Snow Crystal Size Research Articles

Contributions of Clouds, Surface Albedos, and Mixed-Phase Ice Nucleation Schemes to Arctic Radiation Biases in CAM5

The Arctic radiation balance is strongly affected by clouds and surface albedo. Prior work has identified Arctic cloud liquid water path (LWP) and surface radiative flux biases in the Community Atmosphere Model, version 5 (CAM5), and reductions to these biases with improved mixed-phase ice nucleation schemes. Here, CAM5 net top-of-atmosphere (TOA) Arctic radiative flux biases are quantified along with the contributions of clouds, surface albedos, and new mixed-phase ice nucleation schemes to these biases. CAM5 net TOA all-sky shortwave (SW) and outgoing longwave radiation (OLR) fluxes are generally within 10 W m−2 of Clouds and the Earth’s Radiant Energy System Energy Balanced and Filled (CERES-EBAF) observations. However, CAM5 has compensating SW errors: Surface albedos over snow are too high while cloud amount and LWP are too low. Use of a new CAM5 Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar simulator that corrects an error in the treatment of snow crystal size confirms insufficient cloud amount in CAM5 year-round. CAM5 OLR is too low because of low surface temperature in winter, excessive atmospheric water vapor in summer, and excessive cloud heights year-round. Simulations with two new mixed-phase ice nucleation schemes—one based on an empirical fit to ice nuclei observations and one based on classical nucleation theory with prognostic ice nuclei—improve surface climate in winter by increasing cloud amount and LWP. However, net TOA and surface radiation biases remain because of increases in midlevel clouds and a persistent deficit in cloud LWP. These findings highlight challenges with evaluating and modeling Arctic cloud, radiation, and climate processes.

Formaldehyde in the Alaskan Arctic snowpack: Partitioning and physical processes involved in air-snow exchanges

[1] The snowpack is a photochemically active medium which produces numerous key reactive species involved in the atmospheric chemistry of polar regions. Formaldehyde (HCHO) is one such reactive species produced in the snow, and which can be released to the atmospheric boundary layer. Based on atmospheric and snow measurements, this study investigates the physical processes involved in the HCHO air‐snow exchanges observed during the OASIS 2009 field campaign at Barrow, Alaska. HCHO concentration changes in a fresh diamond dust layer are quantitatively explained by the equilibration of a solid solution of HCHO in ice, through solid‐state diffusion of HCHO within snow crystals. Because diffusion of HCHO in ice is slow, the size of snow crystals is a major variable in the kinetics of exchange and the knowledge of the snow specific surface area is therefore crucial. Air‐snow exchanges of HCHO can thus be explained without having to consider processes taking place in the quasi‐liquid layer present at the surface of ice crystals. A flux of HCHO to the atmosphere was observed simultaneously with an increase of HCHO concentration in snow, indicating photochemical production in surface snow. This study also suggests that the difference in bromine chemistry between Alert (Canadian Arctic) and Barrow leads to different snow composition and post‐deposition evolutions. The highly active bromine chemistry at Barrow probably leads to low HCHO concentrations at the altitude where diamond dust formed. Precipitated diamond dust was subsequently undersaturated with respect to thermodynamic equilibrium, which contrasts to what was observed elsewhere in previous studies.

Evaluating the Snow Crystal Size Distribution and Density Assumptions within a Single-Moment Microphysics Scheme

Abstract The Canadian CloudSat/Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Validation Project (C3VP) was a field campaign designed to obtain aircraft, surface, and radar observations of clouds and precipitation in support of improving the simulation of snowfall and cold season precipitation, their microphysical processes represented within forecast models, and radiative properties relevant to remotely sensed retrievals. During the campaign, a midlatitude cyclone tracked along the U.S.–Canadian border on 22 January 2007, producing an extensive area of snowfall. Observations of ice crystals from this event are used to evaluate the assumptions and physical relationships for the snow category within the Goddard six-class, single-moment microphysics scheme, as implemented within the Weather Research and Forecasting (WRF) model. The WRF model forecast generally reproduced the precipitation and cloud structures sampled by radars and aircraft, permitting a comparison between C3VP observations and model snowfall characteristics. Key snowfall assumptions in the Goddard scheme are an exponential size distribution with fixed intercept and effective bulk density, and the relationship between crystal diameter and terminal velocity. Fixed values for the size distribution intercept and density did not represent the vertical variability of naturally occurring populations of aggregates, and the current diameter and fall speed relationship underestimated terminal velocities for all sizes of crystals.

Three examples where the specific surface area of snow increased over time

Abstract. Snow on the ground impacts climate through its high albedo and affects atmospheric composition through its ability to adsorb chemical compounds. The quantification of these effects requires the knowledge of the specific surface area (SSA) of snow and its rate of change. All relevant studies indicate that snow SSA decreases over time. Here, we report for the first time three cases where the SSA of snow increased over time. These are (1) the transformation of a melt-freeze crust into depth hoar, producing an increase in SSA from 3.4 to 8.8 m2 kg−1. (2) The mobilization of surface snow by wind, which reduced the size of snow crystals by sublimation and fragmented them. This formed a surface snow layer with a SSA of 61 m2 kg−1 from layers whose SSAs were originally 42 and 50 m2 kg−1. (3) The sieving of blowing snow by a snow layer, which allowed the smallest crystals to penetrate into open spaces in the snow, leading to an SSA increase from 32 to 61 m2 kg−1. We discuss that other mechanisms for SSA increase are possible. Overall, SSA increases are probably not rare. They lead to enhanced uptake of chemical compounds and to increases in snow albedo, and their inclusion in relevant chemical and climate models deserves consideration.

Using microwave brightness temperature to detect short-term surface air temperature changes in Antarctica: An analytical approach

Because of the importance of detecting climate change signals over Antarctica, we examined the dependence of the snow's effective temperature times its emissivity (brightness temperature) on surface air temperatures over Antarctica. A range of microwave frequencies and various time scales from daily to yearly is considered. Reasonable accuracy of the air temperature should be obtained without including the annual variation of the emissivity. We studied the sensitivity of the inferred air temperature to penetration depth, a determinant parameter for the effective temperature, and used a snow dielectric constant model derived from strong fluctuation theory. Below 10 GHz, the snow cover temperature and layering determine this depth, whereas above 20 GHz, the determinant factor is the snow crystal size; all parameters have an effect at intermediate frequencies. The penetration depth ranges for the Scanning Multichannel Microwave Radiometer (SMMR) channels are the following: 20–40 m at 6.6 GHz, 6–16 m at 10.7 GHz, about 1–5 m at 18 GHz, and 0.1–1.4 m at 37 GHz. Analysis of the effective temperature revealed that the snow effective temperature can explain short-term fluctuations at 37 GHz and annual trends for all SMMR channels. For short-term fluctuations (<10 days), calculations and measurements showed that the brightness temperature at 37 GHz starts to respond immediately to the air temperature changes, even though the time to reach the maximum is delayed by a day or less; furthermore, the amplitude of the resulting fluctuation was sufficiently large to be detectable in most cases. Indeed, short-term fluctuations of the air temperature can clearly be detected from significant variations of the air temperature over 3 days, but small amplitude fluctuations shorter than 1 day will hardly be detectable through the brightness temperature. This limits use of the brightness temperature at 37 GHz, and this is a point to consider for inversion models. Nevertheless, air temperature fluctuations over 2–3 days can be detected; thus, this technique could be used to observe the winter warming periods in Antarctica that often occur, even during cloudy periods.

Influences of Wind and Snow on Northern Tree-Line Environments at Churchill, Manitoba, Canada

The influences of wind and snow distribution at the tree line near Churchill, Manitoba, were studied. Wind profiles above the snow surface, snow crystal size and quantity were examined during abrasion experiments with white spruce branchlets. For white spruce, the abrasion zone was evidently within 80 cm above the snow surface, and needle removal by abrasion was strongly influenced by branch age. Removal by abrasion of similar-aged needles was highest in new branches and declined with branch age up to 7 or 8 years, and then increased again with older branches. Trees that grew 80 cm in height in 7 or 8 years were successful in passing through the abrasion zone. In forest-tundra environments the matted forms of trees were snow covered early in winter and received little abrasion. Sparsely needled islands of trees were largely covered with snow at the base. More densely needled trees and some trees within woodlands were less exposed to abrasion. The blockage of air flow resulted in high-speed jets, which cleared out snow within a "horseshoe-shaped vortex" around the base of the tree. Both in much of the woodlands and in the open forest, snow abrasion was minimal or non-existent and snow load on the branches (qali), becomes the prevalent stress. During winter, a large proportion of the snowfall was initially blown off the exposed surfaces of Hudson Bay and the coastal tundra regions, into the woodlands, and eventually across the tree line and into the open forest. As the woodlands filled up with snow in mid-winter, the rate of snow accumulation in the forest increased from double to triple the snowfall. Variations in the rate of accumulation occurred with wind speed and direction. ...Key words: tree line, wind, snow, white spruce, abrasion

Spatial and Temporal Variation of Sea Ice Geophysical Properties and Microwave Remote Sensing Observations: The SIMS'90 Experiment

In this paper we present results from a sea ice field experiment conducted coincidentally with overflights of orbital and aerial remote sensing instrumentation in Resolute Passage and Barrow Strait, Northwest Territories, Canada. Our principal focus is to describe the spatial and temporal distribution of selected geophysical variables in the context of how microwave energy interacts with this seasonally varying snow-covered sea ice surface. Over the duration of the experiment, snow crystal size, structure, and snow volume salinities changed sufficiently to affect synthetic aperture radar (SAR) scattering; thermal profiles through the snow cover were diurnally driven; ice surface microscale roughness increased due to sublimation of water vapour from the snow pack onto the ice surface; and bulk ice surface; and bulk ice salinities did not change. Results from the SAR data analysis indicate that the geophysical structure of multiyear ice created a larger and more rapid change in the seasonal SAR scattering signature than did the structure for early consolidated smooth first-year ice. These results are considered fundamental to measurement and monitoring of the seasonal evolution of the snow-covered arctic sea ice surface using SAR remote sensing.Key words: snow, sea ice, synthetic aperture radar, seasonal evolution, remote sensing

Propagation of polarized millimeter waves through falling snow

Propagation of coherent linearly polarized waves through falling snow is calculated for two monodisperse and one polydisperse model snowstorms for fixed orientation and for random orientation of the snow crystals, at a 10-mm wavelength, utilizing a theoretical model based on the Foldy-Lax model. Results for linearly polarized waves incident on oriented monodispersions and polydispersions exhibit a marked damped oscillatory behavior as a function of propagation distance for the copolarized and cross-polarized intensities. For the polydispersion, a simple approximation for the dependence of the forward scattering matrix elements on snow crystal size is also obtained.

Visible And Infrared Transmission Through Snow

Visible and infrared extinction in falling snow is a function of snow particle concentration and snow crystal size and shape. Relationships between existing meteorological conditions and these characteristics have been investigated using measurements made during the SNOW-ONE (Scenario Normalization for Operation in Winter-Obscuration and the Natural Environment) Field Experiment. Effects of meteorological conditions upon extinction are discussed, and empirical models are suggested. Causes of the variation of extinction with wavelength in the visible and infrared are identified, and the magnitudes of some of the possible effects are estimated.

Snow Crystal Observations at Mt. Yalung Kang, Kangchenjunga Region, East Nepal

The shape and size of snow crystals were recorded by making plastic replicas of them during the snowfall on April 23 and May 17, 1973, at Camp II (6470 m) along the climbing route to the summit of Mt. Yalung Kang (8505 m). It was found by observation of the photomicrographs of replicas that the shapes of snow crystals at 1705 NST, April 23, were hexagonal plates, crystals with broad branches, ordinary dendritic crystals and others, and that the time variation of crystals shapes during the period from 1000 NST to 1110 NST, May 17, was from plates to ordinary dendritic crystals through crystals with broad branches. In both cases, the altitudes of the tops of the precipitating clouds and the thickness of the cloud layer suitable for formation of snow crystals were estimated on the basis of experiments on artificial snow crystals.

Snow Crystals observed at Lhajung Station in Khumbu Region

The shape and size of snow crystals were recorded by making plastic replicas of them during the snowfall on 1 and 2 January, 1975, at Lhajung (4420 m) in Khumbu region. It was found by observation of the photomicrographs of the replicas that the shapes of snow crystals during the period from 1900 to 2100 NST (Nepal Standard Time), 1 January were hexagonal plates, thick plates of skelton form, plates with simple extension, plates with dendritic entension, crystals with sector like branches, crystals with broad branches, dendritic crystals with plates at ends, hollow columns, columns with plates, side planes and graupel. The meteorological conditions in the precipitating clouds were estimated on the basis of the observations of snow crystals. During the period form 0750 to 0935 NST, 2 January, graupel, graupel-like snow of hexagonal type and rimed crystals were observed.

降雪の結晶形,大きさ,質量の同時観測の方法について

The shadow photograph method was applied to obtain a record of the form, size and mass of snow crystals. The apparatue for this method is very simple and is shown in Fig. 1. Illuminating the glass plate G, on which many snow crystals were received, from above with a small electric torch bulb L for a few seconds, the shadow of crystals is taken directly as a shadow photograph on the photographic paper in the cassette C. Such a shadow photograph is a record of the form and size of snow crystals on the glass. By heating the glass plate mildly, the individual snow crystals are melted into hemispherical droplets. A shadow photograph of these droplets is taken in the same way as above, and gives a record of the mass of the original crystals.By this method, about fifty pairs of records for various kinds of snow crystal shapes were obtained on Mt. Taisetsu in Hokkaidô, in February 1953. The relation between the mass and the dimension of snow crystals was sought from these records. Excepting the case of plane dendritic crystals, the results coincide fairly well with the results of the empirical formulas obtained by Nakaya and Terada. And on this occasion, the empirical formulas for stellar and columnar crystals are newly obtained.