Wind Direction Dependence Research Articles

VOC source apportionment: How monitoring characteristics influence positive matrix factorization (PMF) solutions

Positive matrix factorization (PMF) can be used to develop more targeted air quality mitigation strategies by identifying major sources of a pollutant in an area. This technique is dependent, however, on the ability of PMF to resolve factors that accurately represent all sources of that pollutant in an area. We investigated how the accuracy of PMF solutions might be influenced by monitoring data characteristics, such as temporal resolution, monitoring location, and species composition, to better inform the use of PMF in VOC mitigation strategies. We applied PMF to five VOC monitoring programs collected within a four-year period in Colorado and found generally consistent factors, which we identified as oil extraction, processing, and evaporation; natural gas; vehicle exhaust; and liquid gasoline/short-lived oil and gas. The main determinant influencing whether or not a dataset resolved each of these sources was whether the dataset had a comprehensive list of VOC species covering key species of each source. Pollution spikes were not well-modeled in any of the solutions. Hyperlocal and volatile chemical product factors expected to be resolved in the industrialized, urban location were also missing, highlighting three limitations of PMF analysis. Wind direction dependence and diurnal trends aided in source identification, suggesting that high-time resolution data is important for developing actionable PMF results. Based on these findings, we recommend that air monitoring for PMF-informed VOC mitigation efforts include high temporal resolution and a comprehensive array of VOC species.

Wind-Tunnel Experiments on the Interactions among a Pair/Trio of Closely Spaced Vertical-Axis Wind Turbines

To elucidate the wind-direction dependence of the rotor performance in closely spaced vertical-axis wind turbines, wind-tunnel experiments were performed at a uniform wind velocity. In the experiments, a pair/trio of three-dimensional printed model turbines with a diameter of D = 50 mm was used. The experiments were performed systematically by applying incremental adjustments to the rotor gap g and rotational direction of each rotor and by changing the wind direction. For tandem layouts, the rotational speed of the downwind rotor is 75–80% that of an isolated rotor, even at g/D = 10. For the average rotational speed of the rotor pair, an origin-symmetrical and a line-symmetrical distribution are observed in the co-rotating and inverse-rotating configurations, respectively, thereby demonstrating the wind-direction dependence for the rotor pair. The inverse-rotating trio configuration yields a higher average rotational speed than the co-rotating trio configuration for any rotor spacing under the ideal bidirectional wind conditions. The maximum average rotational speed should be obtained for a wind direction of θ = 0° in the inverse-rotating trio configuration. The wind-direction dependence of the rotational speeds of the three turbines was explained via flow visualization using a smoke-wire method and velocity field study using two-dimensional computational fluid dynamics.

First Quasi-Synchronous Hurricane Quad-Polarization Observations by C-Band Radar Constellation Mission and RADARSAT-2

This is the first presentation of quasi-synchronous spaceborne synthetic aperture radar (SAR) high-resolution images acquired from C-band Radar Constellation Mission (RCM) and RADARSAT-2 consisting of quad-polarization (HH + HV + VH + VV) wide-swath observations of Hurricane Epsilon. These measurements clearly show that the denoised HV- and VH-polarized normalized radar cross sections (NRCSs) have great consistency. NRCS values at HV- and VH-polarizations are more sensitive to wind speeds and less sensitive to incidence angles or wind directions than those at HH and VV for hurricane force winds. For large incidence angles and high wind speeds, the sensitivity of HH-polarized NRCS to wind speed is higher than that of VV. HH- and VV-polarized NRCSs gradually lose wind direction dependency at high winds. It is notable that the time interval between the two SAR acquisitions is only 3 min. This allows for a direct comparison of HV- and VH-polarized images to investigate the variations in high-resolution backscattering within the hurricane vortex, thereby revealing the most dynamical areas. An asymmetric dynamic is observed around the eye of Hurricane Epsilon, based on positive and negative differences (VH–HV) in the western and eastern parts of the eye. The impacts of rain on quad-polarized NRCS are also examined using collocated rain rates from the Global Precipitation Mission (GPM) and wind speeds from the Soil Moisture Active Passive (SMAP). Significant rain-induced NRCS attenuations are about 1.7 dB for HH and VV, and 2.2 dB for HV and VH, when the rain rate is 20 mm/h. These attenuations are associated with rain-induced turbulence and atmospheric absorption. This work shows that the collocated RCM and RADARSAT-2 hurricane observations provide a unique analysis of synoptic and joint C-band measurements of the ocean surface in quad-polarization; this is noteworthy in view of preparations for the next generation of dual-polarization scatterometer (SCA) onboard second-generation meteorological operational satellite program (MetOp-SG).

Study on direction dependence of the fully polarimetric wind-induced ocean emissivity at L-band using a semi-theoretical approach for Aquarius and SMAP observations

The wind direction dependence of the fully polarimetric ocean emissivity at L-band is studied using a semi-theoretical approach. The model, which is tuned and validated against Aquarius and Soil Moisture Active-Passive (SMAP) mission observations, incorporates the two-scale polarimetric ocean emission model, an improved directional wave spectrum, the Anguelova and Gaiser, 2013 (AG13) foam emissivity model and a novel foam coverage model. Wind speeds up to 35 m/s are considered. Comparisons of model simulations to satellite data show that the overall accuracy of model could reach 0.2 K, which is slightly worse than the Aquarius geophysical mode function (GMF) error of about 0.14 K. The negative and positive upwind-crosswind asymmetries of L-band ocean emission and the transition of the phase signatures versus wind speed are well reproduced for all polarizations by the proposed model. The average standard deviations of the differences of polarimetric brightness temperature (TB) between observations and model are less than about 0.16 K for wind speeds up to 15 m/s. Simulations also indicate the anisotropy of L-band ocean emissivities for V- and H- pol would be saturated at high wind speeds. The directional signals at the third Stokes component begin to converge at around the wind speed of 24 m/s. The L-band TB variations due to wind direction for the third Stokes parameter show less sensitivity to wind speed at larger observation angles, while the direction-induced TB variations for the fourth Stokes are more sensitive to wind speed with the increase of observation angle.

External controls on the transition between stable boundary‐layer turbulence regimes

AbstractThe mean wind speed at which the stable boundary layer (SBL) experiences a turbulence regime transition (Vr) depends on other flow characteristics, such as its thermal stability. Here, Vr variability is examined both at a single site and across sites using three multiple‐level datasets: Santa Maria, Cooperative Atmosphere–Surface Exchange Study–1999 (CASES‐99), and Fluxes over Snow Surfaces (FLOSS II). A method to determine Vr is introduced and validated. It is taken as the mean wind speed at which the vertical gradient of the turbulence velocity scale switches sign. Emphasis is given to the control exerted on Vr by quantities that are external to the SBL, such as radiation, roughness, and soil properties. In each of the experiments, Vr increases with net radiative loss at the surface (Rn) at a rate that is site‐dependent. It also increases for smaller roughness lengths, as indicated by its wind direction dependence. No conclusive relationship has been found between Vr and downward longwave radiation observed at the surface. The across‐site comparison indicates that soil heat capacity influences the rate at which Vr increases with Rn.

On the Response of Polarimetric GNSS-Reflectometry to Sea Surface Roughness

Reflectometry of Global Navigation Satellite Systems (GNSS) signals from the ocean surface has provided a new source of observations to study the ocean-atmosphere interaction. We investigate the sensitivity and performance of GNSS-Reflectometry (GNSS-R) data to retrieve sea surface roughness (SSR) as an indicator of sea state. A data set of one-year observations in 2016 is acquired from a coastal GNSS-R experiment in Onsala, Sweden. The experiment exploits two sea-looking antennas with right- and left-hand circular polarizations (RHCP and LHCP). The interference of the direct and reflected signals captured by the antennas is used by a GNSS-R receiver to generate complex interferometric fringes. We process the interferometric observations to estimate the contributions of direct signals and reflections to the total power. The power estimates are inverted to the SSR using the state-of-the-art model. The roughness measurements from the RHCP and LHCP links are evaluated against match-up wind measurements obtained from the nearest meteorological station. The results report on successful roughness retrieval with overall correlations of 0.76 for both links. However, the roughness effect in LHCP observations is more pronounced. The influence of surrounding complex coastlines and the wind direction dependence are discussed. The analysis reveals that the winds blowing from land have minimal impact on the roughness due to limited fetch. A clear improvement of roughness estimates with an overall correlation of 0.82 is observed for combined polarimetric observations from the RHCP and LHCP links. The combined observations can also improve the sensitivity of GNSS-R measurements to the change of sea state.

Surface ozone exceedances in Melbourne, Australia are shown to be under NOx control, as demonstrated using formaldehyde:NO2 and glyoxal:formaldehyde ratios

Two and a half years of multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of nitrogen dioxide (NO2), formaldehyde (HCHO) and glyoxal (CHOCHO) are presented alongside in-situ ozone (O3) measurements in Melbourne, Australia. Seasonal and diurnal cycles, vertical profiles and relationships with key meteorological variables are provided. NO2 and CHOCHO were found at highest concentration for low wind speeds implying that their sources were predominantly localised and anthropogenic. HCHO showed an exponential relationship with temperature and a strong wind direction dependence from the northern and eastern sectors, and therefore most likely originated from oxidation of biogenic volatile organic compounds (VOCs) from surrounding forested and rural areas. The glyoxal:formaldehyde ratio (Rgf), reported for the first time in Australia, was consistently high compared to values elsewhere in the world with a mean of 0.105 ± 0.0503 and tended to increase with increasing anthropogenic influence. The HCHO:NO2 ratio (Rfn) was used to characterise tropospheric ozone formation conditions. A strong relationship was found between high temperature, low Rgf, high Rfn and high ozone surface concentrations. Therefore, we propose that both Rgf and Rfn may be useful indicators of tropospheric ozone production regimes and concentrations. The Rfn showed that the vast majority of high ozone production episodes occurred under NOx-limited conditions, suggesting that surface ozone pollution events in Melbourne could be curtailed using NOx emission controls.

Principal component analysis of summertime ground site measurements in the Athabasca oil sands with a focus on analytically unresolved intermediate-volatility organic compounds

Abstract. In this paper, measurements of air pollutants made at a ground site near Fort McKay in the Athabasca oil sands region as part of a multi-platform campaign in the summer of 2013 are presented. The observations included measurements of selected volatile organic compounds (VOCs) by a gas chromatograph–ion trap mass spectrometer (GC-ITMS). This instrument observed a large, analytically unresolved hydrocarbon peak (with a retention index between 1100 and 1700) associated with intermediate-volatility organic compounds (IVOCs). However, the activities or processes that contribute to the release of these IVOCs in the oil sands region remain unclear. Principal component analysis (PCA) with varimax rotation was applied to elucidate major source types impacting the sampling site in the summer of 2013. The analysis included 28 variables, including concentrations of total odd nitrogen (NOy), carbon dioxide (CO2), methane (CH4), ammonia (NH3), carbon monoxide (CO), sulfur dioxide (SO2), total reduced-sulfur compounds (TRSs), speciated monoterpenes (including α- and β-pinene and limonene), particle volume calculated from measured size distributions of particles less than 10 and 1 µm in diameter (PM10−1 and PM1), particle-surface-bound polycyclic aromatic hydrocarbons (pPAHs), and aerosol mass spectrometer composition measurements, including refractory black carbon (rBC) and organic aerosol components. The PCA was complemented by bivariate polar plots showing the joint wind speed and direction dependence of air pollutant concentrations to illustrate the spatial distribution of sources in the area. Using the 95 % cumulative percentage of variance criterion, 10 components were identified and categorized by source type. These included emissions by wet tailing ponds, vegetation, open pit mining operations, upgrader facilities, and surface dust. Three components correlated with IVOCs, with the largest associated with surface mining and likely caused by the unearthing and processing of raw bitumen.

Effect of Planetary Rotation on Oceanic Surface Boundary Layer Turbulence

AbstractA large-eddy simulation (LES) model is configured to investigate the effect of the horizontal (northward) component of Earth’s rotation on upper-ocean turbulence. The focus is on the variability of the effect with latitude/hemisphere in the presence of surface gravity waves and when capped by a stable stratification beneath the surface layer. When is included, the mean flow, turbulence, and vertical mixing depend on the wind direction. The value and effect of are the largest in the tropics and decrease with increasing latitudes. The variability in turbulent flows to wind direction is different at different latitudes and in opposite hemispheres. When limited by stable stratification, the variability in turbulence intensity to wind direction reduces, but the entrainment rate changes with wind direction. In wave-driven Langmuir turbulence, the variability in mean current to wind direction is reduced, but the variability of turbulence to wind direction is evident. When there is wind-following swell, the variability in the mean current to wind direction is further reduced. When there is strong wind-opposing swell so that the total wave forcing is opposite to the wind, the variability in the mean current to wind direction is reduced, but the variability of turbulence to wind direction is enhanced, compared to in Ekman turbulence. The profiles of eddy viscosity, including its shape and its value, show a strong wind direction dependence for both stratified wind-driven and wave-driven Langmuir turbulence. Our study demonstrates that wind direction is an important parameter to upper-ocean mixing, though it is overlooked in existing ocean models.

Characterization of submicron aerosols and CCN over the Yellow Sea measured onboard the Gisang 1 research vessel using the positive matrix factorization analysis method

Aerosol size distributions and cloud condensation nuclei (CCN) number concentrations were measured in spring 2017 over the Yellow Sea on board the research vessel Gisang 1. The average number concentration of particles larger than 10 nm and CCN at 0.65% supersaturation (S) were 7622 ± 4038 cm−3 and 4821 ± 1763 cm−3, respectively. Characteristics of aerosol size distribution data were analyzed using a positive matrix factorization (PMF) method. It was found that only 6 Factors could explain the aerosol size distribution reasonably well. Factors 1 and 2 indicated nucleation mode particles, Factor 3 indicated Aitken mode particles, and Factors 4, 5, and 6 indicated accumulation mode particles. The concentrations of nucleation and Aitken mode particles showed a clear wind direction dependence; high under westerly winds due to the high concentrations of particles and precursor gases in eastern China. Meanwhile, the concentration of larger particles and CCN showed no significant wind direction dependence. Aerosol size distribution was also significantly influenced by meteorology. Small particles were predominant during clear days. In contrast during mist or fog days, the aerosol size distribution shifted to larger sizes. A CCN closure experiment was conducted using results of the PMF analysis. The assumption of internally mixed particles led to overestimation of predicted CCN concentrations but agreement was significantly better when externally mixed particles were considered. The logarithmic curve fit of NCCN(S) = 4825 ∗ log S + 4933 was found to very well explain measured CCN concentrations at a few S over the Yellow Sea, and therefore is recommended as input CCN spectral data for numerical models that explicitly treat CCN activation.

Real world vehicle fleet emission factors: Seasonal and diurnal variations in traffic related air pollutants

Temporal variations of vehicle emissions are affected by various compounding factors in the real world. The focus of this study is to determine the effects of ambient conditions and post-tailpipe changes on traffic emissions measured in the near-road region. Emission factors allowed for the isolation of the traffic signal and accounted for effects of local meteorology and dilution. Five month-long measurement campaigns were conducted at an urban near-road site that exhibited a broad range of ambient conditions with temperatures ranging between −18 and +30 °C. Particle number emission factors were 2.0× higher in the winter relative to the summer, which was attributed to changes in particles post-tailpipe. Conversely, toluene emissions were 2.5× higher in the summer relative to the winter, attributed to changes in fuel composition. Diurnal trends of emission factors showed substantial increases in emissions during the morning rush hour for black carbon (1.9×), particle number (2.4×), and particle-bound polycyclic aromatic hydrocarbons (3.0×), affected by fleet make-up. In contrast, particle number emission factors were highest midday with mean values 3.7× higher than at night. This midday increase was attributed to particle formation or growth from local traffic emissions and showed different wind direction dependence than regional events.

A Study of Wind Direction Effects on Sea Surface Specular Scattering for GNSS-R Applications

A modeling study investigating the influence of wind direction on spaceborne global navigation satellite system reflectometry (GNSS-R) near-specular observations of the sea surface is reported. The study first focuses on a purely specular geometry under plane wave incidence, for which it is shown using the theorem of reciprocity and reflection symmetry that up-down wind variations are identically zero. It is also shown that “single-scattering” approximations of rough surface scattering predict no variations with wind direction of any kind for a purely specular geometry, while higher order approximations (such as the second-order small-slope approximation) can predict up/cross wind differences. Examples of these variations are reported and found to be small. Because the delay doppler maps (DDMs) measured in GNSS-R include some nonspecular contributions even for “specular” portions of the DDM, the second part of the study performs an examination of near-specular DDM variations with wind direction using the widely used geometrical optics approximation of surface scattering for a surface described with the non-Gaussian Cox–Munk slope probability density function. Variations with wind direction of the normalized radar cross section (NRCS) mapped onto the surface are examined, and again, it is shown that these variations are small for surface portions contributing to the near-specular portion of the DDM. In addition, it is shown that the dependencies of the bistatic NRCS on wind direction are such that differing portions of the surface “glistening zone” have differing phase shifts in their dependence on wind direction, causing the wind dependencies of the final near-specular DDM to be negligible. The final results of the study suggest that any wind direction dependence in spaceborne GNSS-R should be sought only in portions of the DDM away from the specular region. These results provide information to guide analyses of the wind direction information available in current GNSS-R missions such as TDS-1 and Cyclone Global Navigation Satellite System.

A-26 住宅の通風設計及び性能評価に関する実大模型実験 その9 : 薄板開口に関する実験結果及び流量係数合成値の風向変動要因に関する考察

A-26 住宅の通風設計及び性能評価に関する実大模型実験 その9 : 薄板開口に関する実験結果及び流量係数合成値の風向変動要因に関する考察

Statistical meandering wake model and its application to yaw-angle optimisation of wind farms

The wake produced by a wind turbine is dynamically meandering and of rather narrow nature. Only when looking at large time averages, the wake appears to be static and rather broad, and is then well described by simple engineering models like the Jensen wake model (JWM). We generalise the latter deterministic models to a statistical meandering wake model (SMWM), where a random directional deflection is assigned to a narrow wake in such a way that on average it resembles a broad Jensen wake. In a second step, the model is further generalised to wind-farm level, where the deflections of the multiple wakes are treated as independently and identically distributed random variables. When carefully calibrated to the Nysted wind farm, the ensemble average of the statistical model produces the same wind-direction dependence of the power efficiency as obtained from the standard Jensen model. Upon using the JWM to perform a yaw-angle optimisation of wind-farm power output, we find an optimisation gain of 6.7% for the Nysted wind farm when compared to zero yaw angles and averaged over all wind directions. When applying the obtained JWM-based optimised yaw angles to the SMWM, the ensemble-averaged gain is calculated to be 7.5%. This outcome indicates the possible operational robustness of an optimised yaw control for real-life wind farms.

Ocean Surface Wind Retrieval Using SMAP L-Band SAR

The soil moisture active passive (SMAP) L-band synthetic aperture radar (SAR) could continuously provide global km scale ocean surface wind observations, which had a better coverage than other SARs and a higher spatial resolution than scatterometers. This paper investigates SMAP normalized radar cross sections (NRCS) dependence on wind vectors using more than 5 million matchups consisting of Defense Meteorological Satellite Program F17 Special Sensor Microwave Image/Sounder wind speed, National Center for Environmental Predication wind direction and SMAP L-band NRCS. An L-band geophysical model function (GMF) is proposed for SMAP wind retrieval on the basis of these matchups, and it indicates wind speed and direction dependence of SMAP L-band NRCS for about 40° incidence angle and 0–25 m/s wind speed range in both HH and VV polarization. The wind speed dependence increases rapidly with wind speed, and HH-polarized one is greater than VV polarization. The upwind–downwind difference for HH polarization is greater than that for VV polarization. A negative upwind–crosswind asymmetry occurs for HH- and VV-polarized backscatter at lower wind speeds. The retrieved SMAP wind speed using the proposed GMF is validated by using National Data Buoy Center buoy winds. The root mean square differences and biases are 1.77 and 0.19 m/s, respectively. The accuracies of SMAP wind speeds at 0–10 m/s range are better than those at higher wind speed range. In addition, SMAP wind speeds in upwind and downwind directions are relatively more accurate than those in crosswind directions.

Airborne Co-polarization and Cross-Polarization Observations of the Ocean-Surface NRCS at C-Band

Airborne co-polarization and cross-polarization observations of ocean surface normalized radar cross section (NRCS) were conducted over the North Atlantic during January and February 2015. Observations were made using the University of Massachusetts' Imaging Wind and Rain Airborne Profiler (IWRAP) radar system and a prototype antenna for the next-generation European scatterometer aboard MetOp-SG. Both were installed on a National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft to characterize the wind response of the ocean-surface cross-polarization NRCS. During the flights, numerous constant-roll-angle circle maneuvers were performed at several different angles to collect NRCS measurements over a range of incidence angles. Surface winds at speeds between 8 and 34 ms -1 were observed at incidence angles from 20° to 60° at all polarization combinations. The majority of measurements fell between 8 and 20 ms -1 . Wind-direction dependence similar to copolarized NRCS was observed in the cross-polarized (VH) NRCS. The amplitude of the VH NRCS with respect to direction is less than that of copolarized NRCS at all wind speeds. Incidence angle dependence was also observed in the VH NRCS at all wind speeds. As a function of wind speed, the mean VH NRCS (A 0 ) has a similar shape to the VV NRCS. The VH NRCS appears to not saturate at most incidence angles, unlike the VV and HH NRCS. VH and HH geophysical model functions (GMFs) were developed as functions of wind speed, incidence angle, and wind-relative azimuth for the wind speeds and incidence angles observed.

Ocean Wind Study Using Simulated RCM Compact-Polarimetry SAR

. The incidence angle, wind speed, and wind direction dependence of compact polarimetry (CP) synthetic aperture radar (SAR) parameters are presented for open water. Selected CP parameters are related to C-band geophysical model function (CMOD) output. Wind speeds range between 0 and 24 m/s, and incidence angles range between 18 and 50°. SAR data are coincident with meteorological buoy data. CP parameters are simulated from polarimetric RADARSAT-2 data and emulate data available on the pending RADARSAT Constellation Mission (RCM). Analysis is performed for 20 CP parameters, based on RADARSAT-2 images. Five linear-polarization SAR parameters are included for comparison. Results show robust relationships for CP parameters related to CMOD. For CP parameters unrelated to CMOD, results provide first-, second-, or third-order linear regression models, as functions of incidence angle, wind speed, wind direction, or combinations thereof. These models provide a set of values for comparison with sea ice or other features on, or adjacent to, open water. The results of this study are of value to the Canadian Ice Service of the Meteorological Service of Canada, as well as other government of Canada departments, for implementing CP SAR data in their operational workflows once RCM is launched.Résumé. La dépendance à l’angle d’incidence, à la vitesse et à la direction du vent des paramètres mesurés en polarimétrie compacte (CP) est présentée pour l’eau libre. Les paramètres de CP sélectionnés sont liés aux sorties de la fonction du modèle géophysique en bande C (CMOD). Les vitesses du vent varient entre 0 et 24 m/s, et les angles d’incidence varient entre 18 et 50 degrés. Les données SAR coïncident avec les données des bouées météorologiques. Les paramètres en CP sont simulés à partir des données polarimétriques de RADARSAT-2, et ils simulent les données qui seront disponibles à partir de la mission de la Constellation RADARSAT prévue (MCR). L’analyse est effectuée pour 20 paramètres en CP, sur la base des images RADARSAT-2. Cinq paramètres SAR en polarisation linéaire sont inclus pour comparaison. Les résultats montrent des relations robustes pour les paramètres en CP liés à CMOD. Pour les paramètres en CP non liés à la CMOD, les résultats fournissent des modèles de régression linéaire de première, deuxième ou troisième ordre, en fonction de l’angle d’incidence, de la vitesse et de la direction du vent, ou des combinaisons de ceux-ci. Ces modèles fournissent un ensemble de valeurs de comparaison avec la glace de mer; ou d’autres éléments sur, ou à proximité de, l’eau libre. Les résultats de cette étude seront importants pour le Service canadien des glaces du Service météorologique du Canada, ainsi que pour d’autres ministères du gouvernement du Canada, pour mettre en œuvre les données SAR en CP dans leur flux de travaux opérationnels, une fois la MCR lancée.

The annual cycle of snowfall at Summit, Greenland

AbstractWhile snow accumulation over central Greenland has been extensively studied, interannual variability of snowfall in the region is not well understood due to a dearth of observations. The Integrated Characterization of Energy, Clouds, Atmospheric state and Precipitation at Summit (ICECAPS) project at Summit, Greenland, offers a unique, ground‐based opportunity to study precipitation in central Greenland where the surface mass balance is positive. Combining data from a Precipitation Occurrence Sensor System (POSS), Millimeter‐wavelength Cloud Radar (MMCR), and snow stake field, the annual cycle of precipitation at Summit is examined. Average daily snowfall is higher by a factor of 3 from June to October compared to November to May, while surface height change is only higher by 15% during the same timeframes. This reduced variability in surface height is explained by the seasonally varying nature of latent heat flux, compaction, and wind contributions. The ICECAPS remote sensors and stake field measurements do not agree as far as total annual water equivalent. This discrepancy is likely due to a low bias in the POSS and MMCR snowfall retrievals for Summit. To further examine the seasonal cycle, snowfall measurements by the POSS were linked to local meteorological parameters, including wind direction, liquid water path (LWP), 2 m temperature, and precipitable water vapor. An observed wind direction and moisture dependence are consistent with snowfall being linked to pulses of moist air that originate over nearby, ice‐free ocean, a resource that becomes more readily available in summertime as the winter sea ice retreats. LWP is shown to have little relationship to snowfall, indicating that ice‐phase precipitation processes are quite important for snowfall at Summit.

Tropical Cyclone Winds Retrieved From C-Band Cross-Polarized Synthetic Aperture Radar

This paper presents a geophysical model function (GMF) that has been developed to describe the relation of the ocean surface wind with the normalized radar cross section (NRCS) at C-band cross polarization (cross-pol). Synthetic aperture radar (SAR) images have been simultaneously collected at copolarization (co-pol) and cross-pol at moderate to high wind speeds. Using the SAR co-pol retrieved wind fields and an uncertainty estimate of the retrieved wind speeds, the cross-pol dependencies of the NRCS are investigated with respect to wind, incidence angle, and polarization pairs. For wind speeds above 10 m/s, there is a significant dependence of the NRCS on wind speed. However, the SAR cross-pol data are also significantly affected by the noise floor and crosstalk between the channels. Estimates of the noise floor are determined and removed from the NRCS. Three GMFs are developed: the first is for transmission at horizontal (H) polarization and the second at vertical (V) polarization. A third GMF accounts for wind direction dependence. Validation of the GMFs is conducted by comparison with collocated Stepped Frequency Microwave Radiometer (SFMR) data. The resulting bias of -0.7 m/s and standard deviation of 3.7 m/s demonstrate the excellent performance for these GMFs for wind speed retrieval between 10 and 35 m/s. Furthermore, comparisons show that SAR cross-pol retrieved wind speeds are of similar quality as those of SFMR and are significantly better in the moderate to high wind speed regime than SAR co-pol retrieved winds.

Concentrations of selected volatile organic compounds at kerbside and background sites in central London

Ground-level concentrations of nine volatile organic compounds (VOCs) were measured using a proton transfer reaction-mass spectrometer (PTR-MS) in central London at an urban background (North Kensington, NK, during 16th–25th Jan 2012) and a kerbside site (Marylebone Rd, MRd, during 25th Jan–7th Feb 2012) as part of the winter intensive observation period of the ClearfLo project. Site comparisons indicated that VOC concentrations at the urban background site were significantly lower than at the kerbside site (ratio MRd/NK of 2.3). At the kerbside site PTR-MS measurements of aromatics (benzene, toluene, C2- and C3-benzenes) were compared with the gas chromatography – flame ionization detector data from the UK Government's Automatic Hydrocarbon Network. Very good qualitative agreement was observed between the two methods (r = 0.90–0.91, p < 0.001, N = 260), although there was a significant offset between the instruments. This was partly due to issues with humidity dependent background measurements, but possibly also from isobaric interference of other compounds and their fragments, giving a positive bias to the PTR-MS data. Most compounds showed strong indications of traffic-related sources with double rush hour peaks in diurnal profiles and high correlations with known traffic-related compounds (r = 0.68–0.97 at NK, 0.48–0.87 at MRd, p < 0.001, NNK = 2202–2227, NMRd = 2705–2720) and CO (r = 0.80–0.96 at NK, 0.65–0.84 at MRd, p < 0.001, NNK = 223, NMRd = 256–274). Polar plots agreed with statistical analysis of wind direction dependency and identification of potential emission sources was attempted.