High-latitude Measurements Research Articles

Exploring Snowfall Variability through the High-Latitude Measurement of Snowfall (HiLaMS) Field Campaign

Abstract The High-Latitude Measurement of Snowfall (HiLaMS) campaign explored variability in snowfall properties and processes at meteorologically distinct field sites located in Haukeliseter, Norway, and Kiruna, Sweden, during the winters of 2016/17 and 2017/18, respectively. Campaign activities were founded upon the sensitivities of a low-cost, core instrumentation suite consisting of Micro Rain Radar, Precipitation Imaging Package, and Multi-Angle Snow Camera. These instruments are highly portable to remote field sites and, considered together, provide a unique and complementary set of snowfall observations including snowflake habit, particle size distributions, fall speeds, surface snowfall accumulations, and vertical profiles of radar moments and snow water content. These snow-specific parameters, used in combination with existing observations from the field sites such as snow gauge accumulations and ambient weather conditions, allow for advanced studies of snowfall processes. HiLaMS observations were used to 1) successfully develop a combined radar and in situ microphysical property retrieval scheme to estimate both surface snowfall accumulation and the vertical profile of snow water content, 2) identify the predominant snowfall regimes at Haukeliseter and Kiruna and characterize associated macrophysical and microphysical properties, snowfall production, and meteorological conditions, and 3) identify biases in the HARMONIE-AROME numerical weather prediction model for forecasts of snowfall accumulations and vertical profiles of snow water content for the distinct snowfall regimes observed at the mountainous Haukeliseter site. HiLaMS activities and results suggest value in the deployment of this enhanced snow observing instrumentation suite to new and diverse high-latitude locations that may be underrepresented in climate and weather process studies.

Estimation of Snowfall Properties at a Mountainous Site in Norway Using Combined Radar and In Situ Microphysical Observations

AbstractThe ability of in situ snowflake microphysical observations to constrain estimates of surface snowfall accumulations derived from coincident, ground-based radar observations is explored. As part of the High-Latitude Measurement of Snowfall (HiLaMS) field campaign, a Micro Rain Radar (MRR), Precipitation Imaging Package (PIP), and Multi-Angle Snow Camera (MASC) were deployed to the Haukeliseter Test Site run by the Norwegian Meteorological Institute during winter 2016/17. This measurement site lies near an elevation of 1000 m in the mountains of southern Norway and houses a double-fence automated reference (DFAR) snow gauge and a comprehensive set of meteorological observations. MASC and PIP observations provided estimates of particle size distribution (PSD), fall speed, and habit. These properties were used as input for a snowfall retrieval algorithm using coincident MRR reflectivity measurements. Retrieved surface snowfall accumulations were evaluated against DFAR observations to quantify retrieval performance as a function of meteorological conditions for the Haukeliseter site. These analyses found differences of less than 10% between DFAR- and MRR-retrieved estimates over the field season when using either PIP or MASC observations for low wind “upslope” events. Larger biases of at least 50% were found for high wind “pulsed” events likely because of sampling limitations in the in situ observations used to constrain the retrieval. However, assumptions of MRR Doppler velocity for mean particle fall speed and a temperature-based PSD parameterization reduced this difference to +16% for the pulsed events. Although promising, these results ultimately depend upon selection of a snowflake particle model that is well matched to scene environmental conditions.

Using <i>δ</i><sup>13</sup>C-CH<sub>4</sub> and <i>δ</i>D-CH<sub>4</sub> to constrain Arctic methane emissions

Abstract. We present a global methane modelling study assessing the sensitivity of Arctic atmospheric CH4 mole fractions, δ13C-CH4 and δD-CH4 to uncertainties in Arctic methane sources. Model simulations include methane tracers tagged by source and isotopic composition and are compared with atmospheric data at four northern high-latitude measurement sites. We find the model's ability to capture the magnitude and phase of observed seasonal cycles of CH4 mixing ratios, δ13C-CH4 and δD-CH4 at northern high latitudes is much improved using a later spring kick-off and autumn decline in northern high-latitude wetland emissions than predicted by most process models. Results from our model simulations indicate that recent predictions of large methane emissions from thawing submarine permafrost in the East Siberian Arctic Shelf region could only be reconciled with global-scale atmospheric observations by making large adjustments to high-latitude anthropogenic or wetland emission inventories.

Solar-induced 27-day variations of mesospheric temperature and water vapor from the AIM SOFIE experiment: Drivers of polar mesospheric cloud variability

Polar Mesospheric Clouds (PMCs) are known to be influenced by changes in water vapor and temperature in the cold summertime mesopause. Solar variability of these constituents has been held responsible for 11-year and 27-day variability of PMC activity, although the detailed mechanisms are not yet understood. It is also known that the solar influence on PMC variability is a minor contributor to the overall day-to-day variability, which is dominated by effects of gravity waves, planetary waves, and inter-hemispheric coupling. To address this issue, we have analyzed 15 seasons of data taken from the Solar Occultation for Ice Experiment (SOFIE) on the Aeronomy of Ice in the Mesosphere (AIM) satellite. The SOFIE data contain precise measurements of water vapor, temperature and ice water content (among other quantities). These high-latitude measurements are made during the PMC season at the terminator, and therefore directly relate to the simultaneous measurements of mesospheric ice. Using a composite data set of Lyman-α irradiance, we correlated the time variation of the atmospheric variables with the 27-day variability of solar ultraviolet irradiance. We used a combination of time-lagged linear regression and Superposed Epoch Analysis to extract the solar contribution as sensitivity values (response/forcing) vs. height. We compare these results to previously published results, and show that the temperature sensitivity is somewhat higher, whereas the water sensitivity is nearly the same as published values. The time lags are shorter than that expected from direct solar heating and photodissociation, suggesting that the responses are due to 27-day variations of vertical winds. An analytic solution for temperature changes forced by solar irradiance variations suggests that if the response is due purely to Lyman-α heating and Newtonian cooling, the response should vary throughout the summertime season and depend primarily upon the height-dependent column density of molecular oxygen.

Comparative spectral analysis of high-latitude magnetic and radar measurements

Spectral analysis reveals a certain correspondence between the high-latitude Doppler radar measurements and the frequencies of hidden periodicities found in magnetic observations. With the use of a simple spectral model, this correspondence enables us to estimate indirectly the Doppler frequency shifts inherent in high-latitude magnetic measurements. The relations to high-latitude observations of ultra-low-frequency signals and their possible interpretations are discussed.

An approach to refined mapping of the anomalous gravity field in the Earth’s polar caps

This work describes the different sets of instruments and methodic approaches for testing the models of gravity anomalies by repeated airborne gravimetric surveys in the polar cap regions of the Earth. The survey design including the specifications for flying the survey profiles and the arrangement of the base stations in polar areas is described, and the necessary modifications of the airborne gravity metering complexes for high-latitude measurements are suggested.

Meridional-Flow Measurements from 15 Years of GONG Spherical-Harmonic Time Series

We present results of meridional-flow measurements for 1995–2009, using travel-time differences from velocity images reconstructed using GONG spherical harmonic (SH) coefficients after applying phase-velocity and low-m filters. This filtering technique increases the signal-to-noise ratio and thus extends travel-time measurements to relatively high latitudes and deep into the convection zone. Preliminary analyses shows a strong one-year periodicity presumably due to solar pole misalignment and B0-angle artifacts, which makes it difficult to see underlying temporal variations. Removing a simple one-year-period sine wave fit reveals long-term temporal variations of the flow on top of this yearly periodicity. High-latitude measurements are affected more stronger by foreshortening and B0 -angle artifacts. We analyze different B0-angle intervals separately, so in each hemisphere better high-latitude visibility comes six months apart. This approach suggests why at high latitudes travel-time measurements of meridional flow shows a tendency to change sign instead of continuing towards the poles.

Space Weather and the Global Positioning System

Space Weather and the Global Positioning System

Water Vapor Abundances over Mars North High Latitude Regions: 1996–1999

We present column abundances of water vapor in Mars' atmosphere over high north latitudes for L s 46°–146° during 1996–1997 and L s 34°–180° during 1998–1999. These abundances have been obtained by ground-based high-resolving-power spectroscopy in the very near IR (8174–8200 Å) from the 1.5-m Catalina observatory near Tucson, AZ. All of the measurements have been corrected for atmospheric dust opacity, which increases the measured water vapor abundance over the “apparent measurement” by appreciable amounts at long path lengths. Dust measurements in 1998–1999 were made by near simultaneous measurements, with the same instrument, of the CO 2 absorption lines in the nearby spectral region between 8689 and 8692 Å. We use a doubling and adding radiative transfer code to obtain the dust optical thickness. High northern latitude dust opacities were between 0.8 and 1.0 in this data set for 1998–1999. For the high-latitude measurements made in 1996–1997 during and following observed spring dust storm activity, the dust opacity (τ dust=0.8) was adopted. The computations account for the geometry of the observations (incoming airmass, outgoing airmass, and phase angle). Measurements show a strong increase in water vapor abundance from 8 to 76 ppt. μm with onset of spring and early summer ( L s 34°–118°) followed by an abrupt drop to less than 1 ppt. μm by summer's end.

Use of Fe charge state changes as a tracer for solar wind entry to the magnetosphere

We present a comparison of high charge state ion composition measurements from ACE located near the L1 point and POLAR in the mid‐altitude cusp/cleft, during the 13 May 1998 CME event. Using the frozen in charge state of the solar wind origin Fe ions as a tracer, we demonstrate the connectivity between the ACE observations and the high latitude measurements from POLAR. The ion charge state distributions were observed to show abrupt changes with variations in the average charge state of Fe between +6 and +15 corresponding to changes in the coronal source temperature of 4×105 to 2×106 K. A good correlation between the measurements at L1 and in the cusp/cleft was found, indicating direct entry of solar wind material during periods of southward IMF. In addition there is possible evidence for delayed entry during a period of predominantly northward IMF that may indicate a less direct entry mechanism for this configuration.

Intercomparison of methods for parameterising components of the surface energy balance - a summary of Working Group 1 - COST 710 results

Working Group 1 of COST Action 710 investigated the use of meteorological pre-processors for short-range dispersion models for regulatory purposes. Two commonly used schemes for estimating boundary-layer fluxes from standard meteorological observations were intercompared using high latitude measurements from Sweden for spring/summer and winter conditions. Ground heat fluxes estimated with five different schemes were compared for data measured in Switzerland. The main results of these studies are presented.

Modeling of Jovian hectometric radiation source locations: Ulysses observations

The Unified Radio and Plasma Wave (URAP) experiment on Ulysses has provided unique high latitude measurements of Jovian hectometric radiation (HOM) during its encounter with Jupiter in February 1992. URAP was the first radio instrument in the Jovian environment with radio direction‐finding capability, which was previously used to determine the HOM source locations in the Jovian magnetosphere. These initial source location determinations were based on several assumptions, including the neglect of refractive effects, which may be tested. We have, for the first time, combined the measured incident ray‐direction at the spacecraft with a model magnetosphere to directly trace the rays back to the HOM source. We concentrate on the observations of HOM from high northern latitudes when Ulysses was at distances <15 RJ. The three‐dimensional ray‐tracing calculations presented here indicate that the HOM sources probably lie on L shells in the range 3 ≲ L < 7 (tilted dipole magnetic field model) consistent with previous determinations that ignored the effects of refraction. The ray‐tracing results, however, indicate that wave refraction due to the Io torus and the magnetic field can significantly influence the precise source location. We show that constraints on the locations imposed by the gyroemission mechanism suggest that the Io torus density may have experienced temporal and/or spatial fluctuations during the Ulysses observations of HOM. Finally, in the cold plasma approximation we demonstrate that even if the emission were nearly linearly polarized near the source region, almost circular polarization will be observed at Ulysses, in agreement with observations.

Differential flow between solar wind protons and alpha particles: First WIND observations

Alpha particle and proton measurements in the solar wind made using the SWE Faraday cup detectors on the WIND spacecraft are reported. Some overall trends observed confirm past observations: the ratios of alpha particle to proton density Nα/NP, thermal speed Wα/WP, as well as the differential velocity Vα‐VP (hereafter VαP) are generally correlated with bulk solar wind flow speed. The detailed WIND measurements enable us to investigate instances when the alpha‐proton differences deviate from these overall general trends. Occasionally, difference velocities as large as 80 km/s were seen, with the ratio of |VαP| to the Alfvén speed VA near unity, characteristics more typical of observations at solar distances less than 1 AU. An example is presented where |Vα|‐|VP| reverses sign while |VαP| stays nearly constant. Comparison of the vector velocities and the magnetic field suggests that the speed reversal is associated with a localized kink in the magnetic field. Finally we show an instance where |VαP| exceeds the observed wave speed for Alfvénic fluctuations (Vwave=B0ΔV/ΔB) resulting in alpha particle velocity fluctuations that anti‐correlate with the wave. Though this phenomenon has been previously reported in high‐latitude measurements beyond 1 AU, it is shown here to also occur at 1 AU in the ecliptic.

Interplanetary scintillation observations of the high-latitude solar wind

Until the ULYSSES spacecraft reached the polar regions of the solar wind, the only high-latitude measurements available were from indirect techniques. The most productive observations in regions of the solar wind between 5 R⊙ and 200 R⊙ have been the family of radio scattering techniques loosely referred to as Interplanetary Scintillation (IPS) (Coles, 1978). Useful observations can be obtained using a variety of radio sources, for example spacecraft beacons, planetary radar echoes and compact cosmic sources (quasars, active galactic nuclei, pulsars, galactic masers, etc.). However for measurement of the high-latitude solar wind cosmic sources provide the widest coverage and this review will be confined to such observations. IPS observations played a very important role in establishing that polar coronal holes (first observed in soft x-ray emission) were sources of fast solar wind streams which occasionally extend down to the equatorial region and are observed by spacecraft. Here I will review the IPS technique and show the variation of both the velocity and the turbulence level with latitude over the last solar cycle. I will also outline recent work and discuss comparisons that we hope to make between IPS and ULYSSES observations.

A study of the vertical motion field near the high-latitude summer mesopause during MAC/SINE

We present the results of high-resolution observations of the vertical velocity field obtained with the EISCAT and SOUSY VHF radars near the high-latitude summer mesopause during the MAC/SINE campaign in northern Norway in 1987. The data reveal an energetic motion field with maximum amplitudes of ~ 10 m/s and characteristic periods of ~5–30 min. Motions exhibit a high degree of vertical coherence and a quasi-periodic structure, with typical durations of 5–10 cycles. Estimates of the mean vertical velocity are downward at lower levels and are near zero or positive at greater heights. The mean vertical velocity variance is found to be ~5 m 2/s 2, consistent with other high-latitude measurements. Frequency spectra computed for each radar are found to exhibit considerable variability, while vertical wavenumber spectra are seen to be somewhat variable in amplitude and to have slopes approaching −3 at lower wavenumbers. These results are suggestive of an energetic spectrum of gravity wave motions near the mesopause that has a large vertical flux of wave energy, that may have observed wave frequencies differing significantly from intrinsic frequencies due to Doppler shifting by large horizontal winds, and that is consistent with the separability of the frequency and wavenumber dependence of the motion spectrum and with gravity wave saturation at sufficiently small vertical scales.

The contributions of the incoherent scatter radar technique to the development of thermospheric models

The contributions of the incoherent scatter radar technique to the development of thermospheric models is traced from the development of the radar technique in 1958. Emphasis is given to the qualitative information regarding thermospheric processes and behavior provided by the radar studies and the role that these results have played in the structural development of thermospheric models. The complementary nature of radar and satellite data in the development of recent models is stressed. In addition, a summary of pertinent radar developments over the past two years is presented. Primary among these are measurements made by high-latitude radars, multi-radar coordinated observations, the establishment of an incoherent scatter radar data base, and publication of a number of studies concerning high-latitude measurements, solar-cyle maximum results, long-term data sets, and lower thermosphere tides.

High latitude measurements of the atomic sodium concentration and neutral temperature at the mesopause level by the lidar technique

This paper presents preliminary results of a cooperative franco-sovietic program to study the polar atmosphere by the lidar technique. Atomic sodium concentration has been measured together with the neutral atmospheric temperature by means of the absorption method. The total sodium content is 6.10 9 atom cm −2 during February and March 1976. High values of the temperature (∼210 K) have been measured at the mesopause level.

Electric fields, Joule and particle heating in the high latitude thermosphere

A short review of the recent high latitude measurements of ionospheric electric fields is given. The importance of investigating large-scale and slowly-varying electric fields in order to study magnetospheric convection is stressed. The motion of such high energetic phenomena as auroral forms and spread E-region echoes must be treated by extreme caution when interpreted as a manifestation of convection motion. The relationship between the ionospheric source and polarization field is still an unanswered problem. It is indicated that progress can be made in this respect when electric fields and conductivities are measured simultaneously in the ionosphere. Evidence is shown at one occasion that the meridional component during an auroral substorm might be mainly a polarization field. The height-integrated Joule heating rate is occasionally found to be far larger than the solar radiation input at auroral altitudes. The presence of this additional heat source at any time of day is expected to have a strong impact on the global-scale atmospheric dynamics. From comparisons made between Joule and particle heating it appears that the two components are comparable. It is expected that high latitude incoherent radars will contribute substantially to our understanding of these phenomena in the near future.

Solar energetic particles and the configuration of the magnetosphere

Since the original study of PCA riometer measurements, solar energetic particles have been increasingly employed as test particles in the magnetosphere. The characteristics of the access of these particles to the magnetosphere are studied to determine if substantial connection exists between the earth’s dipole field and the solar‐generated interplanetary field. Satellite measurements over the polar caps indicate strong north pole‐south pole proton flux asymmetries that may be related to existing interplanetary particle anisotropies. High‐latitude measurements indicate significant pseudo‐trapping of the solar fluxes on closed magnetospheric field lines. Particle flux comparisons between satellites in interplanetary space and within the magnetosphere suggest that the particle access point is probably not greater than 1000 RE (earth radii) behind the earth in the magnetotail.