead of the cold pool. The maximum pressure and minimum values of temperature, potential temperature, and equivalent potential temperature associated with the cold pool are determined from mesonet observations during a 2-h time window starting immediately after the wind shift. Using the maximum and minimum values of these variables, the pressure rises and temperature deficits associated with the cold pool can be calculated. In addition, the strongest wind gusts during the 2-h period after the wind shift are also determined. Finally, each station time series is examined manually to verify the time of the wind shift and quality control any unusual situations that may alter the results. This examination yields confidence that the resulting calculations apply only to conditions within cold pools. Only a handful of the resulting computations are changed by the manual examination of the time series and most changes result in modifications to the cold pool characteristics by less than 20% of their original values.

Several factors may influence the interpretations that result from these calculations. During the daytime, cloud-shading effects can reduce the 1.5-m temperatures ahead of the MCS (Markowski et al. 1998b; Markowski and Harrington 2005). The effects of cloud shading are seen in Fig. 2 where the equivalent potential temperature slowly decreases for several hours prior to the arrival of the gust front. Visible satellite imagery indicates that the anvils from thunderstorms to the east are located above this mesonet site during this period of cooling (not shown). Nocturnal cooling also has an influence as near-surface temperatures typically decrease faster than temperatures above the surface as the nocturnal boundary layer develops. Since most of the MCSs studied reach maturity after sunset, nocturnal boundary layers often developed in the preconvective environments ahead of the MCSs. In addition, the calculations of pressure rise may be influenced by mesolows or pressure troughs that form in advance of some thunderstorms due to subsidence warming (Hoxit et al. 1976). A mesolow is suggested in Fig. 2 in association with the local pressure minimum near 230 min, just prior to the arrival of the cold pool. However, mesolows are not seen in all events.

A total of 39 MCS events are identified between 2000 and 2007 that meet the established criteria. For these 39 events, observations from 1389 Oklahoma Mesonet stations are collected and analyzed to determine typical cold pool characteristics (i.e., rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool). A total of 47 stations are associated with the first storms life cycle stage, 156 with the MCS initiation stage, 948 with the mature MCS stage, and 238 with the MCS dissipation stage. Since idealized simulations of convective systems often use environmental thermodynamic profiles typical of the late afternoon and early evening, the 200 station time series that had outflow boundary passage between 1800 and 0200 UTC (regardless of the MCS life cycle stage) are also examined.

3. Results

Results from the analysis of Mesonet data indicate that the mean pressure rise associated with the cold pools is over 3 hPa for all MCS life cycle stages (Fig. 4). For the first storms stage, the mean 3.2-hPa pressure rise is slightly larger than the 2.5 hPa found in association with 20 individual thunderstorms by Goff (1976). In a more complete documentation of these events, Goff (1975) shows the range in surface pressure rises to be 0.8–6.6 hPa, which is very comparable to the first storm values in Fig. 4. The mean pressure rise associated with the cold pool increases during the initiation and mature stages, reaching a mean value of 4.5 hPa for the mature stage. Pressure rises of less than 3.0 hPa for the mature life cycle stage are observed in only 25% of the data, indicating that mature MCSs are typically associated with relatively large cold pool pressure rises. The maximum pressure rise reported is 9.4 hPa for the MCS event on 21 July 2000, and 40 station time series have pressure rises in excess of 8 hPa for mature systems. The mean pressure rises decrease for the dissipation life cycle stage, where only 15 station time series have pressure rises in excess of 6 hPa. The distribution of pressure rises associated with convective systems between 1800 and 0200 UTC is most similar to the distribution of the mature MCS life cycle stage. The pressure rises reported in the literature in association with MCSs have a mean value of 3.6 hPa and a maximum value of 7 hPa (Table 1), slightly less than those found from the Mesonet data for mature MCSs.

The deficits in temperature and potential temperature associated with cold pools show a steady decrease through the MCS life cycles (Fig. 4). The mean potential temperature decrease of 9.8 K associated with the first storms is larger than the 4.0–6.1-K mean decreases reported by Goff (1976) for intensifying or mature thunderstorms. However, the mean potential temperature decrease of 8.8 K associated with the initiation life cycle stage agrees well with the 8.0-K value calculated by Evans and Doswell (2001) for derecho-producing convective systems and the 8.9-K value found by averaging the temperature changes reported from convective systems in the literature (Table 2). However, the 13-K maximum temperature change reported in the literature is quite a bit less than the 17-K maximum temperature change seen from the mesonet data. One hypothesis for the difference between the values for the first storms life cycle stage is that colder cold pools are associated with thunderstorms that grow upscale into MCSs, while Goff (1976) examined individual thunderstorms without regard to their future upscale growth.

The mean temperature (potential temperature) deficits decrease by approximately 1 K between each life cycle stage from 9.5 K (9.8 K) at the first storms stage down to 5.4 K (5.6 K) for the dissipation stage (Fig. 4). Much of this decrease is attributed to nocturnal cooling ahead of the MCSs, since the first storms occur on average at 0100 UTC (subtract 5 h for local time), initiation occurs near 0240 UTC, maturity at 0420 UTC, and dissipation at 0445 UTC (note that since not all four life cycle stages are sampled for each MCS event, the time difference between the life cycle stages is not representative). Results from MCS events that have station data at all four life cycle stages indicate that the mean environmental temperature decreases by nearly 8 K between first storms and dissipation, whereas the mean cold pool temperature decreases by less than 2 K over this same time period. Thus, cold pool temperatures are less modulated by the diurnal cycle than the environmental temperatures ahead of the convective systems. It is therefore not surprising that the largest mean temperature (potential temperature) deficits are 10.9 K (11.2 K) from the 200 station time series that had convection pass overhead during and shortly after daylight hours (1800–0200 UTC; Fig. 4).

These results show that between the first storms and mature MCS life cycle stages the mean pressure rise increases while the mean potential temperature deficit becomes smaller. Assuming that much of the cold pool pressure excess increases hydrostatically as a function of the cold pool temperature (Wakimoto 1982), this inverse relationship suggests that the cold pool is deepening as the MCS matures. These results also suggest that cold pool pressure rises are a better indicator of MCS life cycle stage than temperature deficits.

Further examination of Fig. 4 shows that many MCS events produce temperature changes above 10 K for both the initiation and mature life cycle stages regardless of the time of day, indicating that strong temperature changes in association with cold pools can be produced at any time during the MCS life cycle. The maximum temperature (potential temperature) decrease associated with a cold pool is 17.1 K (17.5 K) on 29 May 2006.

The relationship between cold pool pressure rises and potential temperature deficits is explored for the mature MCS life cycle stage using a scatterplot (Fig. 5a). While there is a fair amount of scatter, the data suggest a weak-to-moderate correlation (r = 0.38) between pressure rises and potential temperature deficits. However, for pressure rises between 4 and 6 hPa the associated potential temperature deficit can vary by a factor of 10 or more. The relationship becomes somewhat better, particularly for the smaller pressure rises, when only those stations that had a mature convective line pass overhead between 1800 and 0200 UTC are considered (Fig. 5b). However, a fair amount of scatter remains for the larger pressure rises. The lack of a strong relationship between the surface pressure rises and surface potential temperature deficits likely reflects the complex vertical buoyancy distributions that often occur within and above the cold pool (Bryan et al. 2005; Trier et al. 2006), such that the surface potential temperature deficit is often not correlated to the overlying buoyancy distribution, even for daytime cases.

The mean values of equivalent potential temperature deficits also decrease throughout the MCS life cycle (Fig. 4), starting at 21.6 K for first storms and decreasing to 13.9 K at dissipation. For the first storms, initiation, and mature MCS stages, over 75% of the observations have deficits of more than 12 K associated with the cold pool. The range of values is very similar for the first three life cycle stages, while only deficits of less than 26.5 K are observed during the dissipation stage. The largest mean equivalent potential temperature deficit is again associated with the cold pools observed between 1800 and 0200 UTC.

The mean maximum wind gusts during the four life cycle stages reaches a maximum for the mature stage and is a minimum for the dissipation stage (Fig. 6). Curiously, the strongest winds from individual events increase as the convective system ages. However, this result is likely due to the small sample size, which is particularly a concern for strong wind gusts that can occur in very localized regions. For the initiation stage, only seven wind gusts are greater than 30 m s−1. For the mature MCS stage, 50 observations out of the 948 station time series are greater than 30 m s−1, whereas only 1 wind observation is greater than 30 m s−1 for the dissipation stage—the 50 m s−1 observation on 16 June 2005. The interquartile range is very similar for the first storms, initiation, and mature MCS stages, only showing a distinct lowering for the dissipation stage, suggesting that little difference in the 10-m wind gust strength is expected in the first three MCS life cycle stages. The distribution of wind gusts between 1800 and 0200 UTC are very similar to the gusts from the four MCS life cycle stages.

4. Discussion

While cold pools are a key element in the mesoscale organization of convection, knowledge of typical surface cold pool characteristics is largely based upon anecdotal observations from a handful of cases (Tables 1 and 2). Thus, the results of this study fill an important void in the literature of cold pools through the examination of 39 MCS events using 1389 surface station time series from the Oklahoma Mesonet. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to dissipation stage, with typical decreases of 1 K associated with each advance in the life cycle stage. The mean changes in equivalent potential temperature have a similar trend, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Results also show that cold pool temperatures appear to be less influenced by the diurnal cycle than the environmental temperatures ahead of the convective systems. Mean wind gusts are above 15 m s−1 for all life cycle stages.

The surface pressure rises are found to be weak-to-moderately correlated with the surface potential temperature deficits, with a somewhat better correlation when considering the late afternoon and early evening cases only. Results further show that the mean pressure rise increases between the first storms and mature life cycle stages, while the mean potential temperature deficit becomes smaller, suggesting that the cold pool are deepening as the MCS matures. While these characteristic surface observations are insufficient to determine the typical cold pool depths, they are a starting point from which to better evaluate model results of MCSs and determine if simulated cold pool characteristics span the observed ranges. Further studies that use special field program observations to calculate cold pool depth as done in Bryan et al. (2005) are clearly needed, but until then a better understanding of typical cold pool surface characteristics is helpful.

A comparison of the observed cold pool characteristics with results from idealized numerical simulations of MCSs, upon which many widely discussed theories for squall-line behavior are founded (see Weisman and Rotunno 2004, 2005; Parker and Johnson 2004a; Stensrud et al. 2005; Coniglio et al. 2006), suggests that MCS cold pools are often colder than what is represented in the simulations (Table 3). The mean temperature and potential temperature deficits among the simulations listed in Table 3 is about 8.7 K. If the deficits are separated by the type of microphysics scheme used, it is found that schemes without ice microphysics have a mean temperature deficit of 8.2 K, while those with ice microphysics have a mean temperature deficit of 10.2 K. These results suggest that the inclusion of ice processes leads to colder cold pools. Note that the maximum simulated temperature deficit is 13 K.

Since the vast majority of MCS simulations are performed with thermodynamic profiles representative of a late-afternoon or early-evening boundary layer, it is most appropriate to compare the simulated cold pool characteristics with the cold pool characteristics observed in the late afternoon and early evening. The mean potential temperature deficit for cold pools during this time window is about 11 K, with over 80% of the deficits above 8.7 K (Fig. 4), and a maximum value of 17 K. This mean potential temperature deficit is slightly larger than the 10.2-K mean deficit found in simulations with ice microphysics and much larger than the 8.2-K mean deficit found in simulations that neglect ice microphysics. In addition, 25% of the observed deficits are larger than 13 K, whereas the largest deficit found in the model simulations is 13 K. While many modeling studies have the lowest model level at ∼200 m above ground level, suggesting that surface temperature deficits are underestimated by a few degrees, these models also do not include the cloud shading and nocturnal cooling effects that can act to reduce the observed surface temperature deficits by several degrees or more. The net result of these differences is uncertain, but it appears reasonable to assume that the observed effects of cloud shading and nocturnal cooling are at least as large as the model effects of the lowest model level height. Thus, simulated MCSs with strong cold pools are likely underrepresented in the modeling literature, particularly when ice processes are neglected in the microphysics parameterization. Thunderstorm simulations and forecasts with thin model layers near the surface are needed to obtain better representations of cold pool surface characteristics that can be compared with observations.

In addition to the underrepresentation of cold pools with very cold surface temperatures in the literature, recent studies by Bryan et al. (2005) and simulations by Trier et al. (2006) suggest that observed cold pools are often deeper than simulated cold pools from idealized simulations. In addition, observed cold pools typically do not display the monotonic decrease in negative buoyancy with height often seen in the simulated cold pools in the near-neutral environments of past idealized simulations and instead negative buoyancy is often observed to increase with height above the surface (Bryan et al. 2005). Since cold pools are a key factor in the development and organization of convection, the results presented herein suggest that it is important to determine how cold pool characteristics are represented in the high-resolution numerical forecast models that are used in research and operations. If these models fail to represent cold pool characteristics correctly, then their ability to correctly predict convective system evolution is likely reduced and improvements to the model physical parameterizations may be needed to reach the goal of accurate convective-scale forecasts.

Acknowledgments

We are very grateful to the Oklahoma Mesonet for providing the quality-controlled surface data used as the foundation of this research. The first author (NAE) thanks the NOAA Hollings Scholar program for providing the funding to spend a summer working on this project at the NSSL. Very constructive and helpful comments from Paul Markowski, Matt Parker, and an anonymous reviewer are greatly appreciated. Composite radar images were obtained online (http://www.mmm.ucar.edu/imagearchive/). Partial funding was provided by NOAA/Office of Oceanic and Atmospheric Research under NOAA/University of Oklahoma Cooperative Agreement NA17RJ1227, U.S. Department of Commerce.

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Fig. 1.
Fig. 1.

Composite reflectivity data from (a) first storm, (b) initiation, (c) mature, and (d) dissipation MCS life cycle stages from an event on 12–13 Jun 2002 over OK and TX. Squares indicate the location of mesonet stations selected to sample the cold pool(s) at the four life cycle stages, although they may not correspond to the time of the radar image. Mesonet stations at Foraker (F) and Norman (N), OK, are shown in (b) and (c), respectively. Reflectivity values above 5 dBZ are shaded and change shade every 5 dBZ. Values above 40 dBZ are black, while values above 50 dBZ are a lighter gray surrounded by black. (Courtesy of the image archive at NCAR)

Citation: Monthly Weather Review 136, 12; 10.1175/2008MWR2528.1

Fig. 2.
Fig. 2.

Time series from the Foraker mesonet site of wind direction (°), surface pressure (hPa), and equivalent potential temperature (K) over a 480-min period (8 h) centered approximately at the time of cold pool arrival at the site on 13 Jun 2002. Vertical gray line indicates analyzed time of cold pool arrival. Dark shaded areas indicate the 30-min period prior to the arrival of the cold pool, while the light gray areas indicate the 2-h period after the arrival of the cold pool. Maximum and minimum values of pressure and equivalent potential temperature are indicated.

Citation: Monthly Weather Review 136, 12; 10.1175/2008MWR2528.1

Fig. 3.
Fig. 3.

As in Fig. 2, but for the Norman mesonet site.

Citation: Monthly Weather Review 136, 12; 10.1175/2008MWR2528.1

Fig. 4.
Fig. 4.

Box and whisker plots of (a) pressure rise (hPa) and (b) potential temperature (K), (c) temperature (K), and (d) equivalent potential temperature (K) deficits associated with the analyzed cold pools for first storms, MCS initiation, mature MCS, and MCS dissipation life cycle stages, as well as for the stations that had the convective line pass overhead between 1800 and 0200 UTC. Boxes denote the 50% of values between the 25th and 75th percentiles (the interquartile range), with the thin vertical line indicating the maximum and minimum values and the horizontal line within each box indicating the mean value.

Citation: Monthly Weather Review 136, 12; 10.1175/2008MWR2528.1

Fig. 5.
Fig. 5.

Scatterplots of potential temperature deficits (K) vs surface pressure rise (hPa) associated with (a) the cold pools of all the mature MCSs as sampled using mesonet data and (b) the cold pools of mature MCSs sampled between 1800 and 0200 UTC.

Citation: Monthly Weather Review 136, 12; 10.1175/2008MWR2528.1

Fig. 6.
Fig. 6.

As in Fig. 4, but for maximum wind gusts (m s−1) within the cold pool.

Citation: Monthly Weather Review 136, 12; 10.1175/2008MWR2528.1

Table 1.

A review of surface pressure rises within convective systems obtained from the refereed literature. Values are estimated from figures. The values range from 1 to 7 hPa with a mean value of 3.6 hPa.

Table 1.
Table 2.

A review of surface temperature (or potential temperature) deficits within convective systems obtained from the refereed literature. Values are either reported explicitly or estimated from figures. The values range from 2 to 13 K with a mean value of 8.9 K.

Table 2.
Table 3.

A review of surface temperature (or potential temperature) deficits within simulated convective systems obtained from the refereed literature. Values are estimated from figures, either from plotted values of temperature (or potential temperature) excess or estimated from plotted values of the buoyancy distribution assuming a base-state potential temperature of 300 K. An initial environment described as ideal indicates a horizontally homogeneous environment. An initial environment described as real indicates an environment initialized with real observations. A thermodynamic sounding described as analytical indicates that the profile was derived from a specified function [WK82 refers to the sounding defined in Weisman and Klemp (1982)]. A thermodynamic sounding described as a composite indicates that the profile was derived from a composite of observed MCS environments. Otherwise, the date of the MCS case upon which the sounding is derived is listed. The basic type of microphysics scheme used (ice processes or no ice processes) is indicated.

Table 3.
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