Marine Boundary Layer Cloud Properties Research Articles

Observational Investigation of the Effect of Turbulence on Microphysics and Precipitation in Warm Marine Boundary Layer Clouds

AbstractTurbulence is omnipresent in the marine boundary layer (MBL) and several mechanisms have been proposed regarding its role on MBL cloud microphysics. In particular, past modeling studies suggested that increasing turbulence broaden the droplet size distribution (DSD) and leads to precipitation initiation, this suggested relationship has not been evaluated yet using long‐term observations. Here, comprehensive retrievals from long‐term ground‐based observations are used to explore the impact of turbulence on microphysics and precipitation in drizzling MBL clouds. Our analysis supports that broader DSD and larger droplets are observed with strong turbulence, but interestingly, the precipitation is reduced instead of being enhanced. This suppression mechanism is mainly due to the entrainment and mixing process in which the entrained dry air evaporates cloud water. This study provides observational evidence to show the complex effect of turbulence on MBL cloud properties and implies the potential influence of turbulence on the clouds lifetime.

Linking large-scale circulation patterns to low-cloud properties

Abstract. The North Pacific High (NPH) is a fundamental meteorological feature present during the boreal warm season. Marine boundary layer (MBL) clouds, which are persistent in this oceanic region, are influenced directly by the NPH. In this study, we combine 11 years of reanalysis and an unsupervised machine learning technique to examine the gamut of 850 hPa synoptic-scale circulation patterns. This approach reveals two distinguishable regimes – a dominant NPH setup and a land-falling cyclone – and in between a spectrum of large-scale patterns. We then use satellite retrievals to elucidate for the first time the explicit dependence of MBL cloud properties (namely cloud droplet number concentration, liquid water path, and shortwave cloud radiative effect – CRESW) on 850 hPa circulation patterns over the northeast Pacific Ocean. We find that CRESW spans from −146.8 to −115.5 W m−2, indicating that the range of observed MBL cloud properties must be accounted for in global and regional climate models. Our results demonstrate the value of combining reanalysis and satellite retrievals to help clarify the relationship between synoptic-scale dynamics and cloud physics.

Inter-comparisons of marine boundary layer cloud properties from the ARM CAP-MBL campaign and two MODIS cloud products.

From April 2009 to December 2010, the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program carried out an observational field campaign on Graciosa Island, targeting the marine boundary layer (MBL) clouds over the Azores region. In this paper, we present an inter-comparison of the MBL cloud properties, namely, cloud liquid water path (LWP), cloud optical thickness (COT) and cloud-droplet effective radius (CER), among retrievals from the ARM mobile facility (AMF) and two Moderate Resolution Spectroradiometer (MODIS) cloud products (GSFC-MODIS and CERES-MODIS). A total of 63 daytime single-layer MBL cloud cases are selected for inter-comparison. Comparison of collocated retrievals indicates that the two MODIS cloud products agree well on both COT and CER retrievals, with the correlation coefficient R>0.95. despite their significant difference in spatial sampling. In both MODIS products, the CER retrievals based on the 2.1 µm band (CER2.1) is significantly smaller than that based on the 3.7 µm band (CER3.7). The GSFC-MODIS cloud product is collocated and compared with ground-based ARM observations at several temporal-spatial scales. In general, the correlation increases with more precise collocation. For the 63 selected MBL cloud cases, the GSFC-MODIS LWP and COT retrievals agree reasonably well with the ground-based observations with no apparent bias and correlation coefficient R around 0.85 and 0.70, respectively. However, GSFC-MODIS CER3.7 and CER2.1 retrievals have a lower correlation (R~0.5) with the ground-based retrievals. For the 63 selected cases, they are on average larger than ground observations by about 1.5 µm and 3.0 µm, respectively. Taking into account that the MODIS CER retrievals are only sensitive to cloud top reduces the bias only by 0.5 µm.

Response of Marine Boundary Layer Cloud Properties to Aerosol Perturbations Associated with Meteorological Conditions from the 19-Month AMF-Azores Campaign

Abstract This study investigates the response of marine boundary layer (MBL) cloud properties to aerosol loading by accounting for the contributions of large-scale dynamic and thermodynamic conditions and quantifies the first indirect effect (FIE). It makes use of 19-month measurements of aerosols, clouds, and meteorology acquired during the Atmospheric Radiation Measurement Mobile Facility field campaign over the Azores. Cloud droplet number concentrations and cloud optical depth (COD) significantly increased with increasing aerosol number concentration . Cloud droplet effective radius (DER) significantly decreased with increasing . The correlations between cloud microphysical properties [, liquid water path (LWP), and DER] and were stronger under more stable conditions. The correlations between , LWP, DER, and were stronger under ascending-motion conditions, while the correlation between COD and was stronger under descending-motion conditions. The magnitude and corresponding uncertainty of the FIE ranged from 0.060 ± 0.022 to 0.101 ± 0.006 depending on the different LWP values. Under more stable conditions, cloud-base heights were generally lower than those under less stable conditions. This enabled a more effective interaction with aerosols, resulting in a larger value for the FIE. However, the dependence of the response of cloud properties to aerosol perturbations on stability varied according to whether ground- or satellite-based DER retrievals were used. The magnitude of the FIE had a larger variation with changing LWP under ascending-motion conditions and tended to be higher under ascending-motion conditions for clouds with low LWP and under descending-motion conditions for clouds with high LWP. A contrasting dependence of FIE on atmospheric stability estimated from the surface and satellite cloud properties retrievals reported in this study underscores the importance of assessing all-level properties of clouds in aerosol–cloud interaction studies.

Evaluation of Warm-Rain Microphysical Parameterizations in Mesoscale Simulations of the Cloudy Marine Boundary Layer

Abstract Several warm-rain microphysical parameterizations are evaluated in a regional forecast model setting (using the Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System) by evaluating how accurately the model is able to represent the marine boundary layer (MBL). Cloud properties from a large suite of simulations using different parameterizations and concentrations of cloud condensation nuclei (CCN) are compared to ship-based observations from the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study—Regional Experiment (VOCALS-REx) field campaign conducted over the southeastern Pacific (SEP). As in previous studies, the simulations systematically underestimate liquid water path and MBL cloud depth. On the other hand, the simulations overestimate precipitation rates relative to those derived from the scanning C-band radar on board the ship. Most of the simulations exhibit a diurnal cycle, although details differ somewhat from a recent observational study. In addition to direct comparisons with the observations, the internal microphysical consistency of simulated MBL cloud properties is assessed by comparing simulation output to a number of observationally and theoretically derived scalings for precipitation and coalescence scavenging. Simulation results are broadly consistent with these scalings, suggesting COAMPS is behaving in a microphysically consistent fashion. However, microphysical consistency as defined in the analysis is highly dependent upon the horizontal resolution of the model. Excessive depletion of CCN from large coalescence processing rates suggests the importance of parameterizing a source term for CCN or imposing some form of fixed, climatological background CCN concentration.

Retrieving co‐occurring cloud and precipitation properties of warm marine boundary layer clouds with A‐Train data

AbstractIn marine boundary layer (MBL) clouds the formation of precipitation from the cloud droplet distribution in the presence of variable aerosol plays a fundamental role in determining the coupling of these clouds to their environment and ultimately to the climate system. Here the degree to which A‐Train satellite measurements can diagnose simultaneously occurring cloud and precipitation properties in MBL clouds is examined. Beginning with the measurements provided by CloudSat and Moderate Resolution Imaging Spectroradiometer (including a newly available microwave brightness temperature from CloudSat), and a climatology of MBL cloud properties from past field campaigns, an assumption is made that any hydrometeor volume could contain both cloud droplet and precipitation droplet modes. Bayesian optimal estimation is then used to derive atmospheric states by inverting a measurement vector carefully accounting for uncertainties due to instrument noise, forward model error, and assumptions. It is found that in many cases where significant precipitation coexists with cloud, due to forward model error driven by uncertainties in assumptions, the uncertainty in retrieved cloud properties is greater than the variance in the prior climatology. It is often necessary to average several thousand (hundred) precipitating (weakly precipitating) profiles to obtain meaningful information regarding the properties important to microphysical processes. Regardless, if such process level information is deemed necessary for better constraining predictive models of the climate system, measurement systems specifically designed to accomplish such retrievals must be considered for the future.

Investigation of the marine boundary layer cloud and CCN properties under coupled and decoupled conditions over the Azores

AbstractSix coupled and decoupled marine boundary layer (MBL) clouds were chosen from the 19 month Atmospheric Radiation Measurement Mobile Facility data set over the Azores. Thresholds of liquid water potential temperature difference ΔθL < 0.5 K (>0.5 K) and total water mixing ratio difference Δqt < 0.5 g/kg (>0.5 g/kg) below the cloud base were used for selecting the coupled (decoupled) cases. A schematic diagram was given to demonstrate the coupled and decoupled MBL vertical structures and how they associate with nondrizzle, virga, and rain drizzle events. Out of a total of 2676 5 min samples, 34.5% were classified as coupled and 65.5% as decoupled, 36.2% as nondrizzle and 63.8% as drizzle (47.7% as virga and 16.1% as rain), and 33.4% as daytime and 66.6% as nighttime. The decoupled cloud layer is deeper (0.406 km) than coupled cloud layer (0.304 km), and its liquid water path and cloud droplet effective radius (re) values (122.1 gm−2 and 13.0 µm) are higher than coupled ones (83.7 gm−2 and 10.4 µm). Conversely, decoupled stratocumuli have lower cloud droplet number concentration (Nd) and surface cloud condensation nucleus (CCN) concentration (NCCN) (74.5 cm−3 and 150.9 cm−3) than coupled stratocumuli (111.7 cm−3 and 216.4 cm−3). The linear regressions between re and Nd with NCCN have demonstrated that coupled re and Nd strongly depend on NCCN and have higher correlations (−0.56 and 0.59) with NCCN than decoupled results (−0.14 and 0.25). The MBL cloud properties under nondrizzle and virga drizzle conditions are similar to each other but significantly different to those of rain drizzle.

Comparison of marine boundary layer cloud properties from CERES‐MODIS Edition 4 and DOE ARM AMF measurements at the Azores

AbstractMarine boundary layer (MBL) cloud properties derived from the NASA Clouds and the Earth's Radiant Energy System (CERES) project using Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) data are compared with observations taken at the Department of Energy Atmospheric Radiation Measurement (ARM) Mobile Facility at the Azores (AMF‐Azores) site from June 2009 through December 2010. Cloud properties derived from ARM ground‐based observations were averaged over a 1 h interval centered at the satellite overpass time, while the CERES‐MODIS (CM) results were averaged within a 30 km × 30 km grid box centered over the Azores site. A total of 63 daytime and 92 nighttime single‐layered overcast MBL cloud cases were selected from 19 months of ARM radar‐lidar and satellite observations. The CM cloud top/base heights (Htop/Hbase) were determined from cloud top/base temperatures (Ttop/Tbase) using a regional boundary layer lapse rate method. For daytime comparisons, the CM‐derived Htop (Hbase), on average, is 0.063 km (0.068 km) higher (lower) than its ARM radar‐lidar‐observed counterpart, and the CM‐derived Ttop and Tbase are 0.9 K less and 2.5 K greater than the surface values with high correlations (R2 = 0.82 and 0.84, respectively). In general, the cloud top comparisons agree better than the cloud base comparisons, because the CM cloud base temperatures and heights are secondary products determined from cloud top temperatures and heights. No significant day‐night difference was found in the analyses. The comparisons of MBL cloud microphysical properties reveal that when averaged over a 30 km × 30 km area, the CM‐retrieved cloud droplet effective radius (re) at 3.7 µm is 1.3 µm larger than that from the ARM retrievals (12.8 µm), while the CM‐retrieved cloud liquid water path (LWP) is 13.5 gm−2 less than its ARM counterpart (114.2 gm−2) due to its small optical depth (9.6 versus 13.7). The differences are reduced by 50% when the CM averages are computed only using the MODIS pixel nearest the AMF site. Using the effective radius retrieved using 2.1 µm channel to calculate LWP can reduce the difference between the CM and ARM microwave radiometer retrievals from −13.7 to 2.1 gm−2. The 10% differences between the ARM and CERES‐MODIS LWP and re retrievals are within the uncertainties of the ARM LWP (~20 gm−2) and re (~10%) retrievals; however, the 30% difference in optical depth is significant. Possible reasons contributing to this discrepancy are increased sensitivities in optical depth from both surface retrievals when τ~10 and topography. The τ differences vary with wind direction and are consistent with the island orography. Much better agreement in τ is obtained when using only those data taken when the wind is from the northeast, where topographical effects on the sampled clouds are minimal.

A 19-Month Record of Marine Aerosol–Cloud–Radiation Properties Derived from DOE ARM Mobile Facility Deployment at the Azores. Part I: Cloud Fraction and Single-Layered MBL Cloud Properties

AbstractA 19-month record of total and single-layered low (<3 km), middle (3–6 km), and high (>6 km) cloud fractions (CFs) and the single-layered marine boundary layer (MBL) cloud macrophysical and microphysical properties was generated from ground-based measurements at the Atmospheric Radiation Measurement Program (ARM) Azores site between June 2009 and December 2010. This is the most comprehensive dataset of marine cloud fraction and MBL cloud properties. The annual means of total CF and single-layered low, middle, and high CFs derived from ARM radar and lidar observations are 0.702, 0.271, 0.01, and 0.106, respectively. Greater total and single-layered high (>6 km) CFs occurred during the winter, whereas single-layered low (<3 km) CFs were more prominent during summer. Diurnal cycles for both total and low CFs were stronger during summer than during winter. The CFs are bimodally distributed in the vertical with a lower peak at ~1 km and a higher peak between 8 and 11 km during all seasons, except summer when only the low peak occurs. Persistent high pressure and dry conditions produce more single-layered MBL clouds and fewer total clouds during summer, whereas the low pressure and moist air masses during winter generate more total and multilayered clouds, and deep frontal clouds associated with midlatitude cyclones.The seasonal variations of cloud heights and thickness are also associated with the seasonal synoptic patterns. The MBL cloud layer is low, warm, and thin with large liquid water path (LWP) and liquid water content (LWC) during summer, whereas during winter it is higher, colder, and thicker with reduced LWP and LWC. The cloud LWP and LWC values are greater at night than during daytime. The monthly mean daytime cloud droplet effective radius re values are nearly constant, while the daytime droplet number concentration Nd basically follows the LWC variation. There is a strong correlation between cloud condensation nuclei (CCN) concentration NCCN and Nd during January–May, probably due to the frequent low pressure systems because upward motion brings more surface CCN to cloud base (well-mixed boundary layer). During summer and autumn, the correlation between Nd and NCCN is not as strong as that during January–May because downward motion from high pressure systems is predominant. Compared to the compiled aircraft in situ measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX), the cloud microphysical retrievals in this study agree well with historical aircraft data. Different air mass sources over the ARM Azores site have significant impacts on the cloud microphysical properties and surface CCN as demonstrated by great variability in NCCN and cloud microphysical properties during some months.

Thermodynamic and Aerosol Controls in Southeast Pacific Stratocumulus

Abstract A near-large-eddy simulation approach with size-revolving (bin) microphysics is employed to evaluate the relative sensitivity of southeast Pacific marine boundary layer cloud properties to thermodynamic and aerosol parameters. Simulations are based on a heavily drizzling cloud system observed by the NOAA ship Ronald H. Brown during the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study—Regional Experiment (VOCALS-Rex) field campaign. A suite of numerical experiments examines the sensitivity of drizzle to variations in boundary layer depth and cloud condensation nuclei (CCN) concentration in a manner consistent with the variability of those parameters observed during VOCALS-Rex. All four simulations produce cellular structures and turbulence characteristics of a circulation driven predominantly in a bottom-up fashion. The cloud and subcloud layers are coupled by strong convective updrafts that provide moisture to the cloud layer. Distributions of reflectivity calculated from model droplet spectra agree well with reflectivity distributions from the 5-cm-wavelength scanning radar aboard the ship, and the statistical behavior of cells over the course of the simulation is similar to that documented in previous studies of southeast Pacific stratocumulus. The simulations suggest that increased aerosol concentration delays the onset of drizzle, whereas changes in the boundary layer height are more important in modulating drizzle intensity.

Reply to Comments on “Seasonal Variation of the Physical Properties of Marine Boundary Layer Clouds off the California Coast”

Based on the strong similarity in microphysical differences between summer–winter marine boundary layer (MBL) clouds and the closed–open mesoscale cellular convection (MCC), Liu (2010)’s comments suggest that there is plausibly a microphysical mechanism, in addition to the macrophysical mechanism discussed in Lin et al. (2009), that is responsible for the seasonal contrast of the boundary layer structure and MBL cloud properties. Given the Moderate Resolution Imaging Spectroradiometer–Clouds and the Earth’s Radiant Energy System (MODIS–CERES) retrieval of mean liquid cloud effective radius re, together with the cloud liquid water path (LWP) and cloud thickness H as presented in Lin et al. (2009), the cloud droplet number density N and drizzle rate P can be estimated using (1), (2), and (6) in Liu (2010)’s comments. Without losing generality, we assume the more commonly used be 5 1/3 and use ae 5 66.83 from Martin et al. (1994) for maritime stratocumulus clouds. See Liu’s comments and the cited references for more discussion about the relations and the coefficients. The effective radius and the calculated number density as well as the drizzle rate are shown in Fig. 1. Indeed, as predicted in Liu’s comments, the MBL clouds off the California coast in summer are not only with higher liquid water content L but also with smaller re, larger N, and smaller P than those in winter. Figure 1 additionally shows that the spatial downstream variations of these microphysical properties along the cross section for each season are similar to the seasonal contrast of these variables. The summer-to-winter variation mirrors the transition from closed to open MCCs, as discussed in Liu (2010). Considering that the microphysical sensitivity of the cloudbase precipitation rate becomes weaker for clouds with stronger precipitation (Kubar et al. 2009; Wood et al. 2009), an alternative formulation with weaker dependence on microphysical properties (Van Zanten et al. 2005) is also used to estimate the drizzle rates. The seasonal difference is very similar (not shown). The higher drizzle rate during the winter in this region is also supported by multiyear MODIS retrievals, with drizzle frequency being higher among MBL cloud scenes during off-peak season for stratocumulus clouds (Jensen et al. 2008). The coincidence of higher drizzle rate and higher inversion height during winter appears to be consistent with the positive correlation between cloud-top height and drizzle frequency, as reported in Leon et al. (2008). Note that the equations for the estimation of N and P are for cloud-scale processes and highly nonlinear. The numbers that are derived based on the long-term averaged L and re should be interpreted with caution. However, there are also important cloud properties that cannot be explained in this analogy. First, for MCCs, along the cross section in the summer, the increasing drizzle rate should be associated with decreasing cloud fraction, because in comparison with closed MCCs, open MCCs usually have about 30% less cloud fraction (Wood and Hartmann 2006). But this is not the case in the seasonal variation: higher rate of drizzle in the summer is associated with larger cloud fraction along the cross section (Fig. 5a in Lin et al. 2009). Furthermore, in the winter, * Current affiliation: Brookhaven National Laboratory, Upton, New York.

Investigation of Regional and Seasonal Variations in Marine Boundary Layer Cloud Properties from MODIS Observations

Abstract To aid in understanding the role that marine boundary layer (MBL) clouds play in climate and assist in improving their representations in general circulation models (GCMs), their long-term microphysical and macroscale characteristics are quantified using observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard the National Aeronautics and Space Administration’s (NASA’s) Terra satellite. Six years of MODIS pixel-level cloud products are used from oceanic study regions off the west coasts of California, Peru, the Canary Islands, Angola, and Australia where these cloud types are common. Characterizations are given for their organization (macroscale structure), the associated microphysical properties, and the seasonal dependencies of their variations for scales consistent with the size of a GCM grid box (300 km × 300 km). MBL mesoscale structure is quantified using effective cloud diameter CD, which is introduced here as a simplified measure of bulk cloud organization; it is straightforward to compute and provides descriptive information beyond that offered by cloud fraction. The interrelationships of these characteristics are explored while considering the influences of the MBL state, such as the occurrence of drizzle. Several commonalities emerge for the five study regions. MBL clouds contain the best natural examples of plane-parallel clouds, but overcast clouds occur in only about 25% of the scenes, which emphasizes the importance of representing broken MBL cloud fields in climate models (that are subgrid scale). During the peak months of cloud occurrence, mesoscale organization (larger CD) increases such that the fractions of scenes characterized as “overcast” and “clumped” increase at the expense of the “scattered” scenes. Cloud liquid water path and visible optical depth usually trend strongly with CD, with the largest values occurring for scenes that are drizzling. However, considerable interregional differences exist in these trends, suggesting that different regression functionalities exist for each region. For peak versus off-peak months, the fraction of drizzling scenes (as a function of CD) are similar for California and Angola, which suggests that a single probability distribution function might be used for their drizzle occurrence in climate models. The patterns are strikingly opposite for Peru and Australia; thus, the contrasts among regions may offer a test bed for model simulations of MBL drizzle occurrence.