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  • View in gallery

    Example of time–height mapping of (a) MMCR reflectivity factor during a cumulus-topped event on 22 Jul 2006. Red dots indicate the cloud bases measured from a ceilometer. Black lines indicate the objectively defined hourly ILH. (b) MMCR Doppler velocity for the period 1200–1400 LST. (c) MMCR reflectivity for the period 1200–1400 LST. (d) Diurnal variation of RWP SNR during a clear-sky day (20 Jun 2006). Black diamonds indicate the algorithm-retrieved mixed layer heights based on SNR gradient. Red crosses are the mixed layer heights estimated using the virtual potential temperature profile from radiosondes launched at 1100 and 1700 LST.

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    (top) Daytime evolution of BLH, ILH, and CBH for cumulus-topped condition. (bottom) Daytime evolution of the BLH and ILH for clear-sky condition. Symbols in both panels indicate the mean observed height from all events (clear or cumulus topped) for the same hours. Bars indicate the plus or minus std dev of the hourly averaged values.

  • View in gallery

    Comparison of BLH estimated from the soundings to the profiler-retrieved BLH based on refractivity gradient from the 4-yr dataset (2004–07). Soundings launched at 1100 and 1700 LST are compared with the BLH values nearest in time from the profiler.

  • View in gallery

    (top) Daytime evolution of normalized vertical velocity variance profiles during cumulus-topped condition. (bottom) As in (top), but during clear-sky condition. Profiles for each hour during clear-sky (cumulus) days are computed independently from all the clear-sky (cumulus) events at this hour from the 4-yr dataset. The zi for both clear-sky and shallow cumulus conditions are estimated from the mixed layer depth retrieval using the RWP signal-to-noise gradient. In the figure legends, the mean value represents the average std dev of the computed variable with height and the term after the ± indicates the minimum and maximum value of the std dev of the computed variable with height.

  • View in gallery

    As in Fig. 4, but for vertical velocity skewness during (top) cumulus-topped condition and (bottom) clear-sky condition.

  • View in gallery

    Classification of profiles of (top) normalized vertical velocity variance and (bottom) vertical velocity skewness during clear-sky and cumulus-topped conditions for three different CF regimes: low CF: 0 < CF ≤ 0.2; moderate CF: 0.2 < CF ≤ 0.6; and high CF: 0.6 < CF < 1.0. Profiles for each classification are computed independently from all the cumulus events for different CFs and clear-sky events between 1100 and 1600 LST from the 4-yr dataset. The zi values for both clear-sky and shallow cumulus conditions are estimated from the mixed layer depth retrieval using the RWP signal-to-noise gradient. In the figure legends, the mean value is calculated as in Fig. 4.

  • View in gallery

    (top) Profiles of mass-flux ratio for different times of the day. Profiles for each hour during clear-sky (cumulus) days are computed as in Fig. 4.

  • View in gallery

    As in Fig. 6, but for classification of profiles of mass-flux ratio.

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Long-Term Observations of the Convective Boundary Layer Using Insect Radar Returns at the SGP ARM Climate Research Facility

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  • 1 Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
  • | 2 Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, Livermore, California
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Abstract

A long-term study of the turbulent structure of the convective boundary layer (CBL) at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility is presented. Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004–08) and are classified into cloudy and clear boundary layer conditions. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. A conditional sampling method is applied to the original Doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport. Overall, the derived turbulent statistics are consistent with previous aircraft and lidar observations. The observations provide unique insight into the daytime evolution of the convective boundary layer and the role of increased cloudiness in the turbulent budget of the subcloud layer. Coherent structures (plumes–thermals) are found to be responsible for more than 80% of the total turbulent transport resolved by the cloud radar system. The extended dataset is suitable for evaluating boundary layer parameterizations and testing large-eddy simulations (LESs) for a variety of surface and cloud conditions.

Corresponding author address: Arunchandra S. Chandra, Department of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street, West Montreal, QC H3A 2K6, Canada. Email: arunchandra.chandra@mail.mcgill.ca

Abstract

A long-term study of the turbulent structure of the convective boundary layer (CBL) at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility is presented. Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004–08) and are classified into cloudy and clear boundary layer conditions. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. A conditional sampling method is applied to the original Doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport. Overall, the derived turbulent statistics are consistent with previous aircraft and lidar observations. The observations provide unique insight into the daytime evolution of the convective boundary layer and the role of increased cloudiness in the turbulent budget of the subcloud layer. Coherent structures (plumes–thermals) are found to be responsible for more than 80% of the total turbulent transport resolved by the cloud radar system. The extended dataset is suitable for evaluating boundary layer parameterizations and testing large-eddy simulations (LESs) for a variety of surface and cloud conditions.

Corresponding author address: Arunchandra S. Chandra, Department of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street, West Montreal, QC H3A 2K6, Canada. Email: arunchandra.chandra@mail.mcgill.ca

1. Introduction

The representation of clear and cloudy boundary layer conditions in global climate models (GlCMs) is relatively poor, thereby limiting the predictability of cloud feedback in a changing climate (Tiedtke 1989; Bony et al. 2006; Teixeira et al. 2008). Over land, shallow cumuli coverage is 5%, yet significant to affect the global radiation budget (e.g., Norris 1998). This shallow mode of convection over land also plays an important role in the preconditioning of deep convection (Lenderink et al. 2004). A controlling factor in the transition and evolution of clear and cloudy boundary layers is the diurnal cycle of the convective boundary layer (CBL), where turbulence is the main transport mechanism for the redistribution of momentum, energy, and moisture. The turbulent structure of the CBL exhibits a strong diurnal cycle as a result of the interaction with the surface (Sfc; e.g., Moyer and Young 1991). In the CBL, turbulence occurs at various scales. Small-scale turbulence dominates in the unstable surface layer, while coherent structures (e.g., thermals, plumes) on the order of the height of the boundary layer play a key role in the mixed layer. These thermals (updrafts) are considered the invisible roots of clouds in the subcloud layer and transport heat and moisture upward from the surface layer (LeMone and Pennell 1976). The coupling between the subcloud layer and the cloud distribution is evident in the form of subcloud layer circulations–roll vortices or well-defined updrafts traceable at least 100 m below cloud base (e.g., LeMone and Pennell 1976). One of the key issues is to understand what regulates the mass flux of subcloud air into the cloud layer, its diurnal cycle, and the effect of clouds (e.g., Betts 1975, 1976; Esbensen 1975; Siebesma et al. 2003; Teixeira et al. 2008). Therefore, improving the representation of transport processes (turbulent local transport, thermals, shallow clouds) in the CBL is still an important issue (e.g., Rio and Hourdin 2008; Teixeira et al. 2008).

Past CBL studies have been conducted during both cloud-free and cloudy conditions (e.g., Lenschow 1970; Kaimal et al. 1976; LeMone and Pennell 1976; Young 1988; Rauber et al. 2007a,b,c; Goke et al. 2007). Coupled studies on the cloud and subcloud layer are limited to aircraft and tower measurements (e.g., LeMone and Pennell 1976; Warner 1977), data obtained from tethered balloon systems (e.g., Echternacht and Garstang 1976; Thompson et al. 1980), and multiple radiosonde ascents (e.g., Johnson 1977). These previous efforts emphasize profiling the vertical structure of the CBL, albeit with coarse resolution, and are limited in duration because of inhibiting costs associated with research aircrafts. Standard similarity theory (e.g., Kaimal and Finnigan 1994) based on surface fluxes using tower measurements are not sufficient (e.g., Kaimal and Finnigan 1994; Rao and Narasimha 2006) and clearly fail near the subcloud layer top in shallow cumulus conditions (e.g., Nicholls and LeMone 1980). An additional approach applied to study the interaction of the cloud and subcloud layer is budget studies (e.g., Betts 1976; Brümmer 1978). These methods implicitly consider the average effects of the transport mechanisms at scales of at least 104 km2 and periods in excess of an hour (Nicholls and LeMone 1980). As a consequence, this approach is insufficient to resolve the shallow cumulus clouds of diameters less than a few kilometers.

Large-eddy simulations (LESs) have proved their suitability for modeling the diurnal evolution of clear and cloudy boundary layers (e.g., Schumann and Moeng 1991; Sullivan et al. 1994, 1998; Siebesma and Cuijpers 1995; Wang and Stevens 2000; De Roode and Bretherton 2003). LESs have successfully modeled the temporal evolution of macroscopic features of the CBL, such as the mixed-layer height and cloud fraction (CF) in well-documented case studies (e.g., Deardorff 1974; Sommeria and LeMone 1978; Moeng 1984; Schmidt and Schumann 1989; Cuijpers and Duynkerke 1993; Siebesma et al. 2003). However, the scarcity of in-cloud observations and insufficient knowledge about the turbulent structure of the subcloud layer and its diurnal evolution limit our ability to evaluate higher-order vertical velocity statistics (variance, skewness) and the mass-flux representation of the vertical turbulent fluxes in the subcloud layer (e.g., Lenderink et al. 2004; Brown et al. 2002; Siebesma et al. 2003; Neggers et al. 2003, 2004; Stevens et al. 2001). Long-term high-resolution vertical velocity observations over the entire depth of the CBL are required to evaluate the LES model results and to understand the structure of the CBL under different forcing regimes.

Progress in ground-based remote sensors for atmospheric boundary layer research offers an opportunity for accurate measurements of basic boundary layer parameters (e.g., profiles of temperature, wind, moisture, and surface fluxes). A thorough review of the contributions and limitations of these surface-based instruments up until the 1990s is discussed elsewhere (Wilczak et al. 1996). Cloud radars and Doppler lidar provide measurements in cloud and clear conditions, with resolutions adequate for conditional sampling studies (Kollias et al. 1999; Kollias and Albrecht 2000; Kollias et al. 2001, 2002; French et al. 2000; Hogan et al. 2009); however, to date there are no long-term observational studies on the subcloud structure of shallow cumulus.

Echoes from nonhydrometeor targets—including insects—have been reported within many low-altitude radar observations, both in clear and cloudy boundary layers (e.g., Riley et al. 1983; Achtemeier 1991; Russell and Wilson 1997; Clothiaux et al. 2000; Wood et al. 2009). Cloud radar observations have been previously used in entomology to study flights of small insects (weight ∼2 mg) because of the sensitivity of these systems to detect small insects (Riley 1992; Ka band, 35 GHz). In a meteorological context, echoes from nonhydrometeor scatterers—including insects—should be removed from hydrometeor echoes from the millimeter wavelength cloud radar (MMCR) for the study of clouds.

In the present study, a different approach is adopted for long-term CBL study. Instead of removing the insect MMCR echo contribution to study clouds in the boundary layer, this study capitalizes on insect Doppler content to study the turbulent structure of the subcloud layer. Here, insects are treated largely as passive scatterers that follow the mean vertical air motion (air tracers). Doppler signatures of insect echoes measured from cloud radar (95 GHz, 3 mm) were previously explored to study the characteristics of buoyant eddies within the CBL using 3 days of data during the International H2O Project (IHOP_2002; Miao et al. 2006). For this study, a more extensive set of insect observations from Doppler radar is classified into clear and cloudy boundary layer (nonprecipitating shallow cumulus) conditions, and composite profiles of vertical velocity variance, skewness, and updraft mass-flux ratio (ratio of updraft mass-flux contribution from coherent structures to the total mass flux, capitalizing on all upward values of the vertical velocity) are computed. In addition to the long-term MMCR dataset, supplementary observations from the radar wind profiler (RWP), total sky imager (TSI), and tower observations are used to characterize the surface conditions and the top of the mixed layer.

2. Observations

The U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility (ACRF) in Lamont, Oklahoma (36.605°N, 97.485°W; altitude: 318 m MSL) provides continuous long-term sampling of the atmosphere from synergetic instruments (e.g., Stokes and Schwartz 1994; Ackerman and Stokes 2003; Kollias et al. 2007). This study capitalizes on several routinely generated ARM products, including the Active Remote Sensing Cloud Location (ARSCL) product (Clothiaux et al. 2000), which combines information from the vertically pointing 35-GHz (Ka-band, 8.66-mm wavelength) MMCR (e.g., Moran et al. 1998; Kollias et al. 2007), a Belfort Instrument laser ceilometer, and a micro-pulse lidar at a temporal resolution of 10 s and a vertical (gate) resolution of 45 m.

The MMCR measurements are used to characterize the cloud (scattering from hydrometeors) and subcloud (scattering from insects) layer. Typically, removing insect echoes from hydrometeor populations is problematic because of the similar range of reflectivity of insects to those of the clouds (Clothiaux et al. 2000). Especially during summer months, MMCR data from the SGP ARM Climate Research Facility has dominant insect echoes (Luke et al. 2008). Weakly flying insects are observed in the “finelines” of the clear-air radar echoes, and high-reflectivity regions are attributed to the high density of insects (Russell and Wilson 1997). Figures 1a,b show the diurnal variation of MMCR reflectivity and Doppler velocity during shallow cumulus conditions. In the figure, the cloud bases are associated with the high-reflectivity region and updraft motion beneath. The echo plumes are expected to represent coherent eddies of rising motion and in most of the fair-weather CBL, these echoes are believed to be small insects (<10 mm in flights; e.g., Miao et al. 2006). According to the hypothesis tested and verified in Geerts and Miao 2005 (hereafter GM05), these scatterers are subject to turbulent mixing in the CBL and tend to converge in the regions of sustained ascent, as these insects oppose any updrafts in which they embedded.

A collocated 915-MHz RWP, a TSI, and a CO2 flux system provide supplemental observations. The RWP collects profiles of wind speed/direction and radar reflectivity factor from hydrometeors and refractive index inhomogeneities at a 30-s temporal and 60-m vertical resolution up to 2.5-km AGL. The CO2 flux system at the SGP central facility consists of a 60-m tower with three sets of instruments installed at 4.5 (surface), 25, and 60 m. Each instrument set contains a sonic anemometer at 10 Hz that measures the orthogonal component of wind velocity (u, υ, and w) and a sonic temperature sensor (which approximates virtual temperature in Kelvin). Turbulent fluxes (e.g., sensible, latent) for this study are only computed from the sonic measurements obtained at the surface.

Observations were collected during the May–September (MJJAS) period and for the years 2004–07 at the SGP ACRF. The MJJAS period was selected because of the presence of the deepest insect layer during most of that period and in part for the favorable conditions for undisturbed clear and cloudy boundary layer conditions. Every 1-h period, the observations are classified as clear and cloudy using a conservative combined threshold from the collocated ceilometer and TSI. Hourly periods are classified as clear if the ceilometer-based hourly cloud fraction is zero and the opacity-based TSI hourly cloud fraction is less than 0.1. Extensive visual inspection of the TSI images for the classified clear-sky hourly periods was performed to validate the classification. Cloudy hourly periods are further classified in three categories according to their cloud fraction: low [>(0%–20%)], medium (20%–60%), and high (60%–100%) hourly fraction of clouds. Precipitation periods are removed using the rain gauge measurements and the MMCR first detection height and intensity. Overcast conditions (stratus or stratocumulus) are filtered out using the TSI and ceilometer measurements.

3. Methodology

For every hour of observations, profiles of the vertical velocity variance and skewness, updraft fraction, and mass flux are estimated. In addition, coherent vertical velocity structures are objectively identified and estimates of their dimensions and contribution to the total mass flux are provided. In detail, the data postprocessing includes the following steps.

a. MMCR ILH detection and correction for insect motion

The primary observations in this study are MMCR cloud radar insect echoes in the CBL (e.g., Riley 1992; Russell and Wilson 1997; Clothiaux et al. 2000; Luke et al. 2008). At the SGP ACRF, insect-based MMCR echoes are regularly observed during the warm season (April–October). Using the ceilometer, rain gauge, TSI measurements, and reflectivity thresholds, one can verify that these returns do not originate from hydrometeors. A limiting factor for the vertical extent of the insect echoes is temperature and clouds (Luke et al. 2008). Insects are assumed to capitalize on large thermals in the CBL to change their altitude (Fig. 1b) and in turn, the insect Doppler signatures are useful to track the spatial scales and magnitude of large eddies in the CBL. Nevertheless, since insects are not passive tracers of air motion, it is important to consider the contribution of insect vertical motion to the observed MMCR Doppler velocity.

Long-term velocity observations from the MMCR dataset in clear-sky conditions indicate a net downward velocity of magnitude of 0.2–0.4 m s−1 in the lower half of the CBL that approaches zero near the insect layer top. The insect layer top is determined as the maximum height up to which MMCR echoes from biological/insect media are available 80% or more of the time within an hour of observations. The average diurnal variation of the insect layer top for clear and cloudy conditions is shown in Fig. 2, and it is commonly observed close to the CBL top. The detection of the CBL top height is discussed later in this section. The range of net downward velocities reported are comparable to the insect velocities observed from past studies (GM05; Riley et al. 1991; David and Hardie 1998; Schaefer 1976; Riley and Reynolds 1979; Riley et al. 1983). Similar downward velocity bias has been reported from airborne measurements of the Wyoming cloud radar (95 GHz) during the IHOP_2002 field campaign over the SGP location (GM05), when compared with the vertical velocity measurements from the aircraft gust probe. The excellent discussion in GM05 indicates that insects introduced stronger downward biases in the presence of stronger updrafts. Thus, the effect of the insect motion in the observed Doppler velocity is more than the introduction of a systematic bias. In this study, Eq. (2) in GM05 is adopted to correct the observed MMCR Doppler velocities for insect motion. This study uses Doppler velocities from a 35-GHz radar, and GM05 utilized a 94-GHz radar. Thus, as a preliminary test, we compared a long record of Doppler velocities from the 35- and 94-GHz radar at the ARM SGP site. The study concluded that there is no difference in the observed Doppler velocity values from the two systems. Thus, the Geerts and Miao relationship was directly applied to our long dataset.

b. Conditional sampling of coherent vertical velocity structures

LES models are developed upon the hypothesis that these models have sufficient resolution to resolve the length of eddies that are responsible for most of the turbulent transport in the boundary layer. One of the main objectives of this study is to examine to what extent the turbulent transport in the CBL is performed by large coherent eddies and to investigate the corresponding spatial dimensions of these plumes. Both tasks require the detection of coherent vertical velocity structures (plumes) using an objective conditional sampling (e.g., Greenhut and Khalsa 1987; Khalsa and Greenhut 1987; Williams and Hacker 1992; Kollias and Albrecht 2000). If statistics involving dimensions of coherent structures are required, then attention is to be given to the effects of small-scale turbulence embedded within the identified structures and their environment (e.g., Williams and Hacker 1992).

MMCR observations are provided in time and height. Time is converted to horizontal distance using the hourly consensus estimates of the horizontal wind magnitude from the RWP. To avoid the short time, large-amplitude fluctuations that manifest in the data as coherent structures (e.g., Williams and Hacker 1992), a minimum physical width (horizontal dimension) of 300 m is required for the detection of a coherent velocity structure (plume). In addition, a minimum vertical extent of 225 m (five MMCR range gates) is set. Thus, a coherent updraft and downdraft structure is an area with a minimum size of 225 × 300 m where all Doppler velocity perturbations are positive and negative, respectively. Higher updraft velocity magnitudes (e.g., 0.3–1.0 m s−1) are used to identify more intense plumes. Once the area (time, height) covered with plumes is detected, the widths of updraft and downdraft regions are recorded and are normalized by the boundary layer height (BLH; estimates of these heights are given in section 3c). Best-fit exponential curves are fitted to the distributions of observed plume widths for updraft and downdraft structures and half-widths (distance at which the best-fit exponential curve attains 0.5 times the smallest resolvable scale) are recorded as a measure of the median width in the observed distributions (e.g., as in Miao et al. 2006). Similarly, the plume contribution to the mass flux is estimated for different velocity thresholds (here, a velocity threshold of 0 m s−1 is assumed).

c. Estimation of BLH

Estimates of the BLH are needed to normalize the profiles of the turbulence parameters with respect to the depth of the CBL. Measurements of potential temperature and mixing ratio from soundings can be used to retrieve estimates of the BLH; however, sounding-based estimates of the BLH are not available at the temporal resolution required for hourly scaling. An alternative approach for retrieving the BLH is the use of the 915-MHz RWP (e.g., Grimsdell and Angevine 1998; Lothon et al. 2008), which is sensitive to Bragg scattering from gradients of the atmospheric refractive index. During nonprecipitating conditions, the RWP backscatter exhibits a local maximum at the top of the CBL because of the presence of strong gradients in the temperature and humidity field. An example of RWP-retrieved mixed layer heights (black circles) as compared with the two estimates obtained from collocated ARM radiosonde launches (red crosses) is shown in Fig. 1d. A subset of the retrieved BLH values from the RWP is evaluated using the BLH estimates using data from atmospheric soundings collected within the same hour period (Fig. 3). The comparison exhibits good agreement between the BLH estimates with a correlation coefficient of 0.9. The comparison provided confidence in the estimation of the BLH using the RWP observations. An automatic procedure was developed to trace the regions of the maximum reflectivity gradient in the RWP field as a proxy for the top of the mixed layer.

4. Analysis

A total of 2894 h of observations over 4 yr (2004–07) of summer periods (May–September) was included in this analysis (Tables 1 –4). The events are separated into clear-sky periods (1603 h) and cumulus-topped boundary layer periods (1291 h). The cumulus-topped cases are further classified with respect to cloud fraction as low (cloud fraction less than 20% of the hour, 412 h), moderate (cloud fraction between 20% and 60%, 516 h) and high (cloud fraction above 60%, 363 h). The dataset is also classified according to the time of the day. An average profile at each hour for clear-sky (cumulus) days is computed independently from all the clear-sky (cumulus) days at this hour having clear-sky (cumulus cloud) conditions.

a. Bulk daytime observations from the dataset (Tables 1–4)

During cloud-free conditions, the mixed layer depth increases from 947 m between 1000 and 1100 local standard time (LST = CST = UTC − 6 h) to a maximum of 1654 m between 1600 and 1700 LST. In cumulus-topped conditions, the largest diurnal variability in the cloud-base height (CBH) is during low-cloud fraction conditions (1080–1730 m) and the smallest is during high-cloud fraction conditions (1154–1401 m). In general, the cloud-base height decreases with increasing cloud fraction. The surface buoyancy flux maximum is observed around 1200 LST and decreases from 275 W m−2 during clear-sky conditions to 233, 217, and 154 for low-, moderate-, and high-cloud fraction conditions, respectively.

The convective velocity scale (Lenschow et al. 1980; Stull 1988) in Tables 1 –4 is defined as
i1520-0442-23-21-5699-e1
where g is the acceleration due to gravity, Tυ is the virtual temperature, wT ′υ is the surface kinematic buoyancy flux, and h is the BLH. In (1), the kinematic buoyancy flux is provided directly from sonic measurements of w′ and T ′υ. The calculated convective-velocity-scale maximum for clear-sky, low, and moderate cloud conditions is observed between 1200 and 1300 and 1300 and 1400 LST with values ranging from 1.65 to 2.23 m s−1. The lowest convective-velocity-scale maximum (1.65 m s−1) is observed during high-cloud fraction conditions. This can be attributed to the reduction in both surface buoyancy flux and boundary layer depth (Table 4). The maximum of the convective velocity scale is found close to the maximum of the surface kinematic buoyancy flux (Tables 1 –4).

In addition to the surface conditions, the daytime evolution of the BLH is documented using the RWP returns. The hourly based estimates of the BLH are used to scale the profiles of vertical velocity statistics. The averaged insect layer top and the BLH for both clear-sky and cumulus-topped conditions are shown in Fig. 2. During clear-sky conditions, the BLH rises from 0.95 km (1000–1100 LST) to 1.65 km (1600–1700 LST). During cumulus-topped conditions, BLH values of low-cloud fraction conditions are higher compared to under clear-sky, medium-, and high-cloud fraction conditions during afternoon, but they are nearly comparable before noon. In clear-sky days, the insect layer “jump” is higher than the growing mixed layer early in the morning (in conjunction with the development of daytime convection) similar to the observations of Wood et al. (2009), but it remains very close to the BLH after 1200 LST. In cumulus-topped conditions, the insect-layer-top estimate remains slightly above the BLH during morning times; during the afternoon, the estimate is slightly lower than the BLH. This is consistent with previous studies of insects at the SGP ACRF (Luke et al. 2008).

b. Vertical velocity variance profiles

Profiles of vertical velocity variance and skewness have been frequently explored to characterize convective conditions and the source of turbulent kinetic energy (TKE) in the boundary layer (e.g., LeMone 1990; Moyer and Young 1991; Hogan et al. 2009). These profiles are often scaled with the boundary layer height (zi) to provide nondimensional vertical coordinates for the daytime turbulent statistics in the planetary boundary layer (e.g., Lenschow et al. 1980; Young 1988). As in the previous sections, during both clear-sky and cloudy conditions, the BLH deduced by the RWP is used to scale the vertical coordinate. For each hour of observations and at each MMCR range gate from the surface to the top of the insect layer, the variance of the vertical air motion (w2) is calculated based on the time series of corrected MMCR mean Doppler velocity measurements. The variance values are normalized by the convective velocity scale () as in Eq. (1).

The daytime evolution of the normalized vertical velocity variance profile for cloudy and clear-sky conditions is shown in Fig. 4. For each hour, and separately for cloudy and clear periods, the mean of the hourly estimated normalized vertical velocity variance is shown. In addition, the measurements of normalized vertical velocity variance collected by aircraft (Nicholls and LeMone 1980) and Doppler lidar (Hogan et al. 2009) are also shown for comparison. The mean normalized vertical velocity profiles collected using the insect radar returns are smooth because of the large number of hours included in the analysis and are limited to the depth z/zi ∼ 0.8 to ensure the filtering of erroneous values due to the scarcity of MMCR insect echoes up to zi for all the days considered in the averaging. In addition to the mean, the standard deviation (std dev) of the hourly values is estimated to provide a measure of the one-day-to-another variability for the same hourly period. The height-averaged standard deviation of the reported values for the cumulus-topped and clear-sky conditions is 0.49 and 0.54 m s−1, respectively; and the spread (change) of the standard deviation values with height for the cumulus-topped and clear-sky conditions is 0.08 and 0.09, respectively (Fig. 4). In both clear and cloudy conditions, a gradual increase in the magnitude of the normalized variance is observed with time. The maximum values are observed between 1400 and 1600 LST. In the vertical profile, the maximum variance is observed between 0.2 and 0.4 in normalized height, which is consistent with the asymmetric profile of Lenschow et al. (1980) fitted to the observations and the previous LESs (e.g., Schmidt and Schumann 1989; Shen and Leclerc 1995) and slightly lower compared to the observations of Nicholls and LeMone (1980). The difference may also be attributed to the differences in the spatial extent of the surface inhomogeneities. Above the variance maximum, the normalized variance profiles decrease with height. This observation is consistent with a well-mixed boundary layer, where forcing is provided from the bottom (surface) in the form of surface heat flux (e.g., Garratt 1992).

Clear-sky days exhibit relatively higher normalized variance values compare to cloudy days. Overall, the observed magnitudes are higher than these observed by Nicholls and LeMone (1980) and Hogan et al. (2009), especially during clear-sky periods. The observed differences can be attributed first to the fact that the observed profiles by Nicholls and LeMone (1980) and Hogan et al. (2009) are derived by a small sample of data and under different conditions, and second to the sensitivity of the vertical velocity variance to the GM05 correction. GM05 used a linear relationship with a 1.92 slope to correct the observed Doppler velocities for insect motion. The variance of the vertical velocity increases with the square of the slope. The variance is the only result presented in this study that is sensitive to the insect motion correction. Nevertheless, the vertical velocity data contain rich information with respect to the vertical structure and the daytime evolution of the normalized variance.

During clear-sky periods, there is a systematic shift toward higher value with daytime. This is not clear in the cloudy periods and it is attributed to the averaging of hours with different cloud fraction amount. The impact of increased cloudiness at the top of the CBL in the observed profile of normalized variance is shown (Fig. 6a). The hourly-estimated normalized variance profiles are classified and averaged with respect to the observed cloudiness for clear-sky, low-, medium-, and high-cloud fraction conditions. The composite averages indicate that an increase in cloudiness is associated with higher normalized variance (compared to low- and medium-cloud fraction conditions). Furthermore, in high-cloud fraction conditions, the normalized variance profile reverses its decreasing trend near the upper part of the CBL. This implies a source of turbulence near the top of CBL driven by increased cloudiness.

c. Vertical velocity skewness profiles

In addition to the normalized variance, the hourly profiles of vertical velocity skewness are estimated using the corrected MMCR Doppler velocities for both clear and cloudy periods. The skewness is defined as
i1520-0442-23-21-5699-e2
where w′ is the vertical velocity perturbation. The sign and magnitude of the skewness are driven by asymmetries in the distribution of vertical velocities in a particular height. Positive (negative) skewness indicates the presence of a few narrow strong updrafts (downdrafts) in our observations. In a surface-driven well-mixed boundary layer, one expects the skewness to be positive and increasing with height; in a top-driven boundary layer, one expects the skewness to be negative at the top (e.g., Moyer and Young 1991).

The composite profiles of vertical velocity skewness and their daytime evolution for cloud-topped and clear-sky conditions as a function of the normalized zi are shown in Fig. 5. For comparison, the measurements from Nicholls and LeMone (1980) and Hogan et al. (2009) are shown in the same figure. The limited dataset from aircraft and Doppler lidar observations exhibits great scatter with no evident vertical structure, and it covers the range of the observed values using the insect radar returns. The composite profiles exhibit a smooth vertical structure. This is attributed to the long dataset used in this study. Despite the smooth vertical structure, on average, the day-to-another-day standard deviation of the observed skewness values is high: 0.51 and 0.38 for cloudy and clear-sky conditions, respectively. During clear-sky conditions, skewness values are positive with magnitudes ranging from 0.1 to 0.4. This is consistent with a surface-driven boundary layer. Near the surface (z/zi < 0.25) and the upper part (z/zi > 0.7) of the CBL, we observe the lowest values (0.1–0.25), while the middle part of the CBL exhibits the higher positive values of skewness without a noticeable daytime progression. During cloud-topped conditions, we observed both positive and negative skewness values. The lowest values of skewness are observed before noon (1100–1200 LST) and during late afternoon (1500–1600 LST), while the highest (positive) values are observed between 1200 and 1500 LST. The skewness remains positive in the lower two-thirds of the CBL and negative in the upper part of the CBL. This is a clear difference compared to the clear-sky profiles, and it is another indicator of the effect of clouds in the subcloud layer turbulent kinetic energy budget.

Once again, the effect of increased cloudiness on the vertical velocity skewness profiles is studied by partitioning the hourly estimated skewness profiles with respect to the cloud fraction (Fig. 6b). Overall, increased cloudiness results in lower magnitudes of positive skewness. A transition to negative skewness values in the CBL occurs at middle- and high-cloud fractions at levels higher than z/zi > 0.7 and z/zi > 0.5, respectively. It is evident that skewness, a higher-moment statistic of the vertical velocity field than variance, shows the effect of clouds in the subcloud layer and the turbulent budget in the upper part of the CBL (e.g., Moeng and Rotunno 1990).

d. Plume dimension and updraft mass-flux profiles

Conditional sampling (section 3) was applied on the vertical velocity field to extract the physical dimensions of coherent updraft and downdraft structures and to estimate their contribution to the total updraft mass flux. The observed updraft and downdraft half-widths cover the range of 0.49–0.83 (Table 5). During clear-sky conditions, half-width values for updrafts (downdrafts) are at a maximum 0.73 (0.80) early in the day (1000–1100 LST) and a minimum 0.49 (0.6) in late afternoon (1600–1700 LST). Similarly, during cloud conditions, the half-width values for updrafts (downdrafts) are at a maximum 0.81 (0.83) early (1000–1100 LST) and a minimum 0.53 (0.51) in late afternoon (1600–1700 LST). During cloud-topped conditions, no clear trend is observed between updraft and downdraft half-widths. The statistics of the width of the updraft structures change little when the velocity threshold for the plumes increased from 0 to 0.6 m s−1 (only the threshold of 0 m s−1 is shown in Table 5), indicating the velocity strength in the updraft plumes. Similar results are found for downdraft half-widths.

The updraft mass flux is calculated using both a “direct” and a conditional sampling method. Here, the direct method estimates the total updraft mass flux using all the available upward MMCR velocities at each MMCR range gate for every hour. Thus, in this direct approach, all upward MMCR Doppler velocity points contribute to the updraft mass-flux calculation. Moreover, this calculation does not discriminate as to whether these measurements are associated with an updraft plume.

The “conditional sampling” method estimates the updraft mass flux from observations that have an upward MMCR velocity and are also part of a coherent updraft plume according to the criteria outlined in section 3b. In the conditional sampling method, not all MMCR upward velocity observations contribute to the estimated mass flux (e.g., Kollias and Albrecht 2000). The updraft mass flux estimated using conditional sampling is usually less than the updraft mass flux estimated using direct sampling. The two estimates are equal only in the extreme case of a coherent vertical velocity field (e.g., sinusoidal wave). The ratio of the two estimates of updraft mass flux (conditional to direct) indicates the percent of turbulent transport in the boundary layer from coherent plumes (large eddies). The shape of mass-flux profiles of both total mass flux and mass flux from coherent structures (not shown) was similar to the normalized variance profiles with a maximum located between z/zi = 0.2 and 0.4 and decreases thereafter. This is consistent with the findings of Nicholls and LeMone (1980). On average, the updraft mass-flux magnitudes calculated during cumulus-topped periods are smaller than these observed during clear-sky periods (not shown).

The daytime evolution of the updraft mass-flux conditional-to-direct method ratio during clear-sky and cumulus-topped conditions is shown in Fig. 7. Overall, coherent structures are responsible for more than 80% of the turbulent transport, and the standard deviation of this estimate from one day to another is only 7%. This is a significant finding that validates mass-flux-based parameterizations of the turbulent transport in the CBL and the use of LES models (e.g., Couvreux et al. 2010) for modeling boundary layer processes. The maximum of the updraft mass-flux ratio occurs at z/zi = 0.3 and then decreases with normalized height to reach their lowest value at the maximum height of our observations (z/zi = ∼0.85). No daytime progression of the updraft mass-flux ratio is observed. The shape of the mass-flux profiles during cumulus-topped conditions is similar to the profiles observed for clear skies, except there is a lower mass-flux ratio during later morning (1100–1200 LST) because of relatively small total mass-flux transport. The maximum updraft mass flux is observed at z/zi = 0.3, similar to the clear-sky condition.

The effect of cloudiness in the observed updraft mass-flux ratio in the subcloud layer is shown in Fig. 8. During high-cloud fraction conditions, the mass-flux ratio is relatively lower compare to other cloud fraction regimes and clear-sky conditions above z/zi = 0.3. Another observed feature is that the mass-flux ratio during clear-sky conditions is lower than during the low- and medium-cloud fraction condition.

e. Seasonal variability

The large dataset (spans four warm seasons at the SGP ACRF) was further analyzed for season-to-season and month-to-month variability. Monthly averages of turbulent statistics (normalized vertical velocity variance and skewness) were computed for clear and cumulus-topped conditions in all months (MJJAS) of the dataset. The analysis exhibits that the magnitudes of normalized variance are higher during clear-sky conditions and especially during the month of July. No other noticeable trend regarding monthly variability or year-to-year variability of the turbulence statistics was found.

5. Summary

Insect radar returns at the SGP ACRF have been long considered a nuisance for efforts to use ground-based vertically pointing radars to detect and study boundary layer clouds. In this study, a different approach is adopted wherein the insect radar returns from vertically pointing Doppler cloud radar are used to monitor the properties of boundary layer turbulence. The study makes use of a multiyear summer dataset from millimeter-wavelength cloud radar, a 915-MHz wind profiler, and flux-measuring sensors at the surface. The Doppler velocity measurements from insects were corrected for insect motion using the GM05 formula.

A clear strength of this study is the use of a large dataset of four consecutive warm seasons of vertical velocity observations at the SGP ACRF. The large dataset facilitates acquiring smoothed vertical structures of turbulence statistics and documenting their diurnal evolution. The large dataset (2894 h of daytime CBL observations) is classified into clear-sky and cumulus-topped conditions. During both clear-sky and cumulus-topped conditions, the 915-MHz wind profiler signal-to-noise ratio (SNR) is used to develop an automated algorithm for the detection of the mixed layer top. The daytime evolution of the boundary layer is studied using the profiles of vertical velocity variance, skewness, and mass-flux ratio.

The normalized vertical velocity variance profiles exhibit a smooth daytime evolution with the maximum turbulent activity observed between 1400 and 1500 LST. During clear-sky conditions, the maximum in normalized variance is observed around z/zi = 0.3. This is in agreement with previous aircraft observations for surface-driven turbulence. The magnitude of the variance is relatively higher during clear-sky conditions as compared to cumulus-topped conditions; this is consistent with previous observations. Evidence for the effect of clouds in the subcloud layer turbulence budget is found in the vertical structure of the normalized variance for high-cloud fraction conditions. The vertical velocity skewness during clear-sky conditions is positive and higher compared to cumulus-topped conditions. During cumulus-topped conditions, negative skewness is observed near the top of the subcloud layer, indicating the interaction between the subcloud layer and cloud layer.

Using an objective conditional sampling method, coherent vertical velocity structures in the boundary layer are identified. The observed normalized updraft and downdraft half-width values vary from 0.8 (prenoon hours) to 0.5 (late-afternoon hours). The reported normalized half-width values are dominated by the presence of numerous plumes with small horizontal width that do not scale with the BLH increase throughout the day. However, when the detail hourly normalized half-width distributions are viewed (not presented here), we do see a number of individual plumes with normalized half-width near or higher than the BLH. Updraft mass-flux calculations using a direct sampling technique and the conditional sampling technique suggest that coherent structures (plumes) are responsible for more than 80% of the total turbulent transport observed by the radar in the boundary layer during clear-sky and cumulus-topped conditions.

Overall, the characterization of the boundary layer turbulence using insect radar returns is consistent with previous studies. The cloud radar observations provided a unique daytime evolution of the convective boundary layer and indicate the role of increased cloudiness in the turbulent budget of the subcloud layer. The large SGP MMCR dataset makes the observations suitable for evaluating boundary layer parameterizations for a variety of surface and cloud conditions. The basic analysis of the data provided in this paper gives support to using the cloud radar data in a number of different studies of greater complexity (a variety of surface and cloudy conditions). It is more straightforward to think of using these observations to test LESs. The study also provides the observational evidence to assess the boundary layer parameterizations by including the behavior of CBL statistics under different conditions.

Acknowledgments

Support for this research was funded by the Office of Biological and Environmental Research, Environmental Sciences Division of the U.S. Department of Energy as part of the Atmospheric Radiation Measurement Program. The contribution of S. A. Klein to this work is performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. We thank Dr. Margaret A. LeMone of the National Center for Atmospheric Research and Dr. Robin Hogan of the University of Reading for generously providing the data for comparison.

REFERENCES

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Fig. 1.
Fig. 1.

Example of time–height mapping of (a) MMCR reflectivity factor during a cumulus-topped event on 22 Jul 2006. Red dots indicate the cloud bases measured from a ceilometer. Black lines indicate the objectively defined hourly ILH. (b) MMCR Doppler velocity for the period 1200–1400 LST. (c) MMCR reflectivity for the period 1200–1400 LST. (d) Diurnal variation of RWP SNR during a clear-sky day (20 Jun 2006). Black diamonds indicate the algorithm-retrieved mixed layer heights based on SNR gradient. Red crosses are the mixed layer heights estimated using the virtual potential temperature profile from radiosondes launched at 1100 and 1700 LST.

Citation: Journal of Climate 23, 21; 10.1175/2010JCLI3395.1

Fig. 2.
Fig. 2.

(top) Daytime evolution of BLH, ILH, and CBH for cumulus-topped condition. (bottom) Daytime evolution of the BLH and ILH for clear-sky condition. Symbols in both panels indicate the mean observed height from all events (clear or cumulus topped) for the same hours. Bars indicate the plus or minus std dev of the hourly averaged values.

Citation: Journal of Climate 23, 21; 10.1175/2010JCLI3395.1

Fig. 3.
Fig. 3.

Comparison of BLH estimated from the soundings to the profiler-retrieved BLH based on refractivity gradient from the 4-yr dataset (2004–07). Soundings launched at 1100 and 1700 LST are compared with the BLH values nearest in time from the profiler.

Citation: Journal of Climate 23, 21; 10.1175/2010JCLI3395.1

Fig. 4.
Fig. 4.

(top) Daytime evolution of normalized vertical velocity variance profiles during cumulus-topped condition. (bottom) As in (top), but during clear-sky condition. Profiles for each hour during clear-sky (cumulus) days are computed independently from all the clear-sky (cumulus) events at this hour from the 4-yr dataset. The zi for both clear-sky and shallow cumulus conditions are estimated from the mixed layer depth retrieval using the RWP signal-to-noise gradient. In the figure legends, the mean value represents the average std dev of the computed variable with height and the term after the ± indicates the minimum and maximum value of the std dev of the computed variable with height.

Citation: Journal of Climate 23, 21; 10.1175/2010JCLI3395.1

Fig. 5.
Fig. 5.

As in Fig. 4, but for vertical velocity skewness during (top) cumulus-topped condition and (bottom) clear-sky condition.

Citation: Journal of Climate 23, 21; 10.1175/2010JCLI3395.1

Fig. 6.
Fig. 6.

Classification of profiles of (top) normalized vertical velocity variance and (bottom) vertical velocity skewness during clear-sky and cumulus-topped conditions for three different CF regimes: low CF: 0 < CF ≤ 0.2; moderate CF: 0.2 < CF ≤ 0.6; and high CF: 0.6 < CF < 1.0. Profiles for each classification are computed independently from all the cumulus events for different CFs and clear-sky events between 1100 and 1600 LST from the 4-yr dataset. The zi values for both clear-sky and shallow cumulus conditions are estimated from the mixed layer depth retrieval using the RWP signal-to-noise gradient. In the figure legends, the mean value is calculated as in Fig. 4.

Citation: Journal of Climate 23, 21; 10.1175/2010JCLI3395.1

Fig. 7.
Fig. 7.

(top) Profiles of mass-flux ratio for different times of the day. Profiles for each hour during clear-sky (cumulus) days are computed as in Fig. 4.

Citation: Journal of Climate 23, 21; 10.1175/2010JCLI3395.1

Fig. 8.
Fig. 8.

As in Fig. 6, but for classification of profiles of mass-flux ratio.

Citation: Journal of Climate 23, 21; 10.1175/2010JCLI3395.1

Table 1.

Tabulated hourly values (mean and std dev) of boundary layer and Sfc properties during clear-sky conditions computed from the 4-yr dataset.

Table 1.
Table 2.

As in Table 1, but during low CF (below 20%) cumulus conditions.

Table 2.
Table 3.

As in Table 1, but during moderate CF (20%–60%) cumulus conditions.

Table 3.
Table 4.

As in Table 1, but during high CF (above 60%) cumulus conditions.

Table 4.
Table 5.

Classification of updraft and downdraft half-widths during shallow cumulus and clear-sky events based on time of day computed from the 4-yr dataset.

Table 5.
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