This study uses airborne data from two field campaigns off the California
coast to characterize aerosol size distribution characteristics in the
entrainment interface layer (EIL), a thin and turbulent layer above marine
stratocumulus cloud tops, which separates the stratocumulus-topped boundary
layer (STBL) from the free troposphere (FT). The vertical bounds of the EIL
are defined in this work based on considerations of buoyancy and turbulence
using thermodynamic and dynamic data. Aerosol number concentrations are
examined from three different probes with varying particle diameter
(
Stratocumulus clouds are extensively studied because they are both the dominant cloud type by global area (Warren et al., 1986), covering approximately a fifth of the planet's surface area on an annual basis (Wood, 2012), and they play an important role in the planet's energy balance due to their impact on planetary albedo. The layer separating the stratocumulus-topped boundary layer (STBL) from the free troposphere (FT) aloft is usually tens of meters in vertical extent and referred to as the entrainment interface layer (EIL) (Caughey et al., 1982; Nicholls and Turton, 1986; Wang and Albrecht, 1994; Lenschow et al., 2000). This layer exhibits strong gradients in thermodynamic and dynamic properties. Although numerous airborne and modeling studies have attempted to increase our understanding about the thermodynamic and dynamic nature of the EIL (e.g., Caughey et al., 1982; Moeng et al., 2005; Haman et al., 2007; Wang et al., 2008; Carman et al., 2012; Katzwinkel et al., 2012; Gerber et al., 2013; Malinowski et al., 2013; Jen-La Plante et al., 2016), aerosol characteristics in this thin layer have not been studied in detail.
The nature of the aerosol layer immediately above cloud top is important to understand because particles impact cloud microphysics and also because clouds vertically redistribute particles, remove them via droplet coalescence, and transform their properties through aqueous reactions (e.g., Wonaschuetz et al., 2012). A modeling study showed that aerosol entrainment from the FT can contribute between 69 and 89 % of particle number concentrations in the marine boundary layer (MBL; Katoshevski et al., 1999), and field measurements have confirmed the importance of entrainment in shaping the marine boundary layer aerosol budget (e.g., Clarke et al., 1998). The effects of above-cloud aerosol particles on clouds depend on the physicochemical properties of particles, their vertical distance from cloud top, and the dynamic and thermodynamic conditions around cloud top. Particles closest to the cloud top can entrain into the cloud and change the number concentration and size distribution of droplets (Costantino and Breón, 2010). On the other hand, an aerosol layer more detached from the cloud top and higher aloft can potentially alter the thermodynamic and dynamic structure of the layer below it, such as with absorbing smoke layers that can lead to stabilization and weaker cloud top long wave radiative cooling. This could in turn reduce cloudiness and cloud radiative forcing (Yamaguchi et al., 2015).
The goal of this study is to examine vertically resolved aircraft data in the marine atmosphere off the California coast to characterize aerosol characteristics as a function of altitude, with a focus on the EIL. The results provide insight into the degree of similarity between the aerosol size distribution in the EIL relative to the STBL and FT. The results motivate additional attention to the EIL in terms of acting as an intermediate layer between the STBL and FT, in which there is some combination of cloud-processed aerosol and FT aerosol, in addition to new particle formation.
Aircraft data from the Center for Interdisciplinary Remotely-Piloted Aircraft
Studies (CIRPAS) Twin Otter are analyzed from the Nucleation in California
Experiment (NiCE, 2013) and the Fog and Stratocumulus Evolution experiment
(FASE, 2016), both of which took place between July and August. The flights
examined here typically lasted 4 h and included vertical characterization of
marine aerosol ranging from near the ocean surface (
Navigational, dynamic, and thermodynamic data were obtained from standard
instruments described in a number of previous studies (e.g., Crosbie et al.,
2016; Wang et al., 2016; Dadashazar et al., 2017). Aerosol particle
concentrations were measured using multiple condensation particle counters
(CPCs; TSI Inc.), specifically the CPC 3010 (particle diameter,
Spatial map of spiral soundings examined in this study from the
NiCE (2013) and FASE (2016) field campaigns. The cases are labeled with the
campaign (F
The PVM-100A probe (Gerber et al., 1994) provided measurements of liquid
water content (LWC). A threshold LWC value of 0.02 g m
F07 on 1 August 2016 showing how thermodynamic and dynamic criteria were applied to define the vertical bounds of the EIL, which separates the STBL from the FT. This subset of data is obtained from an upward spiral sounding.
Summary of EIL thickness and particle concentrations (average
(relative standard deviation as a percentage)) for the
sub-cloud layer (SUB), the entrainment interface layer (EIL), and the free
troposphere (FT). The cases are labeled with the campaign (F
A total of 17 spiral soundings were analyzed from FASE and NiCE, with their
locations shown in Fig. 1. The ranges of cloud base heights and tops were
129–403 and 375–729 m, respectively, for these soundings. Three vertical
layers were defined with respect to the cloud layer including the sub-cloud
(SUB) layer, EIL, and FT. The vertical bounds of the EIL are defined based on
considerations of buoyancy and turbulence, similar to past studies (Carman et
al., 2012). An example from FASE research flight 7 (F07) on 1 August 2016
illustrates the criteria used to determine the vertical boundaries of the
EIL, STBL, and FT (Fig. 2). While some studies extend the EIL into the cloud
layer (Malinowski et al., 2013; Jen-La Plante et al., 2016), this work
defines the base of the EIL at cloud top (i.e., uppermost height where LWC
The FT base is considered to be at the EIL top, while the STBL top marks the
EIL base. The FT layer extends up to 400 m above the EIL top for most cases
except for five spirals that only reached
Particle concentrations in different diameter ranges (3–10, 10–110, and 110–3400 nm) for SUB, EIL, and FT vertical layers. The FT is divided into four layers based on 100 m increments above the EIL top. Whiskers represent 1 standard deviation.
Particle concentration in diameter range 110–3400 (PCASP) as a
function of altitude in the EIL. Linear fits and slopes (s, units of
cm
Same as Fig. 5 but for particle concentration in diameter range 10–110 nm (i.e., CPC-PCASP).
Same as Fig. 5 but for particle concentration in diameter range 3–10 nm (UFCPC-CPC).
The sources of pollution impacting the study region vary in terms of the vertical layer being examined. More specifically, the predominant sources in the STBL are marine sea spray and biogenic emissions as well as ship exhaust (e.g., Coggon et al., 2014; Modini et al., 2015), while the major sources impacting the FT originate from the continent, including biogenic emissions, wildfires, anthropogenic emissions, and crustal emissions (e.g., Wang et al., 2014; Crosbie et al., 2016). As it is challenging with the current dataset to separate the relative importance of the pollution type affecting the EIL, instead the focus of the subsequent discussion is on aerosol size distributions. Also, as a way to rule out the presence of a different air mass in the EIL that is distinctly different than those in the STBL and FT, vertical profiles of CO (not shown here) were examined for the cases in Table 1. CO exhibited a smooth transition in concentration in the EIL progressing from lower values in the STBL to higher values in the FT. Based on that result and the shallow depth of EIL, it is concluded that the EIL in the cases examined did not have a distinct air mass affecting it that was different from either that in the STBL or the lower FT.
Table 1 compares particle concentration measurements from the PCASP and CPCs
between the FT, EIL, and SUB layers. CPC concentrations were highest in the
EIL for 8 of the 17 soundings, with the remaining 9 cases exhibiting
peak values in the FT. With ascending altitude, average CPC concentrations
were as follows: 465
Relationship between the slope of particle number concentration
(CN) in the EIL and number concentration differences between the FT1 and SUB
layers. Results are shown for two particle diameter ranges:
Particle concentrations in different diameter ranges (3–10,
10–110, and 110–3400 nm) in the sub-cloud (SUB) layer for thin (thickness
<333 m) and thick (thickness
Vertically resolved aerosol size distributions during spiral
soundings on
Numerous past studies have discussed the occurrence of nucleation in the
marine atmosphere (Hegg et al., 1991; Covert et al., 1992; Raes and Van
Dingenen, 1992; Hoppel et al., 1994; Pandis et al., 1994; Clarke et al.,
1998; Weber et al., 1998; Petters et al., 2006). Discussion in the previous
section about differences between the UFCPC and CPC results suggests that new
particle formation is a common occurrence in the EIL. Otherwise, it is
difficult to explain the enhancements in particle concentrations with
To further examine differences in the aerosol size distribution in different
vertical layers, Fig. 3 shows average number concentrations of particles in
three
Factors promoting nucleation include cool and moist air and low particle
surface area concentrations (e.g., Kerminen and Wexler, 1996; Pirjola et al.,
1999; Clarke et al., 1999; Alam et al., 2003). Figure 4 shows mean values for
these parameters in each vertical layer. Surface area (SA) concentration was
quantified separately for particles with
As it could be argued that the SA concentration in the EIL was still not very
low in an absolute sense and exceeded values in layers above it, it is
important to put the results in the context of other studies. Nucleation
events adjacent to marine clouds have been recorded to occur for SA
concentrations below 2
The combination of cool and moist air, high actinic solar fluxes, relatively
low SA concentrations as compared to other studies with nucleation events
(e.g., Alam et al., 2003; Cai et al., 2017), and several precursor vapor
sources builds a case for why nucleation resulted in the highest number
concentration of particles with
The potential significance of nucleation in the EIL is that these particles
impact the transfer of solar radiation owing to both directly scattering
light and contributing to the marine atmosphere's cloud condensation nuclei
(CCN) budget after growth to sufficiently large sizes. It is not possible
with the current dataset to accurately calculate either nucleation rates in
the EIL or the growth rates of nucleated particles to CCN-relevant sizes.
However, a comparison of particle concentrations for
The vertical profile of aerosol number concentrations in the EIL provides
insight into the level of influence between adjacent vertical layers (i.e.,
STBL and FT). Thirteen of the 17 examined spirals exhibited an increasing
trend of particle concentration in the
The slopes of the number concentrations for two
An interesting feature of the cases with lower number concentrations in the
SUB layer is that they tended to be concurrent with thicker clouds. Figure 9
shows particle concentrations in the SUB layer for the 17 cases divided in
two different categories (thin and thick clouds) using the median cloud
thickness (333 m) as a dividing threshold value. The number concentration
means for
While some studies suggest that the EIL air has properties intermediate to the STBL and FT owing to detrainment of air from the STBL (Deardorff, 1980; Gerber et al., 2005, 2016), others have not found evidence for detrainment (Faloona et al., 2005; Kurowski et al., 2009). Also, the lowering of cloud top height via mechanisms such as evaporation or drop sedimentation can leave a layer of cloud-processed aerosol in the EIL (Sorooshian et al., 2007; Chen et al., 2012). As those studies were not focused on aerosol size distributions, here we address this issue using PCASP size distribution data. Three case studies (Fig. 10) are used to show the range of conditions experienced with reference made to geometric mean diameters of specific PCASP size bins where number concentration modes were observed.
The N16 case exhibited a unimodal size distribution in the SUB layer with a peak near 420 nm. In the FT, there was a clear peak at or below the minimum size limit of the PCASP (110 nm). The EIL exhibited an intermediate aerosol size distribution with the peak at the lowest size, similar to the FT, and a peak at 420 nm, similar to the SUB layer. In addition, the number concentration was most enhanced in the EIL in comparison to the SUB and FT layers. The number concentration and shape of the size distribution above 315 nm was identical between the EIL and SUB layers. However, the number concentration below that size was most enhanced in the EIL, suggestive of accumulation of subsiding FT aerosol. Earlier work showed how subsiding FT aerosol can lead to thin layers of enriched organic acid aerosol concentrations above cloud tops in the study region (Sorooshian et al., 2007).
The F03-4 case exhibited behavior characteristic of the EIL being mainly influenced by the FT and not the SUB layer. The SUB aerosol size distribution was bimodal with peaks at 182 and 223 nm. The FT aerosol exhibited a bimodal distribution but with peaks at smaller sizes, specifically 151 and 182 nm. The EIL showed the same bimodal structure as the FT, with the resemblance closest near the top of the EIL.
Finally, the F10-1 case exhibited behavior suggestive of higher influence from the SUB layer as compared to the FT. The SUB aerosol size distribution was bimodal, similar to the previous case with peaks at 182 and 223 nm. These same peaks were present in the EIL, and the resemblance to the SUB size distribution was closest at the base of the EIL. The FT aerosol size distribution was unimodal with a peak at 182 nm.
These three cases illustrate that EIL aerosol size distributions exhibit
characteristics of both the STBL and FT aerosol to varying degrees depending
on the case examined. An interesting feature of these three cases is that the
strength of the temperature inversion at cloud top was similar (d
This work examined 17 spiral soundings from research flights off the
California coast with a focus on the aerosol characteristics of the EIL
relative to the FT above it and the STBL below it. The main results are as
follows:
Regardless of particle size range, the SUB layer exhibited the lowest
average number concentrations relative to the EIL and FT. Thicker clouds were
coincident with the lowest number concentrations in the SUB layer, especially
for The aerosol number concentration data provide evidence of nucleation in the
EIL, coincident with factors that promote this mechanism including relatively
low aerosol surface area, favorable meteorological conditions (cool and moist
air), and high actinic fluxes. Vertical aerosol number concentration gradients for diameter range 10–110
and 110–3400 nm in the EIL are a good predictor as to the relative behavior
of the aerosol size distribution between the SUB and FT layers. Vertically resolved aerosol size distribution data show that there can be
signatures of cloud-processed air in the EIL.
The implications of this study are multi-fold with regard to research flight planning and the overall effects of aerosol on climate and clouds. More specifically, the results stress that airborne flights that attempt to characterize aerosol characteristics above stratocumulus clouds require caution in terms of how far above cloud tops flight patterns are conducted owing to differences that exist between the EIL and the FT. Careful attention to where the EIL is relative to the FT is recommended as the latter most clearly will represent aerosol conditions from sources other than those below cloud and the former will have the strongest signature of nucleation. Finally, the EIL often exhibits signatures of cloud-processed aerosol that are important to consider with regard to understanding cloud effects on aerosol.
All data used in this work can be found on the Figshare
database (Sorooshian et al., 2017;
The authors declare that they have no conflict of interest.
This work was funded by Office of Naval Research grants N00014-10-1-0811, N00014-11-1-0783, N00014-10-1-0200, N00014-04-1-0118, and N00014-16-1-2567. Edited by: Manabu Shiraiwa Reviewed by: two anonymous referees