James O. Pinto

Autumnal Mixed-Phase Cloudy Boundary Layers in the Arctic

Abstract Two mixed-phase cloudy boundary layer events observed over the Arctic ice pack in autumn are extensively analyzed. The local dynamic and thermodynamic structure of the boundary layers is determined from aircraft measurements including analysis of turbulence, longwave radiative transfer, and cloud microphysics. The large-scale forcing is determined from the National Centers for Environmental Prediction reanalysis fields while mesoscale forcing is estimated from 40-km aircraft box patterns. The two cases differed somewhat in their local static stability, surface characteristics, and large-scale forcing. One case was characterized by a stably stratified cloudy boundary layer over a heterogeneous surface containing numerous open leads. The other case occurred over a fairly homogenous surface of multiyear ice and consisted of a surface-based stable layer surmounted by a low-level jet and a cloud-topped mixed layer. An important large-scale factor in the development of low clouds appears to have been w...

Cloud Resolving Simulations of Mixed-Phase Arctic Stratus Observed during BASE: Sensitivity to Concentration of Ice Crystals and Large-Scale Heat and Moisture Advection

The authors’ previous idealized, two-dimensional cloud resolving model (CRM) simulations of Arctic stratus revealed a surprising sensitivity to the concentrations of ice crystals. In this paper, simulations of an actual case study observed during the Beaufort and Arctic Seas Experiment are performed and the results are compared to the observed data. It is again found in the CRM simulations that the simulated stratus cloud is very sensitive to the concentration of ice crystals. Using midlatitude estimates of the availability of ice forming nuclei (IFN) in the model, the authors find that the concentrations of ice crystals are large enough to result in the almost complete dissipation of otherwise solid, optically thick stratus layers. A tenuous stratus can be maintained in the simulation when the continuous input of moisture through the imposed large-scale advection is strong enough to balance the ice production. However, in association with the large-scale moisture and warm advection, only by reducing the concentration of IFN to 0.3 of the midlatitude estimate values can a persistent, optically thick stratus layer be maintained. The results obtained from the reduced IFN simulation compare reasonably well with observations. The longwave radiative fluxes at the surface are significantly different between the solid stratus and liquidwater-depleted higher ice crystal concentration experiments. This work suggests that transition-season Arctic stratus can be very vulnerable to anthropogenic sources of IFN, which can alter cloud structure sufficiently to affect the rates of melting and freezing of the Arctic Ocean. The authors find that the Hallett‐Mossop riming splintering mechanism is not activated in the simulations because the cloud droplets are very small and cloud temperatures are outside the range supporting efficient rime splintering. Thus, the conclusions drawn from the results presented in this paper may be applicable to only a limited class of Arctic stratus.

Mesoscale Modeling of Springtime Arctic Mixed-Phase Stratiform Clouds Using a New Two-Moment Bulk Microphysics Scheme

A new two-moment bulk microphysics scheme is implemented into the polar version of the fifthgeneration Pennsylvania State University–NCAR Mesoscale Model (MM5) to simulate arctic mixed-phase boundary layer stratiform clouds observed during Surface Heat Budget of the Arctic (SHEBA) First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Arctic Cloud Experiment (ACE). The microphysics scheme predicts the number concentrations and mixing ratios of four hydrometeor species (cloud droplets, small ice, rain, snow) and includes detailed treatments of droplet activation and ice nucleation from a prescribed distribution of aerosol obtained from observations. The model is able to reproduce many features of the observed mixed-phase cloud, including a near-adiabatic liquid water content profile located near the top of a well-mixed boundary layer, droplet number concentrations of about 200–250 cm 3 that were distributed fairly uniformly through the depth of the cloud, and continuous light snow falling from the cloud base to the surface. The impacts of droplet and ice nucleation, radiative transfer, turbulence, large-scale dynamics, and vertical resolution on the simulated mixed-phase stratiform cloud are examined. The cloud layer is largely self-maintained through strong cloud-top radiative cooling that exceeds 40 K day 1 . It persists through extended periods of downward large-scale motion that tend to thin the layer and reduce water contents. Droplet activation rates are highest near cloud base, associated with subgrid vertical motion that is diagnosed from the predicted turbulence kinetic energy. A sensitivity test neglecting subgrid vertical velocity produces only weak activation and small droplet number concentrations (90 cm 3 ). These results highlight the importance of parameterizing the impact of subgrid vertical velocity to generate local supersaturation for aerosol-droplet closure. The primary ice nucleation mode in the simulated mixed-phase cloud is contact freezing of droplets. Sensitivity tests indicate that the assumed number and size of contact nuclei can have a large impact on the evolution and characteristics of mixed-phase cloud, especially the partitioning of condensate between droplets and ice.

Applications of SHEBA/FIRE data to evaluation of snow/ice albedo parameterizations

Climate models use a wide variety of parameterizations for surface albedos of the ice-covered ocean. These range from simple broadband albedo parameterizations that distinguish among snow-covered and bare ice to more sophisticated parameterizations that include dependence on ice and snow depth, solar zenith angle, and spectral resolution. Several sophisticated parameterizations have also been developed for thermodynamic sea ice models that additionally include dependence on ice and snow age, and melt pond characteristics. Observations obtained in the Arctic Ocean during 1997–1998 in conjunction with the Surface Heat Budget of the Arctic Ocean (SHEBA) and FIRE Arctic Clouds Experiment provide a unique data set against which to evaluate parameterizations of sea ice surface albedo. We apply eight different surface albedo parameterizations to the SHEBA/FIRE data set and evaluate the parameterized albedos against the observed albedos. Results show that these parameterizations yield very different representations of the annual cycle of sea ice albedo. The importance of details and functional relationships of the albedo parameterizations is assessed by incorporating into a single-column sea ice model two different albedo parameterizations, one complex and one simple, that have the same annually averaged surface albedo. The baseline sea ice characteristics and strength of the ice-albedo feedback are compared for the simulations of the different surface albedos.

Use of NWP for Nowcasting Convective Precipitation: Recent Progress and Challenges

Traditionally, the nowcasting of precipitation was conducted to a large extent by means of extrapolation of observations, especially of radar ref lectivity. In recent years, the blending of traditional extrapolation-based techniques with high-resolution numerical weather prediction (NWP) is gaining popularity in the nowcasting community. The increased need of NWP products in nowcasting applications poses great challenges to the NWP community because the nowcasting application of high-resolution NWP has higher requirements on the quality and content of the initial conditions compared to longer-range NWP. Considerable progress has been made in the use of NWP for nowcasting thanks to the increase in computational resources, advancement of high-resolution data assimilation techniques, and improvement of convective-permitting numerical modeling. This paper summarizes the recent progress and discusses some of the challenges for future advancement.

Global Distribution and Characteristics of Diurnally Varying Low-Level Jets

Abstract This study documents the global distribution and characteristics of diurnally varying low-level jets (LLJs), including their horizontal, vertical, and temporal structure, with a special emphasis on highlighting the underlying commonalities and unique qualities of the various nocturnal jets. Two tools are developed to accomplish this goal. The first is a 21-yr global reanalysis performed with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) using a horizontal grid spacing of 40 km. A unique characteristic of the reanalysis is the availability of hourly three-dimensional output, which permits the full diurnal cycle to be analyzed. Furthermore, the horizontal grid spacing of 40 km better resolves many physiographic features that host LLJs than other widely used global reanalyses. This makes possible a detailed examination of the systematic onset and cessation of the jets, including time–height representations of the diurnal cycle. The second tool is an index of nocturnal...

Applications of Aerosondes in the Arctic

he U.S. Arctic remains one of the most difficult places on Earth for year-round scientific observations and research. Logistical support is very expensive, and scientists frequently face dangerous, cold sea–ice dynamics, aircraft icing—even polar bears. While satellites can obtain data in remote regions, their application to many arctic environmental problems is hampered by persistent cloudiness and the complexity of the underlying snow/ice surface. One of the major recommendations of the 1997 report, “Logistics Recommendations for an Improved U.S. Arctic Research Capability” (www.arcus.org/logistics/index.html), was to increase use of robotic aircraft to meet the growing need for environmental observing in the region. Unmanned aerial vehicles (UAVs) excel in “dull, dirty, dangerous” missions. Such UAVs, made by Aerosonde (www.aerosonde. com), were first flown in the Arctic (from a base in Barrow, Alaska) in April 1999 to obtain meteorological observations in support of the Department of Energy Atmospheric Radiation Measurement Program (ARM). However, after only 16 h of flying for this mission, the project lost two aircraft due to airframe icing and one aircraft due to carburetor icing. This mission clearly demonstrated the difficulties of flying the Aerosonde in the extreme arctic environment. The National Science Foundation Office of Polar Programs soon followed these tests by funding a project to establish a facility in Barrow, Alaska, adapt the Aerosonde to extreme arctic conditions, adapt and integrate miniature instrumentation, and assimilate Aerosonde data into predictive models. Meanwhile, the Office of Naval Research began supporting the development and integration of a variety of miniature research instruments for remote sensing of the sea ice surface, measurements of radiative fluxes, and in situ measurements of cloud and precipitation characteristics. Since the first Arctic flights, the Aerosonde has undergone a number of engineering modifications (see Table 1 for Aerosonde specifications). As a result, the Aerosonde project is overcoming the engineering and logistical challenges of operating in the polar environment.

Intercomparison of Bulk Cloud Microphysics Schemes in Mesoscale Simulations of Springtime Arctic Mixed-Phase Stratiform Clouds

Abstract A persistent, weakly forced, horizontally extensive mixed-phase boundary layer cloud observed on 4–5 May 1998 during the Surface Heat Budget of the Arctic Ocean (SHEBA)/First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment–Arctic Clouds Experiment (FIRE–ACE) is modeled using three different bulk microphysics parameterizations of varying complexity implemented into the polar version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The two simpler schemes predict mostly ice clouds and very little liquid water, while the complex scheme is able to reproduce the observed persistence and horizontal extent of the mixed-phase stratus deck. This mixed-phase cloud results in radiative warming of the surface, the development of a cloud-topped, surface-based mixed layer, and an enhanced precipitation rate. In contrast, the optically thin ice clouds predicted by the simpler schemes lead to radiative cooling of t...

Nocturnal Low-Level Jet in a Mountain Basin Complex. Part I: Evolution and Effects on Local Flows

A Doppler lidar deployed to the center of the Great Salt Lake (GSL) basin during the Vertical Transport and Mixing (VTMX) field campaign in October 2000 found a diurnal cycle of the along-basin winds with northerly up-basin flow during the day and a southerly down-basin low-level jet at night. The emphasis of VTMX was on stable atmospheric processes in the cold-air pool that formed in the basin at night. During the night the jet was fully formed as it entered the GSL basin from the south. Thus, it was a feature of the complex string of basins draining toward the Great Salt Lake, which included at least the Utah Lake basin to the south. The timing of the evening reversal to down-basin flow was sensitive to the larger-scale north‐south pressure gradient imposed on the basin complex. On nights when the pressure gradient was not too strong, local drainage flow (slope flows and canyon outflow) was well developed along the Wasatch Range to the east and coexisted with the basin jet. The coexistence of these two types of flow generated localized regions of convergence and divergence, in which regions of vertical motion and transport were focused. Mesoscale numerical simulations captured these features and indicated that updrafts on the order of 5 cm s 21 could persist in these localized convergence zones, contributing to vertical displacement of air masses within the basin cold pool.

Cloud-aerosol interactions during autumn over Beaufort Sea

Cloud and aerosol properties were observed by aircraft in autumn over the Beaufort Sea during the 1994 Beaufort and Arctic Storms Experiment (BASE). The microphysical properties (particle size, concentration, mass, and phase) and vertical structure of autumn clouds are examined as a function of height and minimum in-cloud temperature, T min . Below 2 km, liquid clouds were observed at T min between -5° and -9°C, mixed-phase clouds were observed between -5° and -20°C, and clear-sky ice crystal precipitation was observed at T min as warm as -14°C. Between 2 and 5 km all clouds were mixed-phase and typically consisted of a thin layer of liquid with ice extending well below the liquid layer. These mixed-phase clouds were found at T min as low as -32°C. All clouds observed above 5.5 km were composed entirely of ice at T min as warm as -33°C. The concentration of ice crystals is observed to increase exponentially with decreasing T min . The Hallet-Mossop ice multiplication process did not appear to be an important in the production of ice crystals in the mixed-phase cloud observed in this study. The atmosphere was relatively clean with condensation nuclei (CN) concentrations rarely exceeding 300 cm -3 . The smallest CN concentrations (as low as 50 cm -3 ) were observed in the boundary layer and just above the surface where precipitation and nucleation scavenging have cleansed the air. Thin layers of very large CN concentrations were often observed within and just above low-level clouds possibly resulting from gas-to-particle conversion which requires clean and humid air typical of lower Arctic atmosphere.