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  • View in gallery

    Time series plots of Ze interpolated to plane height (blue) and LWC (green) for (a) 0518:00–0524:00, (b) 0601:00–0607:00, and (c) 0452:00–0500:00 UTC 9 Dec 2009. Mean temperatures at plane height over the given time interval are noted in the legend.

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    (a) The Ze (shaded) from the WCR from 0250:00 to 0350:00 UTC 9 Dec 2009 and θei (K; contours) from the RUC initialization valid at 0300:00 UTC along the same cross section. (b) The W (shaded) measured by the WCR, overlain with θei (K; contours) from the RUC initialization. Surface (shaded triangles) and upper-level (open triangles) cold fronts are shown. The horizontal black arrows are the C-130 flight track, and the black dashed line is the tropopause. Contours of θei are at an interval of 1 K. Figure adopted from Rauber et al. (2014b).

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    Level II base reflectivity from La Crosse, WI; Davenport, IA; Des Moines, IA; and Omaha, NE, radars at 0300 UTC 9 Dec 2009. The 0300 UTC RUC analysis of mean sea level pressure (solid) and 700–400-hPa thickness (dashed) is overlaid, with contour intervals of 2 hPa and 20 m, respectively. The black line denotes the separation between the stratiform and convective regions, and the gray straight line represents the 0251–0351 UTC C-130 flight track, corresponding to the vertical cross section in Fig. 2. The area of the analysis is shown as the inner box in the inset. Figure adopted from Rosenow et al. (2014).

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    (a) Sounding taken when the synoptically forced stratiform region (see Fig. 2) was over the rawinsonde launch site at Clinton, Iowa, at 0000 UTC 9 Dec 2009. (b) Sounding taken at Clinton, Iowa, at 0200 UTC 9 Dec 2009 during the transition from the synoptically forced stratiform region to the convective region in Fig. 2. (c) Sounding taken at Davenport, Iowa, at 0200 UTC 9 Dec 2009 in the convective region in Fig. 2.

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    (a) The Ze (shaded) from the WCR from 0600:00 to 0607:00 UTC 9 Dec 2009 and (b) W (shaded) measured by the WCR, both with θei (K; contours) from the RUC initialization along the same cross section. The horizontal black arrows are the C-130 flight track. Labels i, ii, and iii indicate regions used for CFADs in Fig. 6. Slight striping of W values from 0604 to 0605 UTC in (b) is due to slight flexing of the aircraft fuselage in turbulence.

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    CFADs of W measured by the WCR for the three elevated convective cells in Fig. 5, namely (a) 0601:50–0602:25, (b) 0602:30–0602:50, and (c) 0603:50–0606:20 UTC 9 Dec 2009, corresponding to regions i, ii, and iii in Fig. 5, respectively. Color-shaded values indicate percentage of observations at that altitude falling in each 0.2 m s−1 velocity bin. The break in each diagram, ranging from approximately 3 to 6 km, is the aircraft altitude. White contours on the diagrams denote the percentages of observations with values to the left of the contour and have values of 5%, 10%, 25%, 50%, 75%, 90%, and 95%. Dashed vertical lines indicate the location of W = 0 m s−1.

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    The Ze from the WCR (a) from 0450:00 to 0457:00 UTC 9 Dec 2009 and (b) from 2133:00 to 2140:00 UTC 24 Nov 2009. The horizontal black arrows are the C-130 flight track. Average zd for each convective cell is (a) zd = 3425 and (b) zd = 207 m.

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    The distribution of 1-s data within the three velocity categories by (a)–(c) zd and (d)–(f) T. Histogram bin sizes are (a)–(c) 250 m and (d)–(f) 2.5°C.

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    (a) The Ze and (b) W from the WCR, as well as (c) W from the WCR interpolated to plane height from 0455:00 to 0502:00 UTC 9 Dec 2009. Data in (c) taken when the plane was in regions of Ze < −5 dBZ are not shown. Dashed lines in (c) are placed at −2 and 0 m s−1, separating the various velocity categories. The horizontal black arrows in (a) and (b) are the C-130 flight track. Slight striping of W values near 0458 and 0459 UTC in (b) is due to slight flexing of the aircraft fuselage in turbulence.

  • View in gallery

    (a) The Ze from the WCR, as well as (b) IWC, (c) LWC, (d) N>500, and (e) Dmm for the same time period as Fig. 9. The horizontal black arrow in (a) is the C-130 flight track. Noted along the flight track are the 15 times from which particle images were taken (see Fig. 11).

  • View in gallery

    (a) Selected 2D-C particle imagery within 10-s intervals from 0458:20 to 0500:50 UTC 9 Dec 2009 (see Fig. 10) and (b) particle size distributions for selected 10-s intervals from (a). Particles selected in (a) were those most representative of the population within the given time interval. Colored labels for time intervals in (a) correspond to intervals for which particle size distributions are shown in (b).

  • View in gallery

    Median and 5th-, 25th-, 75th-, and 95th-percentile values for (a) IWC, (b) LWC, (c) N>500, and (d) Dmm within updrafts, stratified in 1000-m zd increments for all measurements within elevated convective cells during both research flights. Data points for each sample are shown in black.

  • View in gallery

    As in Fig. 12, but with data relative to three T increments: −40° < T ≤ −25°C, −25° < T ≤ −15°C, and −15° < T < −5°C.

  • View in gallery

    Scatterplot of T vs zd for all 1-s intervals of updraft sampled within elevated convective cells.

  • View in gallery

    As in Fig. 12, but for residual stratiform regions.

  • View in gallery

    (a) The Ze from the WCR, as well as (b) W from the WCR interpolated to plane height (blue) and LWC (green) from 0603:30 to 0606:30 UTC 9 Dec 2009. The mean temperature at plane height over the time interval is noted in the legend. Ze values at plane height were above the minimum threshold for the entire time interval. Dashed lines in (b) are placed at −2 and 0 m s−1, separating the various velocity categories; the dotted line is placed at LWC = 0.05 g m−3. Red vertical bars in (b) denote seconds where W ≤ 0 m s−1 and LWC > 0.05 g m−3. The horizontal black arrow in (a) is the C-130 flight track.

  • View in gallery

    As in Fig. 13, but for residual stratiform regions.

  • View in gallery

    Particle size distributions for eight different particle habits sampled within the elevated convection analyzed herein, stratified based on whether the particles were sampled within updrafts or residual stratiform regions.

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A Microphysical Analysis of Elevated Convection in the Comma Head Region of Continental Winter Cyclones

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  • 1 Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois
  • 2 Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois, and Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming
  • 3 Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois
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Abstract

An analysis of the microphysical structure of elevated convection within the comma head region of two winter cyclones over the midwestern United States is presented using data from the Wyoming Cloud Radar (WCR) and microphysical probes on the NSF/NCAR C-130 aircraft during the Profiling of Winter Storms campaign. The aircraft penetrated 36 elevated convective cells at various temperatures T and distances below cloud top zd. The statistical properties of ice water content (IWC), liquid water content (LWC), ice particle concentration with diameter > 500 μm N>500, and median mass diameter Dmm, as well as particle habits within these cells were determined as functions of zd and T for active updrafts and residual stratiform regions originating from convective towers that ascended through unsaturated air. Insufficient data were available for analysis within downdrafts.

For updrafts stratified by zd, distributions of IWC, N>500, and Dmm for all zd between 1000 and 4000 m proved to be statistically indistinct. These results imply that turbulence and mixing within the updrafts effectively distributed particles throughout their depths. A decrease in IWC and N>500 in the layer closest to cloud top was likely related to cloud-top entrainment.

Within residual stratiform regions, decreases in IWC and N>500 and increases in Dmm were observed with depth below cloud top. These trends are consistent with particles falling and aggregating while entrainment and subsequent sublimation was occurring.

Current affiliation: School of Meteorology, University of Oklahoma, Norman, Oklahoma.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author address: Amanda M. Murphy, Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, 105 S. Gregory St., Urbana, IL 61801. E-mail: amanda.murphy@ou.edu

Abstract

An analysis of the microphysical structure of elevated convection within the comma head region of two winter cyclones over the midwestern United States is presented using data from the Wyoming Cloud Radar (WCR) and microphysical probes on the NSF/NCAR C-130 aircraft during the Profiling of Winter Storms campaign. The aircraft penetrated 36 elevated convective cells at various temperatures T and distances below cloud top zd. The statistical properties of ice water content (IWC), liquid water content (LWC), ice particle concentration with diameter > 500 μm N>500, and median mass diameter Dmm, as well as particle habits within these cells were determined as functions of zd and T for active updrafts and residual stratiform regions originating from convective towers that ascended through unsaturated air. Insufficient data were available for analysis within downdrafts.

For updrafts stratified by zd, distributions of IWC, N>500, and Dmm for all zd between 1000 and 4000 m proved to be statistically indistinct. These results imply that turbulence and mixing within the updrafts effectively distributed particles throughout their depths. A decrease in IWC and N>500 in the layer closest to cloud top was likely related to cloud-top entrainment.

Within residual stratiform regions, decreases in IWC and N>500 and increases in Dmm were observed with depth below cloud top. These trends are consistent with particles falling and aggregating while entrainment and subsequent sublimation was occurring.

Current affiliation: School of Meteorology, University of Oklahoma, Norman, Oklahoma.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author address: Amanda M. Murphy, Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, 105 S. Gregory St., Urbana, IL 61801. E-mail: amanda.murphy@ou.edu

1. Introduction

Observations of lightning and convection within the comma head of wintertime extratropical cyclones have been documented as far back as the late nineteenth century (Herschel 1888; Butterworth 1895; Brooks 1920; Beldon 1927). Rauber et al. (2014b) review past research on the structure of winter cyclones that produce lightning flashes and show that elevated convection originates in the midtroposphere, is typically found in the southern region of the comma head, and is fueled by elevated potential instability. Vertical motions in wintertime, elevated convection have been observed to approach 7 m s−1 in updrafts and −5 m s−1 in downdrafts (Cronce et al. 2007; Rosenow et al. 2014). Because these cells sometimes produce lightning and significant heavy precipitation (Moore and Blakley 1988; Hunter et al. 2001; Rauber et al. 2014b), a thorough understanding of their microphysical structure is needed before forecasts can be improved.

Gunn and Marshall (1958) were some of the first to directly sample snowflakes and compare their microphysical properties to radar signals, collecting flakes on sheets of wool to observe quantities such as average size distribution to derive snowfall rate. Some of the earliest airborne in situ microphysical observations of the comma head of extratropical cyclones were obtained two decades later, during the University of Washington’s Cyclonic Extratropical Storms (CYCLES) project. Multiple features in these cyclones were examined, including cold-frontal (Hobbs et al. 1980) and warm-frontal regions (e.g., Hobbs and Locatelli 1978; Houze et al. 1981). The University of Washington B-23 aircraft flew through clouds associated with warm-frontal mesoscale rainbands of average width of 10–20 km (Herzegh and Hobbs 1980; Businger and Hobbs 1987) with precipitation regions on the order of tens to hundreds of square kilometers (Hobbs and Locatelli 1978). Rainbands were fueled by moisture flowing into the cyclone at low levels from the south-southwest, with clouds enhanced by local mesoscale updrafts (Houze et al. 1976). Houze et al. (1981) found that small mesoscale areas of precipitation existed within these bands and showed how the microphysical properties varied with environmental conditions such as temperature (Houze et al. 1979). Hobbs et al. (1975) also confirmed that mesoscale processes, such as mesoscale banding of precipitation, are important in the production and microphysical characteristics of frontal precipitation.

Many studies incorporating aircraft data have focused on winter storms and orography, and how changes in cloud microphysics are related to topography and embedded convection (e.g., Lin and Colle 2009; Dorsi et al. 2015; Geerts et al. 2015). Aircraft have also been used to investigate ice crystal habits in embedded convection in winter storms in China (Zhu et al. 2015) and to analyze the microphysical structure of kata-cold-frontal precipitation in France and the United Kingdom (Marécal et al. 1993; Crosier et al. 2014; Dearden et al. 2014). Further, Lloyd et al. (2015) examined the microphysical properties of wintertime precipitation near the Swiss Alps.

However, limited in situ aircraft microphysical data have been published for precipitation substructures evident in the comma head of continental winter cyclones. Passarelli (1978a,b) modeled and observed the aggregation of snowflakes using both Doppler radar and aircraft measurements over central Illinois. Lo and Passarelli (1982) used spiral descents advecting with the motion of particles to observe their evolution during their fall toward the surface. More recently, the microphysical properties of cloud-top generating cells have been examined. Generating cells differ from deeper elevated convection in that they exist only at the top of stratiform clouds and are the circulations from which trails of hydrometeors originate (Marshall 1953). A number of studies have used radar measurements to investigate generating cells and the precipitation streamers they produce (Gunn et al. 1954; Wexler and Atlas 1959; Carbone and Bohne 1975; Heymsfield 1975b; Browning 1983; Syrett et al. 1995; Cunningham and Yuter 2014; Rosenow et al. 2014; Rauber et al. 2015; Keeler et al. 2016). The microphysical properties of generating cells, and their resultant precipitation streamers, have been investigated using aircraft in situ measurements (Heymsfield 1975a; Henrion et al. 1978; Bader et al. 1987; Plummer et al. 2014, 2015) and dual-polarization radar measurements (Kumjian et al. 2014).

Even more limited analyses exist for the microphysical properties of deeper convective cells within the comma head of winter cyclones. Quantifying microphysical properties such as ice water content (IWC), liquid water content (LWC), number concentration N, and median mass diameter Dmm are critical for interpretation of remote sensing measurements and estimation of precipitation, particularly from space (Hou et al. 2014). Microphysical measurements such as snow habit and degree of riming have been made using surface observational techniques (Stark et al. 2013; Colle et al. 2014). However, these data were collected at 15–30-min intervals and cannot fully describe the microphysical structure of the cyclone comma head aloft or its substructures. More detailed analyses of microphysical properties of such convective cells have been made using dual-polarization radar (Andrić et al. 2013; Griffin et al. 2014; Picca et al. 2014; Kumjian and Deierling 2015), but such analyses lack the detailed in situ observations of particle shapes and sizes needed to characterize the finescale microphysical properties within elevated convection of wintertime convective cells.

In this paper, the microphysical properties of elevated convection within the comma head region of two continental winter cyclones are examined. Data obtained on 24 November and 9 December 2009 during the Profiling of Winter Storms (PLOWS) field campaign are used to characterize the microphysical properties of convective towers within the comma head of two wintertime cyclones. The goal of this analysis is to determine the microphysical characteristics of regions of updrafts and residual stratiform regions associated with elevated convection, with stratification of the data by aircraft distance below cloud top and by temperature. Insufficient data were available for analysis within downdrafts. Comparison is made with the microphysical properties observed in stratiform regions of wintertime cyclones sampled during the PLOWS campaign.

2. Data and instrumentation

The Wyoming Cloud Radar (WCR; Wang et al. 2012), as well as a suite of in situ microphysical probes, were mounted on the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) C-130 that flew through the elevated convection analyzed here. The data and instrumentation used in this study are described in depth by Plummer et al. (2014) and Plummer et al. (2015); accordingly, only a brief summary of the cloud probes and radar is given here. Data presented in this analysis were processed using the most updated processing techniques, which were updated to accommodate new algorithms for the computation of maximum projected particle dimension (Wu and McFarquhar 2016) and to calculate the sample volume considering the time the probe was active rather than the sum of accepted particle interarrival times. The later change was especially important given the previous calculation for the precipitation probe only used the interarrival times for accepted particles in the two photodiode subsets [see the appendix of Plummer et al. (2014)]. These changes resulted in reductions of N>500 and IWC by up to a maximum factor of 2 compared to Plummer et al. (2014) and Plummer et al. (2015), depending on the time period.

The equivalent radar reflectivity factor Ze (dBZ; Smith 2010) and the vertical Doppler velocity W were derived from the two vertical (up and down) beams of the WCR. WCR data were sampled at a vertical resolution of 15 m and a horizontal resolution of approximately 4–7.5 m given typical C-130 airspeeds during PLOWS. Processing of the WCR data is described by Rosenow et al. (2014), and positive W values are used here to indicate upward motion of cloud particles. For analysis of vertical velocity at the aircraft’s location, W values were interpolated to the aircraft’s location, as described by Plummer et al. (2014). Large errors in the measurements of vertical velocity at plane height w were evident at some times, presumably as a result of icing of the pressure port associated with the measurement. Thus, for consistency, the remotely sensed W values were used for all the analysis to give some indication of vertical motion. Air temperature T at plane height was obtained by either a heated or unheated Rosemount Type 102 sensor mounted on the C-130, with type depending on the flight. Measurements of both cloud-top altitude zt and distance below cloud-top zd were derived from the Ze measurements from the WCR following Plummer et al. (2014).

Four variables are used herein to characterize the microphysical properties of the convective towers, namely the ice water content (IWC), liquid water content (LWC), number concentration of particles with maximum dimensions greater than 500 μm N>500, and median particle dimension with respect to mass Dmm. LWC was measured using a Gerber particle volume monitor (PVM). Figure 1 shows time series of Ze interpolated to plane height and of LWC for three different elevated convective cells. The nonsynchronous values show that the measurement of LWC by the PVM is not a false response to ice. Hydrometeors were measured with two-dimensional cloud and precipitation optical array probes (2D-C and 2D-P, respectively), with nominal size ranges of 25–1600 and 200–6400 μm, respectively. Particles with diameter D < 500 μm were not included to minimize the effects of ice crystal shattering on the probes (Korolev et al. 2013; Jackson and McFarquhar 2014) and uncertainties associated with the depth of field for small particles (Baumgardner and Korolev 1997). Since small particles typically dominate number concentrations, trends in N>500 should not be compared with previous studies that look at variations of number concentrations including the contributions of smaller particles. The N>500 concentrations were derived using 2D-C measurements for particles with 500 ≤ D ≤ 2000 μm and 2D-P measurements for larger particles. Plummer et al. (2014) describes the corrections to the 2D-P data to account for a reduced photodiode response in the center of the 2D-P array. The projected area of each particle with D > 500 μm was used to estimate its mass (Baker and Lawson 2006), from which the IWC and Dmm were derived (Plummer et al. 2014).

Fig. 1.
Fig. 1.

Time series plots of Ze interpolated to plane height (blue) and LWC (green) for (a) 0518:00–0524:00, (b) 0601:00–0607:00, and (c) 0452:00–0500:00 UTC 9 Dec 2009. Mean temperatures at plane height over the given time interval are noted in the legend.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Elevated convective cells were only sampled during two research flights from PLOWS; therefore, the analysis is focused on data collected on those two days. Detailed synoptic, radar, and profiler analyses for both events are described by Rauber et al. (2014b) and are not repeated here.

3. Defining elevated convection

Hogan et al. (2002) examined the microphysical properties of elevated convection in warm-frontal mixed-phase clouds using both radar and aircraft measurements. They defined elevated convection within these clouds visually, identifying convection as distinct turrets. A similar approach is adopted here. In this analysis, elevated convective cells were first identified primarily by examining images of Ze from the WCR data, overlaid with analyses of equivalent potential temperature with respect to ice (Rauber et al. 2014a,b; Rosenow et al. 2014; Rauber et al. 2015). Profiles of were obtained from RUC model initializations and were then interpolated to the flight-track position and overlaid on the WCR cross sections (Figs. 2 and 5). Elevated convective cells were identified as distinct towers of enhanced Ze (Fig. 2) in regions of elevated potential instability with an absence of generating cells on top of the tower. During both cyclones, rawinsondes were launched in the vicinity of the elevated convection, as described by Rauber et al. (2014b). For the 9 December 2009 storm, the most unstable CAPE, calculated for parcels originating directly above the stable frontal boundary, ranged from 26 to 248 J kg−1 on five different soundings (see their Fig. 4). For the 24 November 2009 storm, the most unstable CAPE ranged from 13 to 163 J kg−1 on five different soundings (see their Fig. 12). Figure 2 shows an image of Ze during one of the research flights. A clear distinction is evident between the stratiform region on the left side of the figure, where the vertical extent of the echo is located in stable air, and elevated convection on the right side of the figure, where the air is stable below 3.4 km with potential instability present above these elevations. Figure 3 also shows the relative locations of the stratiform and convective regions within the comma head from a composite reflectivity plot. It is clear that banded structures exist within the stratiform region, whereas more cellular echoes are visible within the convective region.

Fig. 2.
Fig. 2.

(a) The Ze (shaded) from the WCR from 0250:00 to 0350:00 UTC 9 Dec 2009 and θei (K; contours) from the RUC initialization valid at 0300:00 UTC along the same cross section. (b) The W (shaded) measured by the WCR, overlain with θei (K; contours) from the RUC initialization. Surface (shaded triangles) and upper-level (open triangles) cold fronts are shown. The horizontal black arrows are the C-130 flight track, and the black dashed line is the tropopause. Contours of θei are at an interval of 1 K. Figure adopted from Rauber et al. (2014b).

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Fig. 3.
Fig. 3.

Level II base reflectivity from La Crosse, WI; Davenport, IA; Des Moines, IA; and Omaha, NE, radars at 0300 UTC 9 Dec 2009. The 0300 UTC RUC analysis of mean sea level pressure (solid) and 700–400-hPa thickness (dashed) is overlaid, with contour intervals of 2 hPa and 20 m, respectively. The black line denotes the separation between the stratiform and convective regions, and the gray straight line represents the 0251–0351 UTC C-130 flight track, corresponding to the vertical cross section in Fig. 2. The area of the analysis is shown as the inner box in the inset. Figure adopted from Rosenow et al. (2014).

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

The formation of residual stratiform clouds associated with elevated convection, as opposed to large-scale, synoptically forced stratiform regions such as those analyzed in Plummer et al. (2015), should lead to differences in the microphysical structure of these two cloud types. Figure 4 shows three soundings taken during the 9 December 2009 storm that are characteristic of the synoptically forced stratiform region, transition area between this region and the elevated convective region, and within the elevated convective region. Note the transition from saturated air aloft to unsaturated air aloft. As they form, elevated convective cells rise through unsaturated air to the tropopause, with updraft regions generally evolving into residual stratiform regions. Particles within these cells fall through the residual stratiform regions surrounded by unsaturated air. Unlike large-scale, synoptically forced stratiform regions such as those discussed in Plummer et al. (2015), the air within residual stratiform regions has no additional continuous moisture supply. For this reason, particles falling through residual stratiform should not gain any significant amount of mass by vapor deposition as they fall. Collection of these particles through aggregation can still occur similar to that seen in synoptically forced stratiform. However, most particle growth should occur within updrafts.

Fig. 4.
Fig. 4.

(a) Sounding taken when the synoptically forced stratiform region (see Fig. 2) was over the rawinsonde launch site at Clinton, Iowa, at 0000 UTC 9 Dec 2009. (b) Sounding taken at Clinton, Iowa, at 0200 UTC 9 Dec 2009 during the transition from the synoptically forced stratiform region to the convective region in Fig. 2. (c) Sounding taken at Davenport, Iowa, at 0200 UTC 9 Dec 2009 in the convective region in Fig. 2.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Figure 5 shows WCR images of Ze and W for a series of three convective cells penetrated by the aircraft. Two cells are developing, while the third is in a mature state. Figure 6 shows each cell’s corresponding contoured frequency by altitude (CFAD) diagram of W. The first two CFADs show vertical Doppler velocities that vary between −3 and 5 m s−1 at altitudes of the elevated convection, whereas the third suggests a fairly uniform vertical Doppler velocity structure with altitude in the mature cell. The large variation in the shape of these CFADs illustrates the life cycle of an elevated convective cell. A primary objective of this analysis is to analyze the microphysical differences between regions of updrafts, downdrafts, and residual stratiform regions within elevated convection, as well as to compare and contrast the microphysical structure of these regions of varying W with the microphysical structure of stratiform precipitation in the comma head.

Fig. 5.
Fig. 5.

(a) The Ze (shaded) from the WCR from 0600:00 to 0607:00 UTC 9 Dec 2009 and (b) W (shaded) measured by the WCR, both with θei (K; contours) from the RUC initialization along the same cross section. The horizontal black arrows are the C-130 flight track. Labels i, ii, and iii indicate regions used for CFADs in Fig. 6. Slight striping of W values from 0604 to 0605 UTC in (b) is due to slight flexing of the aircraft fuselage in turbulence.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Fig. 6.
Fig. 6.

CFADs of W measured by the WCR for the three elevated convective cells in Fig. 5, namely (a) 0601:50–0602:25, (b) 0602:30–0602:50, and (c) 0603:50–0606:20 UTC 9 Dec 2009, corresponding to regions i, ii, and iii in Fig. 5, respectively. Color-shaded values indicate percentage of observations at that altitude falling in each 0.2 m s−1 velocity bin. The break in each diagram, ranging from approximately 3 to 6 km, is the aircraft altitude. White contours on the diagrams denote the percentages of observations with values to the left of the contour and have values of 5%, 10%, 25%, 50%, 75%, 90%, and 95%. Dashed vertical lines indicate the location of W = 0 m s−1.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

A total of 61 elevated convective cells were identified with the WCR across both storms, but only 36 were penetrated by aircraft. The others were overtopped by the aircraft. The analysis herein concentrates on the microphysical properties of these 36 cells. Entrance and exit times for the plane into and out of the cell were determined from the WCR data, and boundaries were defined by a −5-dBZ threshold. No microphysical data from regions with Ze < −5 dBZ were included in the analysis.

4. Stratification of data

a. Vertical Doppler velocity

One-hertz W measurements ranged from approximately −5 to 7 m s−1. These data are used to distinguish between updrafts, downdrafts, and residual stratiform regions where updrafts are defined as W > 0 m s−1, downdrafts as W < −2 m s−1, and residual stratiform regions as −2 ≤ W ≤ 0 m s−1. Assuming an approximate crystal fall velocity of −1.0 m s−1 for crystals in all regions [e.g., Rosenow et al. (2014) and references therein], this results in updrafts having true air vertical velocities of greater than 1 m s−1, downdrafts having true air vertical velocities of less than −1 m s−1, and residual stratiform regions having true air vertical velocities between −1 and 1 m s−1.

A number of past analyses of microphysical structure of various phenomena have used vertical velocity to stratify data. The approximate use of vertical velocities on the order of ±1 m s−1 to distinguish between updrafts, downdrafts, and stratiform regions is similar to that used in previous studies. For example, Jorgensen et al. (1985) examined vertical velocities within banded structures in mature hurricanes and defined updrafts and downdrafts as regions with continuous positive or negative vertical velocities for at least 500 m, with at least one data point greater than |0.5| m s−1. McFarquhar and Black (2004) used a similar method, defining updrafts and downdrafts as regions where vertical motions were greater than 1 m s−1 or less than −1 m s−1 for at least 4 s, respectively.

Vertical Doppler velocities of varying magnitudes and sign were observed in the elevated convection. During the flights, no specific targeting was done for the elevated convection (the aircraft flew straight flight legs across the comma head), so the number of convective cores penetrated, and the number of passes directly through the center of cores, are limited. Tables 1 and 2 show the number of samples within updrafts, downdrafts, and residual stratiform regions with minimum threshold averaging times of 1, 2, 3, and 5 s as a function of distance below cloud top (Table 1) and temperature (Table 2). It is clear from these tables that the number of samples decreases significantly as the minimum threshold averaging time is increased beyond 1 Hz. The choice was made to maximize the number of samples by conducting all analyses using 1-Hz data. Although this criterion is less strict than that used in past studies of microphysical properties of convection, it is warranted given the nature of this dataset. In addition, the use of the 1-Hz microphysics data limits the statistical significance especially of the concentrations of the larger particles (McFarquhar et al. 2007a); this may somewhat exaggerate the variability of the size distributions and derived parameters. This method of stratification also allows the possibility that small regions between updrafts or downdrafts of dimensions of 120 m (1 s of flight time) can be classified as residual stratiform. Trends in the data were examined for other temporal resolutions. Although the trends were similar, the trends could not be assessed with any statistical significance.

Table 1.

Number of samples within updrafts, residual stratiform regions, and downdrafts for various averaging periods stratified by distance below cloud top.

Table 1.
Table 2.

Number of samples within updrafts, residual stratiform regions, and downdrafts for various averaging periods stratified by temperature.

Table 2.

b. Sampling characteristics

Elevated convection within the comma heads of the two storms was penetrated at a range of altitudes on both flights, covering a range of temperatures T and distances below cloud top zd. Figure 7, for example, shows Ze images of two elevated convective cells, penetrated at different zd. Distance below cloud top was defined as the difference between the WCR determined altitude at cloud top zt and the altitude of the aircraft zac such that zd = ztzac. Figure 8 shows the number of seconds within updrafts, residual stratiform regions, and downdrafts within convective cells as a function of T and zd. The bin sizes in Fig. 8 for T and zd are 2.5°C and 250 m, respectively. The cloud penetrations were in the range 0 < zd < 4948 m. In total, the aircraft spent 296 s in updrafts, 1803 s in residual stratiform regions, and 77 s in downdrafts. Passes were made at a wide range of aircraft measured temperatures. Temperatures were below 0°C for all samples and ranged from −7.5° to −35.3°C.

Fig. 7.
Fig. 7.

The Ze from the WCR (a) from 0450:00 to 0457:00 UTC 9 Dec 2009 and (b) from 2133:00 to 2140:00 UTC 24 Nov 2009. The horizontal black arrows are the C-130 flight track. Average zd for each convective cell is (a) zd = 3425 and (b) zd = 207 m.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Fig. 8.
Fig. 8.

The distribution of 1-s data within the three velocity categories by (a)–(c) zd and (d)–(f) T. Histogram bin sizes are (a)–(c) 250 m and (d)–(f) 2.5°C.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

To have sufficient samples to represent the data statistically, the number of categories of zd and T were reduced so that data were segregated into five categories of zd and three categories of T. The five categories for zd ranged from 0 to 5000 m in 1000-m intervals. Temperature was segregated into three categories, −40° < T ≤ −25°C, −25° < T ≤ −15°C, and −15° < T < −5°C. Data were analyzed statistically if there were at least 40 samples within the category. As shown in Tables 1 and 2, little data existed for updrafts with zd > 4000 m, and there were insufficient data within nearly all zd and T categories for downdrafts. For this reason, forthcoming statistical analyses will be limited to stratified data within updrafts and residual stratiform regions that meet the minimum sample size.

5. Example case study

Figure 9 shows images of Ze and W from the WCR between 0455:00 and 0502:00 UTC 9 December 2009, as well as time series measurements of W interpolated to the aircraft’s location. Vertical Doppler velocities ranging from −3 to 3 m s−1 were sampled by the WCR as the plane traveled through a region of relatively higher reflectivity (Ze ≥ 15 dBZ). At approximately 0459:00 UTC, the aircraft sampled the base of the first of three updraft regions evident within the cell, with maximum W values within the updraft near 6 m s−1. This region of strong W was surrounded on both sides by regions of downdrafts with W nearing −3 m s−1. The plane encountered two more small updraft regions, passing through the center of the second and clipping the top of the third. In the second updraft region, the highest W values at aircraft height were recorded (3 m s−1).

Fig. 9.
Fig. 9.

(a) The Ze and (b) W from the WCR, as well as (c) W from the WCR interpolated to plane height from 0455:00 to 0502:00 UTC 9 Dec 2009. Data in (c) taken when the plane was in regions of Ze < −5 dBZ are not shown. Dashed lines in (c) are placed at −2 and 0 m s−1, separating the various velocity categories. The horizontal black arrows in (a) and (b) are the C-130 flight track. Slight striping of W values near 0458 and 0459 UTC in (b) is due to slight flexing of the aircraft fuselage in turbulence.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Figure 10 shows the microphysical properties of the cell as time series of IWC, LWC, N>500, and Dmm. Note that the altitude scale is converted to temperature on the y axis of Fig. 10a, using data from a sounding launched at Clinton, Iowa, at 0526:13 UTC (Rauber et al. 2014b). Shortly after entering into the cell at approximately 0458:50 UTC, spikes in IWC, LWC, and N>500 are evident, with IWC nearing 0.8 g m−3 and LWC exceeding 0.06 g m−3. These values were collocated with the first updraft region, evident in Figs. 9b and 9c. Subsequent decreases of IWC to near 0.5 g m−3 and LWC to 0.02 g m−3 occur immediately following the first updraft region. Upon penetrating the second of the three updraft regions outlined in Figs. 9b and 9c, there was a second spike in IWC to 1.0 g m−3 and N>500 to approximately 40 L−1 whereas no spike in LWC was present. The presence of such high N>500 values is coincident with the peak in W (~3 m s−1). The values of Dmm were highest in the high-reflectivity region, except in the sharp updraft near 0459:15 UTC where Dmm had a local minimum. This is consistent with smaller particles present in the updraft region. The values of all variables continue to decrease throughout the plane’s duration in the updraft region. Little evidence of the third updraft region’s effect on the microphysical characteristics of the cell is visible, with IWC ~ 0.2 g m−3, LWC ~ 0.02 g m−3, and N>500 ~ 5 L−1 noted.

Fig. 10.
Fig. 10.

(a) The Ze from the WCR, as well as (b) IWC, (c) LWC, (d) N>500, and (e) Dmm for the same time period as Fig. 9. The horizontal black arrow in (a) is the C-130 flight track. Noted along the flight track are the 15 times from which particle images were taken (see Fig. 11).

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Figure 11 shows the evolution of particle characteristics from 0458:20 to 0500:50 UTC, in 10-s time intervals, both as particle imagery (Fig. 11a) and through particle size distributions (Fig. 11b). The locations of each 10-s time interval are also noted along the flight track in Fig. 10a. Particle images in Fig. 11a were chosen to be representative of those sampled within that time interval. As the aircraft traveled through the region of higher reflectivity from 0458:50 to 0459:40 (Fig. 11a, regions iv–viii), large aggregates, rimed dendrites, and capped columns were visible within the sample volumes. Conversely, smaller crystals and aggregates are found outside of this region of enhanced Ze, and outside of the region of largest IWC and LWC (Figs. 10a–c). Particle size distributions show number concentrations first increasing by more than an order of magnitude as the aircraft penetrated the cell and sampled its core, and then decreasing as the plane exited the cell. The final particle size distribution in Fig. 11b, located at the lower part of the convective cell’s anvil, shows a marked difference from those inside the cell as particles were primarily small, with the majority of particles with diameters 1 mm or less and a large reduction in particles with diameters greater than 1 mm. On average, the concentration of particles with D < 1 mm in regions ii, x, and xii were a factor of 13 less than those in regions iv, vi, and viii.

Fig. 11.
Fig. 11.

(a) Selected 2D-C particle imagery within 10-s intervals from 0458:20 to 0500:50 UTC 9 Dec 2009 (see Fig. 10) and (b) particle size distributions for selected 10-s intervals from (a). Particles selected in (a) were those most representative of the population within the given time interval. Colored labels for time intervals in (a) correspond to intervals for which particle size distributions are shown in (b).

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

6. Statistical characteristics of full dataset

The aircraft penetrated 36 elevated convective cells, resulting in approximately 35 min of radar and microphysical data. These cells ranged in width from approximately 0.6 to 22.8 km, in depth from 4.6 to 8.0 km, and had width-to-depth ratios ranging from 0.1 to 3.0, with an average width to depth ratio of 1.1. Synoptic characteristics of these cells have been analyzed in depth by Rauber et al. (2014b) and Rosenow et al. (2014). Data were separated into categories based on W, and stratified by either zd or T, employing means of stratification discussed in section 4. The data are summarized below in four figures (Figs. 12, 13, 15, and 17) that show individual data points with box-and-whisker plots overlaid, showing the median, as well as the 5th, 25th, 75th, and 95th percentiles for IWC, LWC, N>500, and Dmm.

Fig. 12.
Fig. 12.

Median and 5th-, 25th-, 75th-, and 95th-percentile values for (a) IWC, (b) LWC, (c) N>500, and (d) Dmm within updrafts, stratified in 1000-m zd increments for all measurements within elevated convective cells during both research flights. Data points for each sample are shown in black.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Fig. 13.
Fig. 13.

As in Fig. 12, but with data relative to three T increments: −40° < T ≤ −25°C, −25° < T ≤ −15°C, and −15° < T < −5°C.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

a. Updrafts

The characteristics of the updraft regions stratified by zd are shown in Fig. 12. Median values of IWC ranged from 0.33 to 0.48 g m−3. The interquartile range for IWC, a measure of the variability of the data, increased with depth below cloud top from 0.29 to 0.47 g m−3, while 90% of the measurements ranged from 0.01 to 1.01 g m−3. A key characteristic of the data was the decrease in IWC as cloud top was approached. Very little LWC was located within 2000 m of cloud top, and the median value between 2000 and 4000 m below cloud top ranged from 0.03 to 0.04 g m−3. Although not shown on the figure, because of the lack of sufficient samples, the median value 4000–5000 m below cloud top was 0.11 g m−3, indicating that more supercooled water was located at lower altitudes. The interquartile range varied with depth from 0 to 0.11 g m−3, and the 95th percentile ranged from 0.10 to 0.22 g m−3 at zd greater than 2000 m. The median value of N>500 differed in the cloud-top region from other depths below cloud top, in that near cloud top, the median value of N>500 was 7.3 L−1, while at greater depths it ranged from 14.6 to 18.6 L−1. This is similar to the reduction of IWC seen near cloud top. The interquartile range increased with depth from 12.1 L−1 in the cloud-top layer to 19.1 L−1 between 3000 and 4000 m below cloud top, and the 95th-percentile values ranged from 22.5 to 36.3 L−1. The median values of Dmm ranged from 1075 to 1425 μm across all zd categories. The interquartile range decreased below cloud top from 844 to 300 μm, as did 95th-percentile values, ranging from 1550 to 2420 L−1. Overall, the trends in updrafts when stratified by zd were weak, with a slight increase in IWC with depth as well as a reduction in IWC near cloud top, a stronger increase in LWC with depth, and no noticeable trends in N>500 and Dmm with depth, except for a reduction in N>500 in the cloud-top layer similar to the reduction seen with IWC.

Within updrafts, this even distribution of particles across the depth of the cloud by the updraft is best seen through results of Mann–Whitney U tests (Mann and Whitney 1947; Wilcoxon 1945) performed for IWC, LWC, N>500, and Dmm stratified by zd. Such tests were performed to identify whether there are statistically distinct populations (p < 0.05) (Table 3). Data within the three zd categories in Fig. 12 from 1000 to 4000 m are statistically indistinct from each other for IWC, N>500, and Dmm. However, the data in the category closest to cloud top are statistically distinct from every other category farther from cloud top for the three aforementioned variables. The median values for IWC and N>500 closest to cloud top are less than those farther from cloud top, suggesting a relatively even distribution of particles throughout the majority of the updrafts, with potential entrainment at the top of the cloud. The presence of similar N>500, IWC, and Dmm for particles everywhere throughout the updraft indicates that either nucleation was occurring throughout the clouds or that there was significant vertical mixing, or both. McFarquhar et al. (2007b) and McFarquhar et al. (2011) noted similar trends in stratocumulus sampled during the Mixed-Phase Arctic Cloud Experiment (M-PACE) and in arctic stratocumulus observed in the ISDAC experiment, attributing the observed patterns to extensive vertical mixing.

Table 3.

Mann–Whitney U-test p values for comparison of categories within updrafts at varying zd. Blank cells indicate that the test has already been run and p value is shown in another cell.

Table 3.

For tests performed on the LWC data, data within the categories from 0 to 1000 m and from 1000 to 2000 m below cloud top were statistically indistinct, as were data within categories from 2000 to 3000 m and from 3000 to 4000 m below cloud top, but data were statistically distinct when comparing data within any 1000-m range between 0 and 2000 m below cloud top to data within any 1000-m range between 2000 and 4000 m below cloud top. This is indicative of the increase in LWC toward lower, warmer parts of the updrafts, evident in Table 4. The table shows that 16% of measurements in updrafts between zd = 0 and 1000 m had LWC > 0.01 g m−3 and none had LWC > 0.05 g m−3, while 93% of the measurements between zd = 3000 and 4000 m had LWC > 0.01 g m−3 and 30% had LWC > 0.05 g m−3.

Table 4.

Fraction of observations within specific zd categories for updrafts and residual stratiform regions where LWC exceeded 0.01 and 0.05 g m−3. Fractions were not calculated for updrafts with zd from 4000 to 5000 m because of insufficient samples.

Table 4.

The data in updrafts were also stratified by temperature (Fig. 13). IWC was maximized in the −15° to −25°C layer, with a median value of 0.52 g m−3, an interquartile range of 0.41 g m−3, and 5th- and 95th-percentile values of 0.15 and 1.10 g m−3, respectively. The lowest values were found in the −5° to −15°C layer, with a median of 0.24 g m−3 and a 5th–95th-percentile range from 0.01 to 0.65 g m−3. Lower values were also observed in the −25° to −40°C layer, with a median value of 0.44 g m−3 and 5th–95th-percentile range from 0.02 to 0.67 g m−3. LWC was a strong function of temperature, with nearly no supercooled water observed at lowest temperatures, and a 5th–95th-percentile range of values from 0.01 to 0.21 g m−3 in the −5° to −15°C range. Following the same trend as IWC, N>500 had maximum values in the intermediate temperature layer. Within this layer, the median was 17.3 L−1, the interquartile range was 17.8 L−1, and the 5th–95th-percentile values ranged from 4.8 to 37.8 L−1. At lower temperatures, the median value was 16.8 L−1, with 5th–95th-percentile values ranging from 1.2 to 32.4 L−1, while at higher temperatures, the median value was 8.6 L−1 with 5th–95th-percentile values from 0.6 to 26.5 L−1. The median values of Dmm in all three temperature intervals ranged from 1000 to 1275 μm. The 5th–95th-percentile values ranged from 650 to 2400 μm across all three layers. The general trends on Fig. 13 are higher values of IWC and N>500 in the intermediate temperature range, an increase of LWC at the warmest temperature range, and little trend in Dmm across the temperature ranges.

The data, when stratified by T (Fig. 13), are all statistically distinct (Table 5) but do not show clear trends. The problem with stratifying updrafts by T is that the aircraft penetrated cells in various stages of development so that one penetration at a particular T may be near cloud top, while another at the same T may be kilometers below cloud top. Figure 14, a scatterplot of zd versus T for all updraft penetrations, illustrates this point. Therefore, for the bulk quantities chosen to study, in updrafts, temperature proves less important than distance below cloud top.

Table 5.

As in Table 3, but for data sampled at varying T.

Table 5.
Fig. 14.
Fig. 14.

Scatterplot of T vs zd for all 1-s intervals of updraft sampled within elevated convective cells.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

b. Residual stratiform

Residual stratiform data were stratified by zd shown in Fig. 15. Median values of IWC ranged from 0.16 to 0.32 g m−3 and were maximized in the intermediate levels below cloud top. The interquartile range generally increased from 0.24 g m−3 in the category closest to cloud top, to 0.52 g m−3 for data within 3000–4000 m below cloud top, before decreasing to 0.16 g m−3 for the data at the greatest depth below cloud top. The 5th–95th-percentile values ranged from 0 to 0.81 g m−3 across all zd categories. LWC remained fairly uniform with depth, with median values varying from 0 to 0.03 g m−3 across all zd categories, and interquartile ranges between 0 and 0.04 g m−3 in the three categories farthest from cloud top. The 95th-percentile values ranged from 0.05 to 0.09 g m−3, with no discernable trend with zd. Table 4 shows that 25% of the measurements of LWC in the zd = 0–1000-m layer had LWC > 0.01 g m−3 and 10% had LWC > 0.05 g m−3. In comparison, less LWC was observed in the intermediate layers of zd = 1000–3000 m. In the lowest layer of zd = 4000–5000 m, 55% of the measurements had LWC > 0.01 g m−3 and 9% had LWC > 0.05 g m−3.

Fig. 15.
Fig. 15.

As in Fig. 12, but for residual stratiform regions.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

In both residual stratiform regions and downdrafts, there were some measurements of appreciable LWC. Appreciable and negligible LWC values were defined as LWC > 0.05 and LWC ≤ 0.05 g m−3, respectively. A case-by-case analysis of all cells showed that there was negligible LWC 91% of the time within residual stratiform regions and downdrafts, with no appreciable LWC appearing at all within residual stratiform regions or downdrafts in 24 of the 36 cells. The times of appreciable LWC within the other 12 cells represent isolated times with occasional small cells leading to nonzero LWC, a possible consequence of using 1-Hz data. Additionally, mixing from adjacent updrafts may have led to the appreciable LWC values. Figure 16 shows an image of Ze as well as time series plots of both W at plane height and LWC for one of these 12 cells, showing that for three out of four instances with nonnegligible LWC, the appreciable LWC values occurred just before or after the penetration of updrafts. Similar analysis of all cells where appreciable LWC was identified suggested the LWC was in fact associated with small updrafts. For this reason, values of and trends in LWC within residual stratiform regions and downdrafts will not be discussed further.

Fig. 16.
Fig. 16.

(a) The Ze from the WCR, as well as (b) W from the WCR interpolated to plane height (blue) and LWC (green) from 0603:30 to 0606:30 UTC 9 Dec 2009. The mean temperature at plane height over the time interval is noted in the legend. Ze values at plane height were above the minimum threshold for the entire time interval. Dashed lines in (b) are placed at −2 and 0 m s−1, separating the various velocity categories; the dotted line is placed at LWC = 0.05 g m−3. Red vertical bars in (b) denote seconds where W ≤ 0 m s−1 and LWC > 0.05 g m−3. The horizontal black arrow in (a) is the C-130 flight track.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Median values of N>500 were maximized in the midlevels, ranging from 3.7 to 9.6 L−1, and reaching a maximum in the 1000–2000- and 2000–3000-m zd categories. The interquartile range varied broadly over all cloud depths, with values between 4.7 and 16.0 L−1. The 95th-percentile values ranged from 10.8 to 32.7 L−1 and were minimized in the categories closest to and farthest from cloud top. Median values of Dmm remained fairly constant throughout the depth of the clouds, ranging from 1300 to 1450 μm. The interquartile range decreased fairly steadily with zd, with values between 225 and 650 μm, and 95th-percentile values ranging from 1741 to 2800 μm across all depths. General trends in these data were weak, with a small decrease in IWC and N>500, and a small increase in Dmm with zd.

For data within residual stratiform regions, similar tests were done between data within the five different zd intervals for all four microphysical variables (Fig. 15). For N>500, data from zd = 1000–4000 m were statistically indistinct, pointing to an even distribution of particles throughout that depth (Table 6). Additionally, for IWC, data between some categories were statistically distinct, with only data from zd = 1000–2000 and zd = 3000–4000 m statistically indistinct, potentially owing to the decrease of IWC with depth. The median value of IWC slowly decreases with depth, the median value of N>500 also decreases with depth, and the median value of Dmm increases with depth across these same zd ranges. The behavior of these variables is consistent with particles falling and aggregating while entrainment is occurring. Data within the 0–1000-m zd category were statistically distinct from almost all other categories for IWC and N>500, suggesting that cloud-top entrainment reduced IWC and N>500 as smaller particles sublimated.

Table 6.

As in Table 3, but for residual stratiform regions.

Table 6.

The data in the residual stratiform category were also stratified by temperature (Fig. 17). Median IWC values were maximized in the −15° to −25°C temperature range, with a value of 0.31 g m−3 and interquartile range of 0.35 g m−3. Medians and interquartile ranges were smaller in categories of lower and higher temperatures, with median values of 0.22 and 0.16 g m−3, and interquartile ranges of 0.36 and 0.21 g m−3, respectively. The 95th-percentile value of the intermediate temperature range also exceeded that of the other two data categories, with a value of 0.77 g m−3 as opposed to values of 0.62 and 0.42 g m−3 in the lowest and highest temperature ranges, respectively. Median values of N>500 ranged from 3.7 to 11.2 L−1, decreasing with increasing T. The interquartile range and 95th-percentile values also decreased with increasing T, ranging from 5.6 to 15.1 L−1 and from 11.4 to 26.0 L−1, respectively. Values of Dmm showed a clear trend with T, with median values increasing from 1000 μm in the −25° to −40°C temperature range to 1450 μm in the −5° to −15°C temperature range. The interquartile range was maximized in the intermediate temperature level. Across all temperature ranges it varied from 263 to 525 μm, with 5th–95th-percentile values ranging from 678 to 2800 μm. Overall trends observed in these data included an increase in Dmm with increasing temperature and a decrease in N>500 and IWC from the intermediate to highest temperature categories.

Fig. 17.
Fig. 17.

As in Fig. 13, but for residual stratiform regions.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

Unlike updrafts, which could be sampled anywhere below cloud top depending on their stage of development, residual stratiform regions were sampled after the updrafts had weakened and the cells were in a mature state. For this reason, the stratification of the data by temperature (Fig. 17) within residual stratiform regions can be expected to show similar trends to stratification by zd, provided the convective cells ascend to a similar level, in this case, the tropopause. Except for the coldest two categories of N>500 (Fig. 17c) and warmest two categories of Dmm (Fig. 17d), all other categories were statistically distinct from one another (Table 7). The trends in the medians were similar to the stratification by zd, with IWC decreasing from the −15° to −25°C range to the −5° to −15°C range. Similarly, the median values of N>500 decreased, and Dmm increased over the same ranges. These trends are all consistent with particles falling and aggregating while entrainment is occurring.

Table 7.

As in Table 6, but for data sampled at varying T.

Table 7.

Plummer et al. (2015) examined the microphysical structure of stratiform regions associated with large-scale ascent as a function of T. Their analysis was only stratified by T, but in their stratiform regions, T is a reasonable proxy for zd. Figure 9 in Plummer et al. (2015) shows box-and-whisker plots similar to those in Figs. 12, 13, 15, and 17. In their synoptically forced stratiform clouds, N>500, IWC, and Dmm increase with increasing temperature. This behavior in their data reflects the fact that there was a constant source of moisture within synoptically forced stratiform, allowing particles to grow as they fall through the cloud. Furthermore, because the clouds were widespread, entrainment was minimal. In contrast, within residual stratiform regions formed by elevated convection (Fig. 15), a slight decrease in IWC and N>500 and increase in Dmm with depth below cloud top was found. Without a large-scale source of moisture, particles simply fall through the cloud, and there is no substantial particle growth, except by aggregation, or increase in mass of the particles as they fall.

c. Downdrafts

For downdrafts, not enough data existed to perform a statistical analysis. However, for completeness, general trends will be discussed here. As with the residual stratiform, IWC was maximized at intermediate to large depths below cloud top, as well as at intermediate temperature levels, and values were not substantially different from the residual stratiform values. Median values of N>500 ranged from 4.8 to 11.7 L−1 at various zd and showed a fairly uniform distribution of T. No discernable trends in Dmm were visible when stratified by zd, with median values ranging from 913 to 1425 μm. Values of Dmm were maximized at intermediate temperatures, with median values ranging from 1175 to 1475 μm.

d. Particle habits

Particle habits derived from 2D-C imagery using the Holroyd classification (Holroyd 1987) were classified into eight particle types for updrafts and residual stratiform regions. These particle types included hexagonal, oriented, linear, spherical, and irregular particles, as well as dendrites, graupel, and aggregates. Particle size distributions for both updrafts and residual stratiform regions are shown in Fig. 18. Several distinctions between updrafts and residual stratiform regions can be seen in these plots. The concentration of hexagonal, oriented, linear, irregular, and dendritic particles with sizes smaller than approximately 1000 μm were all higher in updrafts compared to residual stratiform regions. However, concentrations of larger particles were about the same in both updrafts and residual stratiform regions. Concentrations of graupel were consistently higher across the full size range in updrafts, consistent with the higher liquid water contents observed in updrafts (Figs. 12b and 13b). The concentrations of spherical particles and aggregates were similar in both updrafts and residual stratiform regions, except for a larger concentration of spherical particles in updrafts at very small sizes (D < 200 μm). Although the shapes of the distributions for each habit mirror each other fairly well, concentrations of particles in updrafts generally exceed those in residual stratiform regions except for spherical particles and aggregates (Figs. 18d and 18h). Aggregates and graupel appear in far lower concentrations than the other habits.

Fig. 18.
Fig. 18.

Particle size distributions for eight different particle habits sampled within the elevated convection analyzed herein, stratified based on whether the particles were sampled within updrafts or residual stratiform regions.

Citation: Journal of the Atmospheric Sciences 74, 1; 10.1175/JAS-D-16-0204.1

7. Summary

Samples of 36 elevated convective cells were collected in two research flights during the Profiling of Winter Storms (PLOWS) field campaign, using in situ microphysical probes and airborne cloud radar mounted on the NSF/NCAR C-130 flown during the campaign. Measurements of equivalent radar reflectivity Ze and vertical Doppler velocity W were made using the Wyoming Cloud Radar, with Ze measurements used primarily to identify elevated convection. Various aircraft probes measured quantities including temperature T and liquid water content (LWC) and derived quantities such as ice water content (IWC), number concentration N>500, and median mass diameter Dmm. Data were stratified by W into three distinct categories, namely updrafts (W > 0 m s−1), downdrafts (W < −2 m s−1), and residual stratiform regions (−2 ≤ W ≤ 0 m s−1). Data were also stratified by both aircraft distance below cloud top zd and T. Comparisons of IWC, LWC, N>500, and Dmm were made between data categories of the same vertical velocity, with Mann–Whitney U Tests used to determine the statistical independence of these data categories. Since elevated convective cells were only penetrated during two flights, it is difficult to make generalities based solely on the data shown here. This work complements that done in Plummer et al. (2014) and Plummer et al. (2015) for cloud-top generating cells and for widespread stratiform regions. The primary conclusions of this study are as follows:

  1. For updrafts stratified by zd, data within the 1000–4000-m zd range proved to be statistically indistinct for IWC, N>500, and Dmm. This shows that the updrafts are effectively distributing particles evenly throughout their depths. Decreases in IWC and N>500 closest to cloud top were likely related to cloud-top entrainment.
  2. Within residual stratiform regions, decreases in IWC and N>500 and increases in Dmm were observed with depth below cloud top. These trends were consistent with particles falling and aggregating while entrainment and subsequent sublimation is occurring.
  3. The convectively generated residual stratiform regions in this study resulted from convective towers that ascended through unsaturated air. As such these residual stratiform regions did not have a constant supply of moisture. In this sense, they were fundamentally different from the synoptically forced stratiform regions, as discussed in Plummer et al. (2015). For this reason, a fundamentally different microphysical evolution occurred, with N>500 and IWC decreasing with increasing T in residual stratiform regions.

Acknowledgments

The authors thank the staff at the National Center for Atmospheric Research Environmental Observing Laboratory, particularly Allen Schanot, Jorgen Jensen, and the Research Aviation Facility staff for their efforts with the C-130, and the staff of the University of Wyoming King Air facility for their support of the WCR deployment. We thank Major Donald K. Carpenter and the U.S. Air Force Peoria National Guard for housing the C-130 during the project. Rapid Update Cycle data were provided by the National Climatic Data Center’s NOMADS. The comments of Dr. Andy Heymsfield and two anonymous reviewers were instrumental in improving the manuscript. This work was funded under National Science Foundation Grant AGS-1247404 to the University of Illinois.

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