Airborne Radar Observations Research Articles

A Study of Simultaneous Assimilation of Coastal Ground‐Based and Airborne Radar Observations on the Prediction of Harvey (2017) With the Hourly 3DEnVar System for HWRF

AbstractThis study investigates the relative impact of assimilating the ground‐based WSR‐88D radar (GBR) and the airborne Tail Doppler Radar (TDR) observations on the analysis and prediction of Hurricane Harvey (2017) during its landfalling stage. Results show that the assimilation of GBR (experiment “DAG”) outperforms the assimilation of TDR (experiment “DAT”) in multiple aspects. For example, “DAG” produces better predictions in Vmax, radial velocity, brightness temperature, and precipitation than “DAT.” The advantages of “DAG” over “DAT” are likely from the better analyzed thermodynamical structures in addition to the better data availability. The average Minimum Sea Level Pressure (MSLP) prediction error is the only aspect of “DAG” that is inferior to “DAT.” Diagnostics show that such an inferior performance of MSLP for “DAG” is associated with the systematic bias from the Hurricane Weather Research and Forecasting model. The combined assimilation of both observations (experiment “DAB”) shows complementary effects and performs the best overall among all experiments.

A Physically Based, Meshless Lagrangian Approach to Simulate Melting Precipitation

Abstract An outstanding challenge in modeling the radiative properties of stratiform rain systems is an accurate representation of the mixed-phase hydrometeors present in the melting layer. The use of ice spheres coated with meltwater or mixed-dielectric spheroids have been used as rough approximations, but more realistic shapes are needed to improve the accuracy of the models. Recently, realistically structured synthetic snowflakes have been computationally generated, with radiative properties that were shown to be consistent with coincident airborne radar and microwave radiometer observations. However, melting such finely structured ice hydrometeors is a challenging problem, and most of the previous efforts have employed heuristic approaches. In the current work, physical laws governing the melting process are applied to the melting of synthetic snowflakes using a meshless-Lagrangian computational approach henceforth referred to as the Snow Meshless Lagrangian Technique (SnowMeLT). SnowMeLT is capable of scaling across large computing clusters, and a collection of synthetic aggregate snowflakes from NASA’s OpenSSP database with diameters ranging from 2 to 10.5 mm are melted as a demonstration of the method. To properly capture the flow of meltwater, the simulations are carried out at relatively high resolution (15 μm), and a new analytic approximation is developed to simulate heat transfer from the environment without the need to simulate the atmosphere explicitly.

Physical evaluation of hygroscopic cloud seeding in convective clouds using in situ observations and numerical simulations during CAIPEEX

The hygroscopic cloud seeding is conducted over the rain shadow region of the Indian peninsula with an airborne field experiment and radar observations. Physical evaluation of hygroscopic seeding is illustrated with in situ microphysical observations and numerical simulations in convective clouds. Large raindrops were formed near the tops of the cloud, as documented by aircraft observations and model simulations with spectral bin microphysics scheme. Scattering particle number concentration and refractory black carbon in the cloud residue were found up to 4 km in the seeded cloud. The cloud residue measurements of scattering particles and the cloud droplet number concentration showed good agreement in the cloud cores and that has reduced drastically in the cloud edges. Numerical simulations with bin microphysics using observed (background and flare) particle spectra were conducted. Fairly good agreement is noted for the cloud drop size distributions from in situ measurements and numerical simulation. The clouds developed downwind of the seeded area showed further organized convection and longevity and relative enhancement of rainfall as illustrated by the radar observations and the numerical simulations compared to unseeded clouds. The investigation has also brought out the nonlocal impact of the seeded plumes and the new challenges in documenting extra area effects of seeding in the monsoon environment.

Impact of Assimilating Ground-Based and Airborne Radar Observations for the Analysis and Prediction of the Eyewall Replacement Cycle of Hurricane Matthew (2016) Using the HWRF Hybrid 3DEnVar System

Abstract In this study, hourly data assimilation (DA) cycling is performed during a 24-h time period for Hurricane Matthew (2016), assimilating ground-based (GBR) and tail-Doppler radar (TDR) observations together, as well as separately using HWRF and its Hybrid 3DEnVar DA system. The objective is to examine the impacts of assimilating such data on the analysis and prediction of the weakening and re-intensification stages of the eyewall replacement cycle (ERC) of Matthew. Experiments assimilating GBR observations make quicker corrections to the initially inconsistent storm structure than does the TDR experiment, resulting in the primary and secondary eyewalls being realistically represented during the DA cycling period. The TDR experiment analyses show less-realistic concentric eyewall structure before, during, and after TDR observations become available. The forecasts from experiments assimilating GBR observations show more-realistic structural and point intensity changes for the ERC consistently throughout the cycling period when compared with the experiments assimilating TDR observations. Combined assimilation of GBR and TDR observations show similar ERC forecasts, on average, to the GBR experiment. The superior performance of the GBR experiments is shown to be tied to its earlier and longer availability despite its limited low-level coverage especially at the early stage of the cycling. The inferior performance of the TDR experiments even during the availability of TDR is hypothesized to be a result of rapidly changing 3D observational coverage during the high-frequency cycling. Brief mechanism diagnostics additionally suggest the need of properly initializing the TC concentric eyewalls to capture the ERC during the forecasts.

Microphysical Characteristics and Evolution of Seeded Orographic Clouds

AbstractThe spatial distribution and magnitude of snowfall resulting from cloud seeding with silver iodide (AgI) is closely linked to atmospheric conditions, seeding operations, and dynamical, thermodynamical, and microphysical processes. Here, microphysical processes leading to ice and snow production are analyzed in orographic clouds for three cloud-seeding events, each with light or no natural precipitation and well-defined, traceable seeding lines. Airborne and ground-based radar observations are linked to in situ cloud and precipitation measurements to determine the spatiotemporal evolution of ice initiation, particle growth, and snow fallout in seeded clouds. These processes and surface snow amounts are explored as particle plumes evolve from varying amounts of AgI released, and within changing environmental conditions, including changes in liquid water content (LWC) along and downwind of the seeding track, wind speed, and shear. More AgI did not necessarily produce more liquid equivalent snowfall (LESnow). The greatest amount of LESnow, largest area covered by snowfall, and highest peak snowfall produced through seeding occurred on the day with the largest and most widespread occurrence of supercooled drizzle, highest wind shear, and greater LWC along and downwind of the seeding track. The day with the least supercooled drizzle and the lowest LWC downwind of the seeding track produced the smallest amount of LESnow through seeding. The stronger the wind was, the farther away the snowfall occurred from the seeding track.

The regional-scale surface mass balance of Pine Island Glacier, West Antarctica, over the period 2005–2014, derived from airborne radar soundings and neutron probe measurements

Abstract. We derive recent surface mass balance (SMB) estimates from airborne radar observations along the iSTAR traverse (2013, 2014) at Pine Island Glacier (PIG), West Antarctica. Ground-based neutron probe measurements provide information of snow and firn density with depth at 22 locations and were used to date internal annual reflection layers. The 2005 layer was traced for a total distance of 2367 km to determine annual mean SMB for the period 2005–2014. Using complementary SMB estimates from two regional climate models, RACMO2.3p2 and MAR, and a geostatistical kriging scheme, we determine a regional-scale SMB distribution with similar main characteristics to that determined for the period 1985–2009 in previous studies. Local departures exist for the northern PIG slopes, where the orographic precipitation shadow effect appears to be more pronounced in our observations, and the southward interior, where the SMB gradient is more pronounced in previous studies. We derive total mass inputs of 79.9±19.2 and 82.1±19.2 Gt yr−1 to the PIG basin based on complementary ASIRAS–RACMO and ASIRAS–MAR SMB estimates, respectively. These are not significantly different to the value of 78.3±6.8 Gt yr−1 for the period 1985–2009. Thus, there is no evidence of a secular trend at decadal scales in total mass input to the PIG basin. We note, however, that our estimated uncertainty is more than twice the uncertainty for the 1985–2009 estimate on total mass input. Our error analysis indicates that uncertainty estimates on total mass input are highly sensitive to the selected krige methodology and assumptions made on the interpolation error, which we identify as the main cause for the increased uncertainty range compared to the 1985–2009 estimates.

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Abstract. Warm conveyor belts (WCBs) are dynamically important, strongly ascending and mostly stratiform cloud-forming airstreams in extratropical cyclones. Despite the predominantly stratiform character of the WCB's large-scale cloud band, convective clouds can be embedded in it. This embedded convection leads to a heterogeneously structured cloud band with locally enhanced hydrometeor content, intense surface precipitation and substantial amounts of graupel in the middle troposphere. Recent studies showed that embedded convection forms dynamically relevant quasi-horizontal potential vorticity (PV) dipoles in the upper troposphere. Thereby one pole can reach strongly negative PV values associated with inertial or symmetric instability near the upper-level PV waveguide, where it can interact with and modify the upper-level jet. This study analyzes the characteristics of embedded convection in the WCB of cyclone Sanchez based on WCB online trajectories from a convection-permitting simulation and airborne radar observations during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) field campaign (intense observation periods, IOPs, 10 and 11). In the first part, we present the radar reflectivity structure of the WCB and corroborate its heterogeneous cloud structure and the occurrence of embedded convection. Radar observations in three different sub-regions of the WCB cloud band reveal the differing intensity of its embedded convection, which is qualitatively confirmed by the ascent rates of the online WCB trajectories. The detailed ascent behavior of the WCB trajectories reveals that very intense convection with ascent rates of 600 hPa in 30–60 min occurs, in addition to comparatively moderate convection with slower ascent velocities as reported in previous case studies. In the second part of this study, a systematic Lagrangian composite analysis based on online trajectories for two sub-categories of WCB-embedded convection – moderate and intense convection – is performed. Composites of the cloud and precipitation structure confirm the large influence of embedded convection: intense convection produces very intense local surface precipitation with peak values exceeding 6 mm in 15 min and large amounts of graupel of up to 2.8 g kg−1 in the middle troposphere (compared to 3.9 mm and 1.0 g kg−1 for the moderate convective WCB sub-category). In the upper troposphere, both convective WCB trajectory sub-categories form a small-scale and weak PV dipole, with one pole reaching weakly negative PV values. However, for this WCB case study – in contrast to previous case studies reporting convective PV dipoles in the WCB ascent region with the negative PV pole near the upper-level jet – the negative PV pole is located east of the convective ascent region, i.e., away from the upper-level jet. Moreover, the PV dipole formed by the intense convective WCB trajectories is weaker and has a smaller horizontal and vertical extent compared to a previous NAWDEX case study of WCB-embedded convection, despite faster ascent rates in this case. The absence of a strong upper-level jet and the weak vertical shear of the ambient wind in cyclone Sanchez are accountable for the weak diabatic PV modification in the upper troposphere. This implies that the strength of embedded convection alone is not a reliable measure for the effect of embedded convection on upper-level PV modification and its impact on the upper-level jet. Instead, the profile of vertical wind shear and the alignment of embedded convection with a strong upper-level jet play a key role for the formation of coherent negative PV features near the jet. Finally, these results highlight the large case-to-case variability of embedded convection not only in terms of frequency and intensity of embedded convection in WCBs but also in terms of its dynamical implications.

The Soil Moisture Active Passive Experiments: Validation of the SMAP Products in Australia

The fourth and fifth Soil Moisture Active Passive Experiments (SMAPEx-4 and -5) were conducted at the beginning of the SMAP operational phase, May and September 2015, to: 1) evaluate the SMAP microwave observations and derived soil moisture (SM) products and 2) intercompare with the Soil Moisture and Ocean Salinity (SMOS) and Aquarius missions over the Murrumbidgee River Catchment in the southeast of Australia. Airborne radar and radiometer observations at the same microwave frequencies as SMAP were collected over SMAP footprints/grids concurrent with its overpass. In addition, intensive ground sampling of SM, vegetation water content, and surface roughness was carried out, primarily for validation of airborne SM retrieval over six ~ 3 km × 3 km focus areas. In this study, the SMAPEx-4 and -5 data sets were used as independent reference for extensively evaluating the brightness temperature and SM products of SMAP, and intercompared with SMOS and Aquarius under a wide range of SM and vegetation conditions. Importantly, this is the only extensive airborne field campaign that collected data while the SMAP radar was still operational. The SMAP radar, radiometer, and derived SM showed a high agreement with the SMAPEx-4 and -5 data set, with a root-mean-squared error (RMSE) of ~3 K for radiometer brightness temperature, and an RMSE of ~ 0.05 m3 for the radiometer-only SM product. The SMAP radar backscatter had an RMSE of 3.4 dB, while the retrieved SM had an RMSE of 0.11 m3/m3 when compared with the SMAPEx-4 data set.

Firn Clutter Constraints on the Design and Performance of Orbital Radar Ice Sounders

Radar sounding is a powerful tool for constraining subglacial conditions, which influence the mass balance of polar ice sheets and their contributions to global sea-level rise. A satellite-based radar sounder, such as those successfully demonstrated at Mars, would offer unprecedented spatial and temporal coverage of the subsurface. However, airborne sounding studies suggest that poorly constrained radar scattering in polar firn may produce performance-limiting clutter for terrestrial orbital sounders. We develop glaciologically constrained electromagnetic models of radar interactions in firn, test them against in situ data and multifrequency airborne radar observations, and apply the only model we find to be consistent with observation to assess the implications of firn clutter for orbital sounder system design. Our results show that in the very high-frequency (VHF) and ultrahigh-frequency (UHF) bands, radar interactions in the firn are dominated by quasi-specular reflections at the interfaces between layers of different densities and that off-nadir backscatter is likely the result of small-scale roughness in the subsurface density profiles. As a result, high frequency (HF) or low VHF center frequencies offer a significant advantage in near-surface clutter suppression compared to the UHF band. However, the noise power is the dominant constraint in all bands, so the near-surface clutter primarily constrains the extent to which the transmit power, pulselength, or antenna gain can be engineered to improve the signal-to-noise ratio. Our analysis suggests that the deep interior of terrestrial ice sheets is a difficult target for orbital sounding, which may require optimizations in azimuth processing and cross-track clutter suppression which complement existing requirements for sounding at the margins.

Impact of Mass–Size Parameterizations of Frozen Hydrometeors on Microphysical Retrievals: Evaluation by Matching Radar to In Situ Observations from GCPEx and OLYMPEx

AbstractThis article illustrates how multifrequency radar observations can refine the mass–size parameterization of frozen hydrometeors in scattering models and improve the correlation between the radar observations and in situ measurements of microphysical properties of ice and snow. The data presented in this article were collected during the GPM Cold Season Precipitation Experiment (GCPEx) (2012) and Olympic Mountain Experiment (OLYMPEx) (2015) field campaigns, where the true mass–size relationship was not measured. Starting from size and shape distributions of ice particles measured in situ, scattering models are used to simulate an ensemble of reflectivity factors for various assumed mass–size parameterizations (MSP) of the power-law type. This ensemble is then collocated to airborne and ground-based radar observations, and the MSPs are refined by retaining only those that reproduce the radar observations to a prescribed level of accuracy. A versatile “retrieval dashboard” is built to jointly analyze the optimal MSPs and associated retrievals. The analysis shows that the optimality of an MSP depends on the physical assumptions made in the scattering simulators. This work confirms also the existence of a relationship between parameters of the optimal MSPs. Through the MSP optimization, the retrievals of ice water contentMand mean diameterDmseem robust to the change in meteorological regime (between GCPEx and OLYMPEx); whereas the retrieval of the diameter spreadSmseems more campaign dependent.

The Rapid Intensification and Eyewall Replacement Cycles of Hurricane Irma (2017)

Abstract The initiation of a rapid intensification (RI) event for a tropical cyclone (TC) at major hurricane intensity is a rare event in the North Atlantic basin. This study examined the environmental and vortex-scale processes related to such an RI event observed in Hurricane Irma (2017) using a combination of flight-level and airborne radar aircraft reconnaissance observations, microwave satellite observations, and model environmental analyses. The onset of RI was linked to an increase in sea surface temperatures and ocean heat content toward levels more commonly associated with North Atlantic RI episodes. Remarkably, Irma’s RI event comprised two rapidly evolving eyewall replacement cycle (ERC) episodes that each completed in less than 12 h. The two ERC events displayed different secondary eyewall formation (SEF) mechanisms and vortex evolutions. During the first SEF event, a secondary maximum in ascent and tangential wind was observed at the leading edge of a mesoscale descending inflow jet. During the ensuing ERC event, the primary eyewall weakened and ultimately collapsed, resulting in a brief period of weakening. The second SEF event displayed characteristics consistent with unbalanced boundary layer dynamics. Additionally, it is plausible that both SEF events were affected by the stagnation and axisymmeterization of outward-propagating vortex Rossby waves. During the second ERC event, the TC continued to rapidly intensify, which is a stark contrast to the ERC paradigm described in the literature. The differing ERC evolutions appear linked to the vortex response to changing environmental conditions. The results presented here underscore the utility of frequent aircraft reconnaissance observations for an improved understanding of TC dynamics.

Comparison of GPM DPR and Airborne Radar Observations in OLYMPEX

Previous work has shown that the relatively large resolution volume of spaceborne precipitation radars can affect precipitation measurements in several ways. During the Olympic Mountains Experiment (OLYMPEX), Ku- and Ka-band airborne radar measurements of precipitation were obtained during two Global Precipitation Measurement (GPM) overpasses, one case over land and one case over ocean. The authors compare the GPM-measured radar quantities with the same quantities inferred from the higher spatial resolution airborne radar data. Differences include smaller maximum reflectivity and path attenuation and more significant surface clutter in mountainous areas for GPM.

Convective activity in an extratropical cyclone and its warm conveyor belt – a case‐study combining observations and a convection‐permitting model simulation

Warm conveyor belts (WCBs) are important Lagrangian features in extratropical cyclones for the evolution of clouds, precipitation and flow dynamics. According to the classical concept, WCBs rise continuously from the boundary layer to the upper troposphere with ascent rates of less than 50 hPa/hr. Recent studies identified embedded convection in WCBs with ascent rates exceeding 50 hPa/hr, however, its significance and characteristics have not yet been analysed systematically. This study presents a detailed analysis of a frontal wave cyclone that occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) and investigates the occurrence of convection with ascent rates exceeding 100–200 hPa/hr embedded in the WCB. A set of diagnostics, based on the combination of Meteosat products, ECMWF data, and a convection‐permitting simulation, reveals consistently that convection occurs frequently in the warm sector of the investigated cyclone, in particular in the region of the WCB. These convective regions are characterized by increased surface precipitation, low values of convective available potential energy, and significant large‐scale forcing for ascent, indicating that this type of convection embedded in WCBs differs from classical air mass convection with higher vertical velocities. This is qualitatively confirmed by airborne radar observations of the considered cyclone with reflectivities hardly exceeding 30 dBZ. In the investigated WCB, the ascent is not continuous, but characterized by intermittent periods of very strong or even convective ascent and occasionally by short periods of descent. Together, these results provide a refined view on the concept of WCBs and its embedded convection.

Retrieval of snowflake microphysical properties from multifrequency radar observations

Abstract. We have developed an algorithm that retrieves the size, number concentration and density of falling snow from multifrequency radar observations. This work builds on previous studies that have indicated that three-frequency radars can provide information on snow density, potentially improving the accuracy of snow parameter estimates. The algorithm is based on a Bayesian framework, using lookup tables mapping the measurement space to the state space, which allows fast and robust retrieval. In the forward model, we calculate the radar reflectivities using recently published snow scattering databases. We demonstrate the algorithm using multifrequency airborne radar observations from the OLYMPEX–RADEX field campaign, comparing the retrieval results to hydrometeor identification using ground-based polarimetric radar and also to collocated in situ observations made using another aircraft. Using these data, we examine how the availability of multiple frequencies affects the retrieval accuracy, and we test the sensitivity of the algorithm to the prior assumptions. The results suggest that multifrequency radars are substantially better than single-frequency radars at retrieving snow microphysical properties. Meanwhile, triple-frequency radars can retrieve wider ranges of snow density than dual-frequency radars and better locate regions of high-density snow such as graupel, although these benefits are relatively modest compared to the difference in retrieval performance between dual- and single-frequency radars. We also examine the sensitivity of the retrieval results to the fixed a priori assumptions in the algorithm, showing that the multifrequency method can reliably retrieve snowflake size, while the retrieved number concentration and density are affected significantly by the assumptions.

Estimating water volume stored in the south-eastern Greenland firn aquifer using magnetic-resonance soundings

Recent observations of the Greenland ice sheet show an increase of the area affected by progressive melt of snow and ice, thus resulting in production of the additional meltwater. In 2011, an important storage of meltwater in the firn has been observed in the S-E Greenland. This water does not freeze during the wintertime and forms a perennial firn aquifer. The aquifer spatial extent has been initially monitored with combined ground and airborne radar observations, but these geophysical techniques are not able to inform us on the amount of meltwater stored at depth. In this study, we use the magnetic resonance soundings (MRS) method for estimating the volume of water stored in the Greenland ice sheet firn and mapping its spatial variability. Our study area covers a firn aquifer along a 16-km E-W transect, ranging between elevations of 1520 and 1760 m. In July 2015 and July 2016, we performed MRS measurements that allow estimating the water volume in the studied area as well as the one-year water volume evolution. Water storage is not homogeneous, fluctuating between 0.2 and 2 m3/m2, and contains discontinuities in the hydrodynamic properties. We estimate an average volume of water stored in the firn in 2016 to be 0.76 m3/m2, which corresponds to a 0.76-m-thick layer of bulk water. MRS monitoring reveals that from April 2015 to July 2016 the volume of water stored at the location of our transect increases by about 36%. We found MRS-estimated depth to water in a good agreement with that obtained with the ground penetrating radar (GPR).

Subrainband Structure and Dynamic Characteristics in the Principal Rainband of Typhoon Hagupit (2008)

Abstract The principal rainband in tropical cyclones is currently depicted as a solitary and continuous precipitation region. However, the airborne radar observations of the principal rainband in Typhoon Hagupit (2008) reveal multiple subrainband structures. These subbands possess many characteristics of the squall lines with trailing stratiform in the midlatitudes and are different from those documented in previous principal rainband studies. The updraft and reflectivity cores are upright and elevated. The updraft is fed by a low-level radial outflow from the inner side. The tangential wind speed shows a clear midlevel jet on the inner side of the reflectivity core. Except for the structural similarities, the dynamics of the subbands is also similar to the squall lines. The local environment near the subbands shows little convective inhibition, modest instability, and vertical wind shear. The temperature retrieval shows a cold pool structure in the stratiform precipitation region. The estimated vertical wind shear induced by the cold pool is close to that of the local environment. The structural and dynamic similarities to the squall lines imply that the variation of principal rainbands is subjected to convective-scale dynamics related to the local environment in addition to storm-scale dynamics. The subbands show positive impacts to the vortex intensity in terms of potential vorticity redistribution and absolute angular momentum advection. The positive impacts are closely related to specific structural characteristics of the subbands, which suggests the importance of understanding the convective-scale structure and dynamics of the principal rainband.

Retrieval of Vertical Air Motion in Precipitating Clouds Using Mie Scattering and Comparison with In Situ Measurements

AbstractFor the first time, the Mie notch retrieval technique is applied to airborne cloud Doppler radar observations in warm precipitating clouds to retrieve the vertical air velocity profile above the aircraft. The retrieval algorithm prescribed here accounts for two major sources of bias: aircraft motion and horizontal wind. The retrieval methodology is evaluated using the aircraft in situ vertical air velocity measurements. The standard deviations of the residuals for the retrieved and in situ measured data for an 18-s time segment are 0.21 and 0.24 m s−1, respectively; the mean difference between the two is 0.01 m s−1. For the studied cases, the total theoretical uncertainty is less than 0.19 m s−1 and the actual retrieval uncertainty is about 0.1 m s−1. These results demonstrate that the Mie notch technique combined with the bias removal procedure described in this paper can successfully retrieve vertical air velocity from airborne radar observations with low spectral broadening due to Doppler fading, which enables new opportunities in cloud and precipitation research. A separate spectral peak due to returns from the cloud droplets is also observed in the same radar Doppler spectra and is also used to retrieve vertical air motion. The vertical air velocities retrieved using the two different methods agree well with each other, and the correlation coefficient is as high as 0.996, which indicates that the spectral peak due to cloud droplets might provide another way to retrieve vertical air velocity in clouds when the Mie notch is not detected but the cloud droplets’ spectral peak is discernable.

GSI‐based ensemble‐variational hybrid data assimilation for HWRF for hurricane initialization and prediction: impact of various error covariances for airborne radar observation assimilation

The hybrid Ensemble Kalman Filter ‐ Variational (EnKF‐Var) data assimilation (DA) system based on Grid‐point Statistical Interpolation (GSI) is extended for the Hurricane‐WRF model (HWRF). Background ensemble forecasts initialized by the EnKF are used to provide the flow‐dependent error covariance to be ingested by GSI using the extended control variable method. The hybrid system is then applied to assimilate airborne radar data.In this article, the newly developed HWRF hybrid system capable of assimilating airborne radar observations is introduced. The impact of using variously estimated background error covariances on tropical cyclone (TC) core analyses and subsequent forecasts is explored by a detailed study of Hurricane Sandy (2012) and by systematic comparison of various sensitivity experiments for multiple cases during the 2012–2013 seasons. The hybrid system using the HWRF EnKF ensemble covariance (Hybrid‐HENS) is able to correct both the wind and mass fields in a dynamically and thermodynamically coherent fashion. In contrast, the wind and pressure adjustments by GSI three‐dimensional variation (GSI3DVar) using the static covariance are inconsistent. The wind and pressure relation in the covariances derived from the GFS ensemble (Hybrid‐GENS) improves upon the static covariance, but is still inconsistent compared to that of HWRF. Verifications against independent flight‐level and Stepped Frequency Microwave Radiometer (SFMR) wind data, and Hurricane Research Division (HRD) radar wind composite reveal that the Hybrid‐HENS system improves the analysed TC structure upon both GSI3DVar and Hybrid‐GENS. Hybrid‐HENS and Hybrid‐GENS improve the track, minimum sea‐level pressure (MSLP) and Vmax forecast relative to GSI3DVar. Hybrid‐HENS further improves track forecasts compared to Hybrid‐GENS. Hybrid‐HENS provides the largest positive impact of the airborne radar data. In comparison, GSI3DVar shows consistently negative impact of the data when analysing the structure and verifying track forecasts. Blending the static background error covariance in the hybrid system improves the maximum wind forecast while little benefit is found in the analysed structures and the MSLP and track forecasts.

A polarimetric scattering database for non-spherical ice particles at microwave wavelengths

Abstract. The atmospheric science community has entered a period in which electromagnetic scattering properties at microwave frequencies of realistically constructed ice particles are necessary for making progress on a number of fronts. One front includes retrieval of ice-particle properties and signatures from ground-based, airborne, and satellite-based radar and radiometer observations. Another front is evaluation of model microphysics by application of forward operators to their outputs and comparison to observations during case study periods. Yet a third front is data assimilation, where again forward operators are applied to databases of ice-particle scattering properties and the results compared to observations, with their differences leading to corrections of the model state. Over the past decade investigators have developed databases of ice-particle scattering properties at microwave frequencies and made them openly available. Motivated by and complementing these earlier efforts, a database containing polarimetric single-scattering properties of various types of ice particles at millimeter to centimeter wavelengths is presented. While the database presented here contains only single-scattering properties of ice particles in a fixed orientation, ice-particle scattering properties are computed for many different directions of the radiation incident on them. These results are useful for understanding the dependence of ice-particle scattering properties on ice-particle orientation with respect to the incident radiation. For ice particles that are small compared to the wavelength, the number of incident directions of the radiation is sufficient to compute reasonable estimates of their (randomly) orientation-averaged scattering properties. This database is complementary to earlier ones in that it contains complete (polarimetric) scattering property information for each ice particle – 44 plates, 30 columns, 405 branched planar crystals, 660 aggregates, and 640 conical graupel – and direction of incident radiation but is limited to four frequencies (X-, Ku-, Ka-, and W-bands), does not include temperature dependencies of the single-scattering properties, and does not include scattering properties averaged over randomly oriented ice particles. Rules for constructing the morphologies of ice particles from one database to the next often differ; consequently, analyses that incorporate all of the different databases will contain the most variability, while illuminating important differences between them. Publication of this database is in support of future analyses of this nature and comes with the hope that doing so helps contribute to the development of a database standard for ice-particle scattering properties, like the NetCDF (Network Common Data Form) CF (Climate and Forecast) or NetCDF CF/Radial metadata conventions.

Satellite Remote Sensing of Snow Depth on Antarctic Sea Ice: An Inter-Comparison of Two Empirical Approaches

Snow on Antarctic sea ice plays a key role for sea ice physical processes and complicates retrieval of sea ice thickness using altimetry. Current methods of snow depth retrieval are based on satellite microwave radiometry, which perform best for dry, homogeneous snow packs on level sea ice. We introduce an alternative approach based on in-situ measurements of total (sea ice plus snow) freeboard and snow depth, which we use to compute snow depth on sea ice from Ice, Cloud, and land Elevation Satellite (ICESat) total freeboard observations. We compare ICESat snow depth for early winter and spring of the years 2004 through 2006 with the Advanced Scanning Microwave Radiometer aboard EOS (AMSR-E) snow depth product. We find ICESat snow depths agree more closely with ship-based visual and air-borne snow radar observations than AMSR-E snow depths. We obtain average modal and mean ICESat snow depths, which exceed AMSR-E snow depths by 5–10 cm in winter and 10–15 cm in spring. We observe an increase in ICESat snow depth from winter to spring for most Antarctic regions in accordance with ground-based observations, in contrast to AMSR-E snow depths, which we find to stay constant or to decrease. We suggest satellite laser altimetry as an alternative method to derive snow depth on Antarctic sea ice, which is independent of snow physical properties.