Abstract

Dual-antenna radar designs avoid using a transmit/receive switch. In order to measure radar reflectivity accurately and to avoid a general decrease in system sensitivity, these systems require precise alignment of their high-gain/narrow-beamwidth antennas, which is difficult. Given precisely aligned antennas, a parallax correction to account for antenna beam overlap, which is range-dependent, must be used with the correct alignment information to produce accurate reflectivities. Calculations show that dual-antenna parallax errors are extremely sensitive to the alignment of the two antennas, especially for the current generation of W-band radars, which tend to use 0.91- and 1.21-m Cassegrain antennas with half-power beamwidths of typically ≤0.25°. For example, the minimum detectable reflectivity of a W-band radar system may be degraded by more than an order of magnitude for alignment errors on the order of the antenna half-power beamwidth. Moreover, parallax errors are essentially independent of range at cirrus altitudes, and it is not possible to separate parallax effects from offsets in calibration at these far ranges. Observations from a field experiment that include both single- and dual-antenna radar measurements are used to demonstrate these points. Alignment problems have led to the abandonment of dual-antenna pulsed W-band systems in the cloud remote sensing community, and the current generation of millimeter-wave frequency-modulated continuous wave systems must properly take these problems into consideration.

Abstract

The dynamics and predictability of the rapid intensification (RI) of Hurricane Harvey (2017) were examined using convection-permitting initialization, analysis, and prediction from a cycling ensemble Kalman filter (EnKF) that assimilated all-sky infrared radiances from the Advanced Baseline Imager on GOES-16. The EnKF analyses were able to evolve the various scales of the radiance fields associated with Harvey close to those observed, including those associated with scattered individual convective cells before the onset of rapid intensification (RI) and the organized vortex-scale convective system during and after RI. This was true for more than 3 days of a continuous assimilation cycling. Deterministic forecasts initialized from the EnKF analyses captured the rapidly deepening intensity of Harvey more than 24 h prior to its onset. To explore the predictability of Harvey’s intensity during RI, ensemble probabilistic forecasts and sensitivity analyses were conducted. It was found that significant ensemble spread growth was induced by initial perturbations individually in either the wind or moisture fields. The nonlinear interactions between wind and moisture perturbations further limited the predictability of the intensification process of Harvey by increasing the uncertainty in the simulated wind and moisture distributions and modifying the convective activity and its feedback on vortex flow. This study highlights both the importance of better initializing the dynamic and moisture state variables simultaneously and the potential contribution of satellite all-sky radiance assimilation on constraining them and their associated convective activity that impacts RI of tropical cyclones.

Abstract

The vertical structure of clouds associated with a developing midlatitude cyclone was studied using a 94-GHz cloud radar accompanied by a host of other surface-based instruments and rawinsondes. A thickening cirrostratus deck was observed as the storm approached. As the storm drew near, low-level moisture advection increased, and a drizzle-producing stratus deck quickly developed. A rather lengthy period of light to occasionally moderate rain accompanied the passage of the storm. When the storm pulled away to the northeast and the rain ended, the character of the stratus deck changed markedly, with no drizzle production evident.

Other cloud features observed included generating cells and their resultant fallstreaks and an “eye” that apparently accompanied the passage of a negatively tilted upper-level trough, as evidenced by measurements from a 50-MHz wind profiler located near the cloud radar. Rawinsonde measurements showed that the cloud radar also traced the descent of the melting layer. Satellite observations indicated that attenuation often limited the ability of the radar to detect cloud top when precipitation was occurring. As a result, the radar-reported cloud tops were 2–5 km lower than those indicated from the satellite cloud-top temperatures during the heaviest precipitation. During very light precipitation and precipitation-free periods, the satellite brightness temperatures yielded slight underestimates of cloud-top height. In spite of the attenuation, the cloud radar revealed many detailed structures of the clouds of a fairly typical midlatitude cyclone and captured the entire 3-day event.

Abstract

Radiosonde, in situ, and surface-based remote sensor data from the Atlantic Stratocumulus Transition Experiment are used to study the diurnal cycle of cloud and thermodynamic structure. A cloud layer and decoupled subcloud layer separated by a stable transition layer, often observed in the vicinity of cumulus cloud base, characterizes the thermodynamic structure during the study period. The mode of cloud structure is cumulus with bases below decoupled stratus. Data are presented that support the hypothesis that diurnal variations in cumulus development are modulated by the stability in the transition layer.

The frequency of cumulus convection decreases during the afternoon, but mesoscale regions of vigorous cumulus with cloud tops overshooting the base of the trade inversion and increased surface drizzle rates are present during the late afternoon and early evening, when the transition layer is the most stable. It is postulated that mesoscale organization may be required to accumulate enough water vapor in the subcloud layer to produce the convective available potential energy needed for developing cumulus to overcome transition layer stability. The mesoscale regions appear to fit the description of cyclic cumulus convection proposed in a previous study, and this theory is expanded to account for diurnal variations in the stability of the transition layer. The occurrence of these mesoscale clusters of vigorous convection makes it difficult to determine if the latent heat flux in the cloud layer has actually decreased in the late afternoon and early evening, when the transition layer is the most stable.

Liquid water structure was examined and no pronounced diurnal signal was found. Results showed that clouds thicker than approximately 450 m tended to have subadiabatic integrated liquid water contents, presumably due to evaporation of drizzle in the subcloud layer, removal of liquid water at the surface, and the evaporation of cloud water at cloud top. A significant fraction of clouds less than 450 m thick produced liquid water contents that were greater than adiabatic, and there may be a physical mechanism that could produce such values in this cloud system (i.e., lateral detrainment of cloud water from convective elements mixing with existing liquid water in decoupled stratus or with liquid water detrained by nearby convective elements). Unfortunately, instrument limitations may have also produced these greater-than-adiabatic values and the extent of instrument artifacts in these results is unclear.

Abstract

When stratiform-cloud-integrated radiative flux divergence (heating) is dependent on liquid water path (LWP) and droplet concentration N_d , feedbacks between cloud dynamics and this heating can exist. These feedbacks can be particularly strong for low LWP stratiform clouds, in which cloud-integrated longwave cooling is sensitive to LWP and N_d . Large-eddy simulations reveal that these radiative–dynamical feedbacks can substantially modify low LWP stratiform cloud evolution when N_d is perturbed.

At night, more rapid initial evaporation of the cloud layer occurs when N_d is high, leading to more cloud breaks and lower LWP values that both result in less total cloud longwave cooling. Weakened circulations result from this reduced longwave cooling and entrainment drying is able to counteract cloud growth. When N_d is low, the cloud layer is better maintained because cloud longwave cooling is still relatively strong.

During the day, the addition of shortwave warming leads to reduced LWP for all values of N_d and, consequently, further reduced longwave cooling and weakened circulations. For high N_d , these reductions are such that the cloud layer cannot be maintained. For lower N_d , the reductions are smaller and the cloud layer thins but does not dissipate.

These results suggest that low LWP cloud layers are more tenuous when N_d is high and are more prone to dissipating during the day. Comparison with other studies suggests the modeled low LWP cloud response may be sensitive to the initial thermodynamic profile and model configuration.

Abstract

A database of stratus cloud droplet (diameter <50 μm) size distribution parameters, derived from in situ data reported in the existing literature, was created, facilitating intercomparison among datasets and quantifying typical values and their variability. From the datasets, which were divided into marine and continental groups, several parameters are presented, including the total number concentration, effective diameter, mean diameter, standard deviation of the droplet diameters about the mean diameter, and liquid water content, as well as the parameters of modified gamma and lognormal distributions. In light of these results, the appropriateness of common assumptions used in remote sensing of cloud droplet size distributions is discussed. For example, vertical profiles of mean diameter, effective diameter, and liquid water content agreed qualitatively with expectations based on the current paradigm of cloud formation. Whereas parcel theory predicts that the standard deviation about the mean diameter should decrease with height, the results illustrated that the standard deviation generally increases with height. A feature common to all marine clouds was their approximately constant total number concentration profiles; however, the total number concentration profiles of continental clouds were highly variable. Without cloud condensation nuclei spectra, classification of clouds into marine and continental groups is based on indirect methods. After reclassification of four sets of measurements in the database, there was a fairly clear dichotomy between marine and continental clouds, but a great deal of variability within each classification.

The relevant applications of this study lie in radiative transfer and climate issues, rather than in cloud formation and dynamics. Techniques that invert remotely sensed measurements into cloud droplet size distributions frequently rely on a priori assumptions, such as constant number concentration profiles and constant spectral width. The results of this paper provide a basis for evaluating the sensitivity of these techniques. In particular, there were large enough differences in observed droplet spectral widths to significantly affect remotely sensed determinations of cloud microphysics.

Abstract

Scattering properties of a large collection of pristine ice crystals at millimeter and centimeter wavelengths are calculated using the generalized multiparticle Mie method. Millimeter- and centimeter-wavelength radar observables are also calculated by employing particle size distributions (PSDs) that ensure the bulk properties (e.g., ice water content and total number concentration) fall within physically realistic ranges. The relationships between radar reflectivity Z and ice water content (IWC) are shown to be sensitive (from one to two orders of magnitude in variability) to the PSDs used and are thus not recommended for IWC retrievals. The relationships between IWC and specific differential phase K _DP are less dependent on PSDs. Simple relationships between IWC and K _DP at different radar elevation angles and wavelengths are given. If only the general crystal type is known (i.e., planar vs columnar), IWC retrieval errors based on K _DP mostly fall within 30%. If more detailed ice crystal type is known, the retrieval errors are reduced to mostly within 10%. These results are similar to earlier reports in the literature but are based on a more extensive collection of model ice crystals and electromagnetic-scattering computations at four wavelengths: X, Ku, Ka, and W bands. The applicability of K _DP in retrieving IWC is limited by the measurement accuracy of K _DP, which usually requires averaging over several kilometers in range. Given the same noise level, the shorter wavelengths may have relatively smaller fractional errors than the longer wavelengths in K _DP-based IWC retrievals and are promising wavelengths for further investigation.

Abstract

Testing the often-made assumption that ice particle aggregates (snowflakes) are well represented by oblate spheroids, ellipsoid fits are applied to aggregate images. An algorithm to retrieve both the ellipsoidal parameters and the orientations of the fitted ellipsoids is applied to Multi-Angle Snowflake Camera measurements of ice particle aggregates observed in Alaska. The resulting ellipsoids have shapes closer to prolate spheroids than the oft-assumed oblate spheroids. A robust linear relationship exists between the two characteristic aspect ratios of the ellipsoids. The most probable orientation of the maximum dimension of the retrieved ellipsoids is not in the horizontal plane, and the rotational angles of the maximum dimensions in the horizontal plane are not uniform, but instead display some correlation with the wind direction at the times of the measurements. The retrieval results can be used to improve the representation of aggregates in microphysics and/or electromagnetic radiation scattering models applicable to radar and satellite measurements.

Abstract

A 4-yr climatology (1997–2000) of warm boundary layer cloud properties is developed for the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site. Parameters in the climatology include cloud liquid water path, cloud-base height, and surface solar flux. These parameters are retrieved from measurements produced by a dual-channel microwave radiometer, a millimeter-wave cloud radar, a micropulse lidar, a Belfort ceilometer, shortwave radiometers, and atmospheric temperature profiles amalgamated from multiple sources, including radiosondes. While no significant interannual differences are observed in the datasets, there are diurnal variations with nighttime liquid water paths consistently higher than daytime values. The summer months of June, July, and August have the lowest liquid water paths and the highest cloud-base heights. Model outputs of cloud liquid water paths from the European Centre for Medium-Range Weather Forecasts (ECMWF) model and the Eta Model for 104 model output location time series (MOLTS) stations in the environs of the SGP central facility are compared to observations. The ECMWF and MOLTS median liquid water paths are greater than 3 times the observed values. The MOLTS data show lower liquid water paths in summer, which is consistent with observations, while the ECMWF data exhibit the opposite tendency. A parameterization of normalized cloud forcing that requires only cloud liquid water path and solar zenith angle is developed from the observations. The parameterization, which has a correlation coefficient of 0.81 with the observations, provides estimates of surface solar flux that are comparable to values obtained from explicit radiative transfer calculations based on plane-parallel theory. This parameterization is used to estimate the impact on the surface solar flux of differences in the liquid water paths between models and observations. Overall, there is a low bias of 50% in modeled normalized cloud forcing resulting from the excess liquid water paths in the two models. Splitting the liquid water path into two components, cloud thickness and liquid water content, shows that the higher liquid water paths in the model outputs are primarily a result of higher liquid water contents, although cloud thickness may a play a role, especially for the ECMWF model results.

Abstract

The properties of midlatitude cirrus clouds are examined using one year of continuous vertically pointing millimeter-wave cloud radar data collected at the Atmospheric Radiation Measurement Program Southern Great Plains site in Oklahoma. The goal of this analysis is to present the cloud characteristics in a manner that will aid in the evaluation and improvement of cirrus parameterizations in large-scale models. Using a temperature- and radar reflectivity–based definition of cirrus, the occurrence frequency of cirrus, the vertical location and thickness of cirrus layers, and other fundamental statistics are examined. Also the bulk microphysical properties of optically thin cirrus layers that occur in isolation from other cloud layers are examined. During 1997, it is found that cirrus were present 22% of the time, had a mean layer thickness of 2.0 km, and were most likely to occur in the 8.5–10-km height range. On average, the cirrus clouds tended to be found in layers in which the synoptic-scale vertical velocity was weakly ascending. The mean synoptic-scale vertical motion in the upper troposphere as derived from Rapid Update Cycle model output was +0.2 cm s⁻¹. However, a significant fraction of the layers (33%) were found where the upper-tropospheric large-scale vertical velocity was clearly descending (w < −1.5 cm s⁻¹). Microphysical properties were computed for that subset of cirrus events that were optically thin (infrared emissivity < 0.85) and occurred with no lower cloud layers. This subset of cirrus had mean values of ice water path, effective radius, and ice crystal concentration of 8 g m⁻², 35 μm, and 100 L⁻¹, respectively. Although all the cloud properties demonstrated a high degree of variability during the period considered, the statistics of these properties were fairly steady throughout the annual cycle. Consistent with previous studies, it is found that the cloud microphysical properties appear to be strongly correlated to the cloud layer thickness and mean temperature. Use of these results for parameterization of cirrus properties in large-scale models is discussed.