High-resolution Radar Measurements Research Articles

Microscale Updrafts within Northeast U.S. Coastal Snowstorms Using High-Resolution Cloud Radar Measurements

Abstract Limited knowledge exists about ∼100-m-scale precipitation processes within U.S. northeast coastal snowstorms because of a lack of high-resolution observations. We investigate characteristics of microscale updraft regions within the cyclone comma head and their relationships with snowbands, wind shear, frontogenesis, and vertical mass flux using high-spatiotemporal-resolution vertically pointing Ka-band radar measurements, soundings, and reanalysis data for four snowstorms observed at Stony Brook, New York. Updraft regions are defined as contiguous time–height plotted areas with upward Doppler velocity without hydrometeor sedimentation that is equal to or greater than 0.4 m s−1. Most updraft regions in the time–height data occur on a time scale of seconds (<20 s), which is equivalent to spatial scales < 500 m. These small updraft regions within cloud echo occur more than 30% of the time for three of the four cases and 18% for the other case. They are found at all altitudes and can occur with or without frontogenesis and with or without snowbands. The updraft regions with relatively large Doppler spectrum width (>0.4 m s−1) occur more frequently within midlevels of the storms, where there are strong wind shear layers and moist shear instability layers. This suggests that the dominant forcing for the updrafts appears to be turbulence associated with the vertical shear instability. The updraft regions can be responsible for upward mass flux when they are closer together in space and time. The higher values of column mean upward mass flux often occur during snowband periods. Significance Statement Small-scale (<500 m) upward motions within four snowstorms along the U.S. northeast coast are analyzed for the first time using high-spatiotemporal-resolution millimeter-wavelength cloud radar pointed vertically. The analysis reveals that updrafts appear in the storms regardless of whether snowbands are present or whether there is larger-scale forcing for ascent. The more turbulent and stronger updrafts frequently occur in midlevels of storms associated with instability from vertical shear and contribute to upward mass flux during snowband periods when they are closer together in space and time.

A Generative Deep Learning Approach to Stochastic Downscaling of Precipitation Forecasts.

Despite continuous improvements, precipitation forecasts are still not as accurate and reliable as those of other meteorological variables. A major contributing factor to this is that several key processes affecting precipitation distribution and intensity occur below the resolved scale of global weather models. Generative adversarial networks (GANs) have been demonstrated by the computer vision community to be successful at super-resolution problems, that is, learning to add fine-scale structure to coarse images. Leinonen etal. (2020, https://doi.org/10.1109/TGRS.2020.3032790) previously applied a GAN to produce ensembles of reconstructed high-resolution atmospheric fields, given coarsened input data. In this paper, we demonstrate this approach can be extended to the more challenging problem of increasing the accuracy and resolution of comparatively low-resolution input from a weather forecasting model, using high-resolution radar measurements as a "ground truth." The neural network must learn to add resolution and structure whilst accounting for non-negligible forecast error. We show that GANs and VAE-GANs can match the statistical properties of state-of-the-art pointwise post-processing methods whilst creating high-resolution, spatially coherent precipitation maps. Our model compares favorably to the best existing downscaling methods in both pixel-wise and pooled CRPS scores, power spectrum information and rank histograms (used to assess calibration). We test our models and show that they perform in a range of scenarios, including heavy rainfall.

Cloud radar perspective on tropical warm clouds associated with summer monsoon rainfall and warm rain onset process

The role of warm clouds in monsoon rainfall and the warm rain onset process are investigated using high-resolution cloud-sensitive radar measurements. Quality controlled data from Ka-band radar consisting of 1.8 million profiles during July–August 2015, have been explored. A method is devised using the Ze profiles, for identifying warm clouds whose cloud tops are confined below 5.3 km. Warm rain microphysical parameters are examined utilizing the complemented measurements from Disdrometer, rain gauge, and CloudSat data. The strong temperature inversion of 2 to 4 K km−1 at 3 km explained the generation of shallow warm clouds. Strong low-level updraft and subsidence above the warm clouds are also evident. Contoured frequency by altitude diagram infers that 75% of Ze (velocity) dominates at −20 dBZe (−1 m s−1). Simultaneous Disdrometer measurements revealed 78% of cases with zero-drop count and zero rain rates. The dominating non-precipitating characteristics of warm clouds are also confirmed from the CloudSat inferred cloud droplet effective radius below 10 μm. During the rainy episodes, peak at 0.91 mm in the drop size distribution suggests prominence of drizzle-sized drops. The precipitation caused by warm clouds accounts for a small fraction (28%) of the total rainfall. The warm rain onset process involving the transformation of smaller cloud droplets into big raindrops is investigated using the radar's three lower-order moments. Before cloud droplet to raindrop transition an intense spike in the spectral width (SW) is observed for a brief period in 98% of the cases. Therefore, SW, which infers turbulence, triggers the collision-coalescence of cloud droplets to form raindrops. This study provides a multi-observation-based perspective on the macro- and micro-physical characteristics of monsoon warm clouds and the onset of warm rain for a better representation in the global climate models.

Cloud radar observations of multi-scale variability of cloud vertical structure associated with Indian summer monsoon over a tropical location

Tropics nurture three different types of convective clouds, i.e., shallow cumulus, cumulus congestus, and deep cumulonimbus. The vertical structure of clouds holds a crucial metric in studying tropical clouds. Ground-based high-resolution cloud radar measurements are the potential candidate in exploring the characteristics of various types of tropical clouds and their evolution. Quality-controlled cloud radar data containing a total of five million vertical profiles of equivalent reflectivity factor (VPR) are used to examine the intra-seasonal variation of cloud vertical structure (VSC) during the Indian summer monsoon (ISM) over Mandhardev (18.04° N, 73.87° E, and ~ 1.3 km AMSL) in the Indian Western Ghats. The cumulus congestus (Cc) in the transition of shallow to deep clouds is investigated for the first time using the hourly VPR data for 60 consecutive ISM days. Mid-level moistening plays a vital role in this non-precipitating shallow to precipitating congestus transformation and increment of the rain accumulation. Low cloud reflectivity distribution can distinguish precipitating and non-precipitating clouds that help to classify the observed monsoon as normal or below normal. More than 150 mm of rain accumulation during ISM is associated with more than 22% of high clouds. This particular aspect indicates that cold rain processes are essential to assess the ISM over the observational site. FFT analysis on the time series of low-, mid-, and high-level cloud regions with the VPR shows prominent intra-seasonal variability of 5–10, 10–20, and 30–60 days periodicities. This study highlights the significance of VSC over tropics pertinent to the monsoon large scale atmospheric condition.

A Delay-Correlating Direct-Sequence Spread-Spectrum (DS/SS) Radar System-on-Chip Operating at 183–205 GHz in 28 nm CMOS

This article presents delay-correlating direct-sequence spread-spectrum (DS/SS) radars at mm-wave frequencies, which transmit a bit sequence waveform instead of frequency chirps or pulses to estimate time of flight between the radar and scatterers within a scene. The simple modulation and demodulation architecture of the proposed DS/SS radar enables wideband and high-resolution radar measurements without the need for broadband A/D and D/A conversion, making the delay-correlating approach attractive for low-power radar applications. The demonstrated radar system operates at 205 GHz and offers a 3.0 cm range resolution while consuming 614 mW of dc power.

Quantifying stratosphere-troposphere transport of ozone using balloon-borne ozonesondes, radar windprofilers and trajectory models

In a series of 10-day campaigns in Ontario and Quebec, Canada, between 2005 and 2007, ozonesondes were launched twice daily in conjunction with continuous high-resolution wind-profiling radar measurements. Windprofilers can measure rapid changes in the height of the tropopause, and in some cases follow stratospheric intrusions. Observed stratospheric intrusions were studied with the aid of a Lagrangian particle dispersion model and the Canadian operational weather forecast system. Definite stratosphere-troposphere transport (STT) events occurred approximately every 2–3 days during the spring and summer campaigns, whereas during autumn and winter, the frequency was reduced to every 4–5 days. Although most events reached the lower troposphere, only three events appear to have significantly contributed to ozone amounts in the surface boundary layer. Detailed calculations find that STT, while highly variable, is responsible for an average, over the seven campaigns, of 3.1% of boundary layer ozone (1.2 ppb), but 13% (5.4 ppb) in the lower troposphere and 34% (22 ppb) in the middle and upper troposphere, where these layers are defined as 0–1 km, 1–3 km, and 3–8 km respectively. Estimates based on counting laminae in ozonesonde profiles, with judicious choices of ozone and relative humidity thresholds, compare moderately well, on average, with these values. The lamina detection algorithm is then applied to a large dataset from four summer ozonesonde campaigns at 18 North American sites between 2006 and 2011. The results show some site-to-site and year-to-year variability, but stratospheric ozone contributions average 4.6% (boundary layer), 15% (lower troposphere) and 26% (middle/upper troposphere). Calculations were also performed based on the TOST global 3D trajectory-mapped ozone data product. Maps of STT in the same three layers of the troposphere suggest that the STT ozone flux is greater over the North American continent than Europe, and much greater in winter and spring than in summer or fall. When averaged over all seasons, magnitudes over North America show similar ratios between levels to the previous calculations, but are overall 3–4 times smaller. This may be because of limitations (trajectory length and vertical resolution) to the current TOST-based calculation.

High-Resolution Ice-Sounding Radar Measurements of Ice Thickness Over East Antarctic Ice Sheet as a Part of Chinese National Antarctic Research Expedition

This paper presents the ice thickness, fine resolution internal reflecting horizons (IRHs), and distinct bottom topography measurements of Chinese Kunlun Station and Grove Mountains, Antarctica, derived from sounding these glaciers with a high-resolution radar. To enable the development of next-generation ice-sheet models, we need information on IRHs, bottom topography, and basal conditions. To this end, we performed measurements with the progressively improved ice-sounding radar system, currently known as the high-resolution ice-sounding radar developed by the Key Laboratory of Electromagnetic Radiation and Sensing Technology of Institute of Electronics, Chinese Academy of Sciences, Beijing, China. We processed the collected data using focused synthetic aperture radar (SAR) algorithm named the modified range migration algorithm using curvelets and the modified nonlinear chirp scaling algorithm to improve radar sensitivity and reduce along-track surface clutter. Representative results from selected transects indicate that we successfully sounded 3-km-thick ice with a fine resolution of 0.75 m. In this paper, we provide a brief description of the radar system, discuss the focused SAR processing algorithms, and provide sample results to demonstrate the successful sounding of the ice sheet in Antarctica.

Mesoscale observational analysis of lifting mechanism of a warm-sector convective system producing the maximal daily precipitation in China mainland during pre-summer rainy season of 2015

A long-lived, quasi-stationary mesoscale convective system (MCS) producing extreme rainfall (maximum of 542 mm) over the eastern coastal area of Guangdong Province on 20 May 2015 is analyzed by using high-resolution surface observations, sounding data, and radar measurements. New convective cells are continuously initiated along a mesoscale boundary at the surface, leading to formation and maintenance of the quasi-linear-shaped MCS from about 2000 BT 19 to 1200 BT 20 May. The boundary is originally formed between a cold dome generated by previous convection and southwesterly flow from the ocean carrying higher equivalent potential temperature (θe) air. The boundary is subsequently maintained and reinforced by the contrast between the MCS-generated cold outflow and the oceanic higher-θe air. The cold outflow is weak (wind speed ≤ 5 m s −1), which is attributable to the characteristic environmental conditions, i.e., high humidity in the lower troposphere and weak horizontal winds in the middle and lower troposphere. The low speed of the cold outflow is comparable to that of the near surface southerly flow from the ocean, resulting in very slow southward movement of the boundary. The boundary features temperature contrasts of 2–3°C and is roughly 500-m deep. Despite its shallowness, the boundary appears to exert a profound influence on continuous convection initiation because of the very low level of free convection and small convection inhibition of the near surface oceanic air, building several parallel rainbands (of about 50-km length) that move slowly eastward along the MCS and produce about 80% of the total rainfall. Another MCS moves into the area from the northwest and merges with the local MCS at about 1200 BT. The cold outflow subsequently strengthens and the boundary moves more rapidly toward the southeast, leading to end of the event in 3 h.

High-resolution X-Band radar measurements of currents, bathymetry and sea state in highly inhomogeneous coastal areas

In this paper, high-resolution wave, current and water depth fields derived by marine X-Band radar are presented for a coastal region of extreme tidal currents in the presence of inhomogeneous bathymetry at the south coast of New Zealand’s North Island. The current and water depth information for the presented location covers an area of approximately 13 km2 with a spatial resolution of 225 m and an update rate of 3 min. The sea state data provides a spatial representation of coastal effects like wave shoaling and refraction forced by bathymetry and current interaction. The near-surface current measurements about 3 km off the coast show expected tidal current pattern with maximum northwest/southeast current of 1.5–2 m/s alongshore. This is in agreement with currents from the RiCOM hydrodynamic model. The spatial resolution of the observed current field exhibits in addition small-scale current features caused by the influence of the local bathymetry. These data demonstrate the insight to be gained in complex, high-energy coastal situations through the use of high-resolution remote sensing techniques.

Seasonal thaw settlement at drained thermokarst lake basins, Arctic Alaska

Abstract. Drained thermokarst lake basins (DTLBs) are ubiquitous landforms on Arctic tundra lowland. Their dynamic states are seldom investigated, despite their importance for landscape stability, hydrology, nutrient fluxes, and carbon cycling. Here we report results based on high-resolution Interferometric Synthetic Aperture Radar (InSAR) measurements using space-borne data for a study area located on the North Slope of Alaska near Prudhoe Bay, where we focus on the seasonal thaw settlement within DTLBs, averaged between 2006 and 2010. The majority (14) of the 18 DTLBs in the study area exhibited seasonal thaw settlement of 3–4 cm. However, four of the DTLBs examined exceeded 4 cm of thaw settlement, with one basin experiencing up to 12 cm. Combining the InSAR observations with the in situ active layer thickness measured using ground penetrating radar and mechanical probing, we calculated thaw strain, an index of thaw settlement strength along a transect across the basin that underwent large thaw settlement. We found thaw strains of 10–35% at the basin center, suggesting the seasonal melting of ground ice as a possible mechanism for the large settlement. These findings emphasize the dynamic nature of permafrost landforms, demonstrate the capability of the InSAR technique to remotely monitor surface deformation of individual DTLBs, and illustrate the combination of ground-based and remote sensing observations to estimate thaw strain. Our study highlights the need for better description of the spatial heterogeneity of landscape-scale processes for regional assessment of surface dynamics on Arctic coastal lowlands.

Adaptive Compressive Sensing and Processing of Delay-Doppler Radar Waveforms

Compressive sensing and processing handles high-resolution delay-Doppler radar measurements using a low sampling rate, simple receiver design, and inexpensive processing when Nyquist rate sensing and processing becomes impractical. The benefits of compressive sensing and processing are, however, offset by an increase in estimation error that is introduced when processing compressed measurements versus measurements sampled at the Nyquist rate. In this work, an adaptive compressive sensing and processing method is proposed for the radar tracking problem. The adaptive scheme naturally incorporates information on target state that is readily available from a particle filter based tracker. The proposed method is shown to improve tracking performance over nonadaptive compressive sensing and processing, while maintaining low-sampling rates, a computationally inexpensive operation, and a simple receiver design.

Multi-scale features of solar terrestrial coupling in the cusp ionosphere

The large scale dynamics of the cusp ionosphere is directly controlled by the solar wind and the orientation of the interplanetary magnetic field. The IMF BY controlled east–west movement of the poleward moving forms (PMAFs), separating from the cusp, is consistent with the magnetic tension pull of newly opened flux, which is a key feature tying the auroral phenomenon of PMAFs to flux transfer events (FTEs) at the magnetopause. The central region of an FTE is a mesoscale flow channel, bounded by a pair of Birkeland current sheets on each flank. Systematic observations by EISCAT Svalbard Radar, meridian scanning photometers and all-sky imagers have provided us further insight into key processes of the cusp ionosphere such as how FTEs/PMAFs control ion upflow events, and how transient magnetopause reconnection processes segment a stream of high density solar EUV plasma into polar cap patches at the polar cap boundary. We have found a close causal relationship between formation of plasma patches and onset of scintillation by a previously unrecognized process. We have discovered a new category of flow channels, reversed flow channel events (RFEs) that may appear related to a signature of Birkeland currents arcs, and may be signature of the MI-coupling/inverted V rather than mapping directly to the magnetopause. This review paper concentrates on a niche of cusp studies made by high-resolution radar and optical measurements.

Estimation of the turbulent heat flux in the lower stratosphere from high resolution radar measurements

Two estimates of the turbulent diffusivity (i.e., the heat flux per unit gradient) in the lower stratosphere are inferred from high‐resolution radar measurements and compared. First, the local heat flux (within the turbulent patches) is evaluated from the dissipation rate of turbulent kinetic energy εk under the basic assumptions of local homogeneity and stationarity of the fluctuations. We then estimated the effective heat flux per unit gradient as the time‐averaged flux for a given altitude during the measurement period (six hours), taking into account the observed turbulence intermittence. The time‐averaged heat flux per unit gradient is found to be ∼2 × 10−2 m2s−1 typically, in good agreement with some of the weakest estimates of diffusivity already published. The observed ratio between the local and the time averaged fluxes can reach about one order of magnitude. This last result stresses the fact that turbulent diffusivity inferred from MST radars measurements cannot generally be directly interpreted as an effective diffusivity, since radar estimates, in most cases, do not take into account the turbulence intermittence.

Estimation of the Turbulent Fraction in the Free Atmosphere from MST Radar Measurements

Abstract Small-scale turbulence in the free atmosphere is known to be intermittent in space and time. The turbulence fraction of the atmosphere is a key parameter in order to evaluate the transport properties of small-scale motions and to interpret clear-air radar measurements as well. Mesosphere–stratosphere–troposphere (MST)/stratosphere–troposphere (ST) radars provide two independent methods for the estimation of energetic parameters of turbulence. First, the Doppler spectral width σ2 is related to the dissipation rate of kinetic energy εk. Second, the radar reflectivity, or C2n, relates to the dissipation rate of available potential energy εp. However, these two measures yield estimates that differ with respect to an important point. The Doppler width measurements, and related εk, are reflectivity-weighted averages. On the other hand, the reflectivity estimate is a volume-averaged quantity. The values of εp depend on both the turbulence intensity and the turbulent fraction within the radar sampling volume. Now, the two dissipation rates εp and εk are related quantities as shown by various measurements within stratified fluids (atmosphere, ocean, lakes, or laboratory). Therefore, by assuming a “canonical” value for the ratio of dissipation rates, an indirect method is proposed to infer the turbulent fraction from simultaneous radar measurements of reflectivity and Doppler broadening within a sampling volume. This method is checked by using very high resolution radar measurements (30 m and 51 s), obtained by the PROUST radar during a field campaign. The method is found to provide an unbiased estimation of the turbulent fraction, within a factor of 2 or less.

Temporal and spatial resolution of rainfall measurements required for urban hydrology

The objective of the paper is to provide recommendations on the temporal and spatial resolution of rainfall measurements required for urban hydrological applications, based on quantitative investigations of the space-time scales of urban catchments and rainfall. First the temporal rainfall–runoff dynamics is studied using lag time values derived from rainfall and discharge time series for a set of urban catchments. Then the temporal and spatial structure of rainfall is analysed with high resolution rain gauge and radar measurements from the HYDROMET Integrated Radar Experiment '98 experiment for three typical intense Mediterranean rain events. In particular the evolution of the structure of the rainfall field at different time steps is investigated using geostatistics. Finally the required space-time resolution of rainfall for Mediterranean regions is estimated as a function of the surface of the catchments. According to the results, hydrological applications for urban catchments of the order of 1000 ha require a temporal resolution of about 5 min and a spatial resolution of about 3 km. For urban catchments of the order of 100 ha, it becomes a resolution of about 3 min and 2 km, that common operational networks or radars cannot provide. These results complement the scarce recommendations on resolution of rainfall for urban hydrology reported in the literature.

Experiments in Rainfall Estimation with a Polarimetric Radar in a Subtropical Environment

A unique dataset consisting of high-resolution polarimetric radar measurements and dense rain gauge and disdrometer observations collected in east-central Florida during the summer of 1998 was examined. Comparison of the radar measurements and radar parameters computed from the disdrometer observations supported previous studies, which indicate that oscillating drops in the free atmosphere have more spherical apparent shapes in the mean than equilibrium shapes. Radar–disdrometer comparisons improved markedly when using an empirical axis ratio relation developed from observational studies and representing more spherical drop shapes. Fixed-form power-law rainfall estimators for radar reflectivity (ZH), specific differential phase (KDP), specific differential phase–differential reflectivity (KDP, ZDR), and radar reflectivity–differential reflectivity (ZH, ZDR) were then determined using the disdrometer observations. Relations were produced for both equilibrium shapes and the empirical axis ratios. Polarimetric rainfall estimators based on more spherical shapes gave significantly improved performance. However, the improvement was largely in bias mitigation. Rainfall estimates with the ZH–ZDR measurement pair had the highest correlation with rain gauge observations, the smallest range in bias factors from storm to storm, and the smallest root-mean-square error.

Energetics of small scale turbulence in the lower stratosphere from high resolution radar measurements

Abstract. Very high resolution radar measurements were performed in the troposphere and lower stratosphere by means of the PROUST radar. The PROUST radar operates in the UHF band (961 MHz) and is located in St. Santin, France (44°39’ N, 2°12’ E). A field campaign involving high resolution balloon measurements and the PROUST radar was conducted during April 1998. Under the classical hypothesis that refractive index inhomogeneities at half radar wavelength lie within the inertial subrange, assumed to be isotropic, kinetic energy and temperature variance dissipation rates were estimated independently in the lower stratosphere. The dissipation rate of temperature variance is proportional to the dissipation rate of available potential energy. We therefore estimate the ratio of dissipation rates of potential to kinetic energy. This ratio is a key parameter of atmospheric turbulence which, in locally homogeneous and stationary conditions, is simply related to the flux Richardson number, Rf .Key words. Meteorology and atmospheric dynamics (turbulence) – Radio science (remote sensing)

High Range Resolution Radar Measurements of the Speed Distribution of Breaking Events in Wind-Generated Ocean Waves: Surface Impulse and Wave Energy Dissipation Rates

Abstract A set of X-band radar measurements, backscattered from the sea surface at near grazing incidence with very high spatial and temporal resolution (30 cm in range and 2000-Hz pulse repetition frequency) in moderate wind conditions, are dominated by moving discrete events (sea spikes). They have radar cross sections of up to about 1 m2 and are found to possess the characteristics of breaking wave fronts. Contributions from Bragg backscattering appear to be at least two orders of magnitude smaller. The number of events detected per unit area per unit time was of the same order as found by Ding and Farmer at almost the same wind speed, but the distribution of event speeds was narrower—the fastest breaking wave events observed had line-of-sight speeds of about 0.6 of the dominant wave speed. The measured histograms of number of events versus event speed c suggested that the smaller events with c < 3 m s−1 were only incompletely counted so that the characteristics of only the faster events (3–6 m s−1) we...

Gamma mixture models for target recognition

This paper considers a mixture model approach to automatic target recognition using high-resolution radar measurements. The mixture model approach is motivated from several perspectives including requirements that the target classifier is robust to uncertainty in amplitude scaling, rotation and translation of the target. Estimation of the model parameters is achieved using the expectation-maximisation (EM) algorithm. Gamma mixtures are introduced and the re-estimation equations derived. The models are applied to the classification of high-resolution radar range profiles of ships and results compared with a previously published self-organising map approach.

High-Resolution VHF Radar Measurements in the Troposphere with a Vertically Pointing Beam

Abstract Measurements using the Sounding System (SOUSY) VHF radar in the Harz Mountains, Germany, were carried out in the troposphere and lower stratosphere in the summer of 1996. To study the structure and the dynamics with high temporal and spatial resolution, the beam was pointing continuously in the vertical direction, and a 75-m range resolution was used. Three case studies are analyzed in detail: one within a frontal passage, one during a developing ridge, and one during a thunderstorm. Fine structures in the echo intensity that could not be resolved with the standard resolution of 300 m are observed during the frontal passage and are related to humidity fluctuations. Integrating the high-resolution velocity data with time makes it possible to estimate vertical displacements that are used to indicate vertical transport. The velocities are analyzed harmonically and interpreted as waves with periods of about 6 h and amplitudes of a few centimeters per second. Stable layers evident in the troposphere a...