Melanie A. Wetzel

Dynamics and chemistry of marine stratocumulus - DYCOMS II

The second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is described. The field program consisted of nine flights in marine stratocumulus west-southwest of San Diego, California. The objective of the program was to better understand the physics a n d dynamics of marine stratocumulus. Toward this end special flight strategies, including predominantly nocturnal flights, were employed to optimize estimates of entrainment velocities at cloud-top, large-scale divergence within the boundary layer, drizzle processes in the cloud, cloud microstructure, and aerosol–cloud interactions. Cloud conditions during DYCOMS-II were excellent with almost every flight having uniformly overcast clouds topping a well-mixed boundary layer. Although the emphasis of the manuscript is on the goals and methodologies of DYCOMS-II, some preliminary findings are also presented—the most significant being that the cloud layers appear to entrain less and drizzle more than previous theoretical work led investigat...

Evaluation of the aerosol indirect effect in marine stratocumulus clouds: Droplet number, size, liquid water path, and radiative impact

Received 9 June 2004; revised 24 September 2004; accepted 10 February 2005; published 20 April 2005. [1] Data from nine stratocumulus clouds in the northeastern Pacific Ocean were analyzed to determine the effect of aerosol particles on cloud microphysical and radiative properties. Seven nighttime and two daytime cases were included. The number concentration of below-cloud aerosol particles (>0.10 mm diameter) was highly correlated with cloud droplet number concentration. Droplet number concentrations were typically about 75% of particle number concentration in the range of particle concentrations studied (� 400 cm � 3 ). Particle number was anticorrelated with droplet size and with liquid water content in drizzle-sized drops. Radiative impact also depends upon cloud liquid water content and geometric thickness. Although most variability in these macroscopic properties of the clouds could be attributed to variability in the large-scale environment, a weak anticorrelation between particle concentration and cloud geometric thickness was observed. Because of these variations, no correlation between calculated cloud optical thickness or albedo and particle concentration was detectable for the data set as a whole. For regions with comparable liquid water contents in an individual cloud, higher particle concentrations did correspond to increased cloud optical thickness. These results verify that higher particle concentrations do directly affect the microphysics of stratiform clouds. However, the constant liquid water path assumption usually invoked in the Twomey aerosol indirect effect may not be valid.

Storm Peak Laboratory: A Research, Teaching, and Service Facility for the Atmospheric Sciences

The Storm Peak Laboratory (SPL), operated by the Atmospheric Sciences Center of the Desert Research Institute, is now located in a newly constructed permanent building at elevation 3210 m (10 530 ft) above mean sea level in the northwestern Colorado Rocky Mountains. The laboratory provides a site for the conduct of basic and applied research in the atmospheric sciences, hands-on instruction in meteorology for students ranging from middle school through graduate school, and high-elevation atmospheric measurement programs for various scientific groups, agencies, and private companies. This article provides a background of the history of SPL, its past and current activities, and a description of the facilities and opportunities available at the laboratory.

Satellite‐observed patterns in stratus microphysics, aerosol optical thickness, and shortwave radiative forcing

A long-term global daily data set of satellite-derived radiances has been applied over oceans to investigate the effect of aerosol particles on marine stratus cloud physical parameters and cloud shortwave radiative forcing. Tropospheric aerosol can indirectly influence cloud reflectance and radiative forcing by acting as cloud condensation nuclei, thus modifying the droplet size distribution and cloud optical properties. The NOAA polar-orbiting satellite advanced very high resolution radiometer Pathfinder Atmosphere (PATMOS) data provide estimates of aerosol optical thickness (AOT) which were utilized to represent the amount of aerosol available to modify marine stratus. Periods of stratospheric aerosol loading were avoided for this analysis. PATMOS multichannel radiances for cloudy pixels were combined with radiative transfer modeling to estimate cloud droplet effective radius and cloud optical thickness for several months and years in (110 km)2 grid cells between 50°S and 50°N. The results indicate an inverse relationship between aerosol burden and cloud droplet size, including seasonal and regional variability that compares well with known aerosol sources. Analyses suggest an AOT value above which the stratus properties do not change; that is, the indirect effect reaches a “microphysical saturation limit.” Radiative flux modeling was used to evaluate the impact of observed cloud characteristics on shortwave forcing. Decreases in droplet size and increases in cloud albedo associated with enhanced AOT produce significantly increased shortwave radiative forcing estimates as compared with “clean background” cloud and aerosol conditions. These results provide observational evidence for the global influence of aerosol on the shortwave radiative forcing by stratus.

Influence of Cloud Condensation Nuclei on Orographic Snowfall

Abstract Pollution aerosols acting as cloud condensation nuclei (CCN) have the potential to alter warm rain clouds via the aerosol first and second indirect effects in which they modify the cloud droplet population, cloud lifetime and size, rainfall efficiency, and radiation balance from increased albedo. For constant liquid water content, an increase in CCN concentration (NCCN) tends to produce an increased concentration of droplets with smaller diameters. This reduces the collision and coalescence rate, and thus there is a local reduction in rainfall. While this process applies to warm clouds, it does not identically carry over to mixed-phase clouds in which crystal nucleation, crystal riming, crystal versus droplet fall speed, and collection efficiency play active roles in determining precipitation amount. Sulfate-based aerosols serve as very efficient cloud nuclei but are not effective as ice-forming nuclei. In clouds where precipitation formation is dominated by the ice phase, NCCN influences precipi...

Chemical and microphysical properties of marine stratiform cloud in the North Atlantic

The chemical and microphysical properties of marine stratiform cloud were measured at a ridgetop elevation of 992 m above mean sea level (AMSL) on Tenerife in the Canary Islands in the eastern North Atlantic during the summers of 1995 and 1996. The results show an inverse relationship between hourly-averaged cloud droplet diameter and droplet number concentration, which ranged from 116 to 1355 cm−3. Strong relationships were observed between droplet number and equivalent clear air concentrations of non-sea-salt sulfate, nitrate, and elemental carbon in the droplets. Droplet sizes inferred from radiances measured by satellite for clouds offshore and upwind agreed with droplet sizes derived for clouds over the mountain sampling site, and also with those measured in cloud 4–5 hours later. Estimated cloud short-wave radiative forcing was enhanced by 8% in radiative model studies of polluted versus clean clouds with droplet concentrations of 786 and 127 cm−3 and droplet effective radii of 6 and 10 μm, respectively.

Satellite Microphysical Retrievals for Land-Based Fog with Validation by Balloon Profiling

Abstract Digital data from the National Oceanic and Atmospheric Administration Advanced very High Resolution Radiometer (AVHRR) satellite instrument provides multispectral images in visible near-infrared and thermal infrared wave bands, which have been utilized to develop retrieval techniques for estimating the droplet effective radius and optical depth of land-based fog. The retrieval methods are based on multiple scattering calculations that simulate the increased near-infrared absorption by fog layers with increasing droplet size and liquid water path. The AVHRR thermal window channels are utilized to remove the effects of thermal emission in the near-infrared band. New instrumentation and field sampling methods have been developed for obtaining detailed vertical profiles of fog droplet size distributions and thermodynamic conditions in fog decks. The in situ measurements derived from the field observations were employed to test the satellite retrieval techniques. Intercomparison shows a close correspo...

Mesoscale Snowfall Prediction and Verification in Mountainous Terrain

Short-term forecasting of precipitation often relies on meteorological radar coverage to provide information on the intensity, extent, and motion of approaching mesoscale features. However, in significant portions of mountainous regions, radar coverage is lacking because of topographic blocking, and the absence of radar signatures in sections of the radar scan produces uncertain or even misleading information to the public and operational forecasters. In addition, echo characteristics within the radar volume scan are often influenced by the vertical extent and type of precipitation. Each of these conditions limits the opportunity for accurate snowfall prediction and studies of precipitation climatology. To improve both short-term forecasting and postevent verification studies, much greater use can be made of specifically sited surface observations, tailored graphical output from mesoscale models, satellite remote sensing, and case study knowledge of local topographic influences. In this paper, methods to support snowfall forecasts and verification in radar-limited mountainous terrain are demonstrated that include matching the output parameters and graphics from high-resolution mesoscale models to surface mesonets and snowfall observations, analysis of continuous and event-based measurements of snow density, application of multispectral satellite data for verification and trend analysis, and characterization of orographic influences in different winter storm scenarios. The advantages of improved wintertime quantitative precipitation forecasting (QPF) in mountain regions include public safety responsibilities that are critical to National Weather Service (NWS) operations, and are relevant to any mountainous region with radar scan limitations or during periods of radar data outages.

Retrieval of Marine Stratus Cloud Droplet Size from NOAA-AVHRR Nighttime Imagery

Abstract A method for retrieval of the droplet radius and temperature of oceanic stratocumulus is presented. It is based on night imagery obtained from the infrared channels of NOAA–AVHRR and an atmospheric radiative transfer model that makes use of the discrete ordinate method DISORT. It uses the observed satellite brightness temperature differences (BTD) between channels 4 and 5 to obtain the cloud temperature and between channels 3 and 4 to extract the effective radius of the cloud droplets. We also studied the peculiarities of the method, taking into account the behavior of the single scattering parameters, deduced from Mie theory, with droplet size. Results obtained are compared with in situ data collected at the Canary Islands (Spain) during summer 1996.

Theoretical development and sensitivity tests of a stratus cloud droplet size retrieval method for AVHRR-K/L/M

Abstract A modified AVHRR multispectral imaging instrument is to be flown on the NOAA polar-orbiting satellites beginning with NOAA-K, and is designated AVHRR-K/L/M. The instrument will include a narrowed near-visible Channel 2 centered at 0.85 μm, and a new near-infrared Channel 3A centered at 1.6 μm. In this study, we describe a retrieval method for marine stratus cloud droplet size which utilizes the radiance measurements in these channels. Factors which influence the sensitivity and accuracy of the estimation method are discussed, and results of model simulations designed to test the method are presented. Potential retrieval errors due to haze within the boundary layer are identified. It is concluded that the effective radius of the cloud droplet size distribution can be retrieved for clouds of optical depth seven or greater which fill the AVHRR radiometer field of view.