Abstract

The 50-MHz profiler operating near Darwin, Northwest Territory, Australia, is sensitive to both turbulent clear-air (Bragg) and hydrometeor (Rayleigh) scattering processes. Below the radar bright band, the two scattering peaks are observed as two well-separated peaks in the Doppler velocity spectra. The Bragg scattering peak corresponds to the vertical air motion and the Rayleigh scattering peak corresponds to the hydrometeor motion. Within the radar bright band, the Rayleigh scattering peak intensity increases and the downward velocity decreases causing the hydrometeor peak to overlap or merge with the air motion peak. If the overlap of the two peaks is not taken into account, then the vertical air motion estimate will be biased downward. This study describes a filtering procedure that identifies and removes the downward bias in vertical air motions caused by hydrometeor contamination. This procedure uses a second collocated profiler sensitive to hydrometeor motion to identify contamination in the 50-MHz profiler spectra. When applied to four rain events during the Tropical Warm Pool-International Cloud Experiment (TPW-ICE), this dual-frequency filtering method showed that approximately 50% of the single-frequency method vertical air motion estimates within the radar bright band were biased downward due to hydrometeor contamination.

Abstract

This study consists of two parts. The first part describes the way in which vertical air motions and raindrop size distributions (DSDs) were retrieved from 449-MHz and 2.835-GHz (UHF and S band) vertically pointing radars (VPRs) deployed side by side during the Midlatitude Continental Convective Clouds Experiment (MC3E) held in northern Oklahoma. The 449-MHz VPR can measure both vertical air motion and raindrop motion. The S-band VPR can measure only raindrop motion. These differences in VPR sensitivities facilitates the identification of two peaks in 449-MHz VPR reflectivity-weighted Doppler velocity spectra and the retrieval of vertical air motion and DSD parameters from near the surface to just below the melting layer.

The second part of this study used the retrieved DSD parameters to decompose reflectivity and liquid water content (LWC) into two terms, one representing number concentration and the other representing DSD shape. Reflectivity and LWC vertical decomposition diagrams (Z-VDDs and LWC-VDDs, respectively) are introduced to highlight interactions between raindrop number and DSD shape in the vertical column. Analysis of Z-VDDs provides indirect measure of microphysical processes through radar reflectivity. Analysis of LWC-VDDs provides direct investigation of microphysical processes in the vertical column, including net raindrop evaporation or accretion and net raindrop breakup or coalescence. During a stratiform rain event (20 May 2011), LWC-VDDs exhibited signatures of net evaporation and net raindrop coalescence as the raindrops fell a distance of 2 km under a well-defined radar bright band. The LWC-VDD is a tool to characterize rain microphysics with quantities related to number-controlled and size-controlled processes.

Abstract

Principal component analysis (PCA) is applied to wind profiler observations to study the vertical profile of the wind field and its temporal evolution. The rationale for decomposing time–height wind profiler data using PCA is twofold. The orthogonal vertical profile vectors are determined empirically from the variance of the observations, and the time evolutions of these vectors are not simple sinusoids, but are temporal varying signals that can be directly related to other measurements. As an example of its utility, PCA is used to compare the annual and interannual variation of zonal wind measured with a 50-MHz VHF wind profiler above Christmas Island, Kiribati, with the difference between surface pressures measured at Tahiti, French Polynesia, and Darwin, Australia. The high correlation coefficients relate the vertical profile of zonal wind observed in the central Pacific with the variation of convection in the western Pacific. Complex PCA (C-PCA) allows the analysis of data consisting of amplitude and phase information. It can describe the phase progression of oscillations embedded within the data. The C-PCA is applied to VHF wind profiler observations to study the seasonal behavior of the diurnal meridional wind observed above Biak, Indonesia, and the oscillatory structures of the vertical wind during a convective precipitation event observed above Darwin.

Abstract

Polarimetric weather radars offer the promise of accurate rainfall measurements by including polarimetric measurements in rainfall estimation algorithms. Questions still remain on how accurately polarimetric measurements represent the parameters of the raindrop size distribution (DSD). In particular, this study propagates polarimetric radar measurement uncertainties through a power-law median raindrop diameter D ₀ algorithm to quantify the statistical uncertainties of the power-law regression. For this study, the power-law statistical uncertainty of D ₀ ranged from 0.11 to 0.17 mm. Also, the polarimetric scanning radar D ₀ estimates were compared with the median raindrop diameters retrieved from two vertically pointing profilers observing the same radar volume as the scanning radar. Based on over 900 observations, the standard deviation of the differences between the two radar estimates was approximately 0.16 mm. Thus, propagating polarimetric measurement uncertainties through D ₀ power-law regressions is comparable to uncertainties between polarimeteric and profiler D ₀ estimates.

Abstract

This study provides a very clear picture of the microphysics and flow field in a convective storm in the Rondonia region of Brazil through a synthesis of observations from two unique radars, measurements of the surface drop size distribution (DSD), and particle types and sizes from an aircraft penetration. The primary findings are 1) the growth of rain by the collision–coalescence–breakup (CCB) process to equilibrium drop size distributions entirely below the 0°C level; 2) the subsequent growth of larger ice particles (graupel and hail) at subfreezing temperatures; 3) the paucity of lightning activity during the former process, and the increased lightning frequency during the latter; 4) the occurrence of strong downdrafts and a downburst during the latter phase of the storm resulting from cooling by melting and evaporation; 5) the occurrence of turbulence along the main streamlines of the storm; and 6) the confirmation of the large drops reached during the CCB growth by polarimetric radar observations. These interpretations have been made possible by estimating the updraft magnitude using the “lower bound” of the Doppler spectrum at vertical incidence, and identifying the “balance level” at which particles are supported for growth. The combination of these methods shows where raindrops are supported for extended periods to allow their growth to equilibrium drop size distributions, while smaller drops ascend and large ones descend. A hypothesis worthy of pursuit is the control of the storm motion by the winds at the balance level, which is the effective precipitation generating level. Above the 0°C level the balance level separates the small ascending ice crystals from the large descending graupel and hail. Collisions between the two cause electrical charging, while gravity and the updrafts separate the charges to cause lightning. Below the 0°C level, large downward velocities (caused by the above-mentioned cooling) in excess of the terminal fall speeds of raindrops represent the downbursts, which are manifested in the surface winds.

Abstract

Doppler radar measurements at different frequencies (50 and 2835 MHz) are used to characterize the terminal fall speed of hydrometeors and the vertical air motion in tropical ice clouds and to evaluate statistical methods for retrieving these two parameters using a single vertically pointing cloud radar. For the observed vertical air motions, it is found that the mean vertical air velocity in ice clouds is small on average, as is assumed in terminal fall speed retrieval methods. The mean vertical air motions are slightly negative (downdraft) between the melting layer (5-km height) and 6.3-km height, and positive (updraft) above this altitude, with two peaks of 6 and 7 cm s⁻¹ at 7.7- and 9.7-km height. For the retrieved hydrometeor terminal fall speeds, it is found that the variability of terminal fall speeds within narrow reflectivity ranges is typically within the acceptable uncertainties for using terminal fall speeds in ice cloud microphysical retrievals. This study also evaluates the performance of previously published statistical methods of separating terminal fall speed and vertical air velocity from vertically pointing Doppler radar measurements using the 50-/2835-MHz radar retrievals as a reference. It is found that the variability of the terminal fall speed–radar reflectivity relationship (V_t –Z_e ) is large in ice clouds and cannot be parameterized accurately with a single relationship. A well-defined linear relationship is found between the two coefficients of a power-law V_t –Z_e relationship, but a more accurate microphysical retrieval is obtained using Doppler velocity measurements to better constrain the V_t –Z_e relationship for each cloud. When comparing the different statistical methods to the reference, the distribution of terminal fall speed residual is wide, with most residuals being in the ±30–40 cm s⁻¹ range about the mean. The typical mean residual ranged from 15 to 20 cm s⁻¹, with different methods having mean residuals of <10 cm s⁻¹ at some heights, but not at the same heights for all methods. The so-called V_t –Z_e technique was the most accurate above 9-km height, and the running-mean technique outperformed the other techniques below 9-km height. Sensitivity tests of the running-mean technique indicate that the 20-min average is the best trade-off for the type of ice clouds considered in this analysis. A new technique is proposed that incorporates simple averages of Doppler velocity for each (Z_e , H) couple in a given cloud. This technique, referred to as DOP–Z_e –H, was found to outperform the three other methods at most heights, with a mean terminal fall residual of <10 cm s⁻¹ at all heights. This error magnitude is compatible with the use of such retrieved terminal fall speeds for the retrieval of microphysical properties.

Abstract

Kinematic and microphysical characteristics of a stratiform rainband within Tropical Storm Gabrielle during landfall on 14 September 2001 were investigated using data from a collocated 915-MHz wind profiler and scanning Doppler radar. The curved 60-km-wide rainband was relatively intense with mesoscale updrafts and downdrafts exceeding ±1 m s⁻¹. The bright band is classified as strong, as indicated by reflectivity factors in excess of 50 dBZ and rainfall rates below the bright band peaking at 10–20 mm h⁻¹. The melting layer microphysical processes were examined to understand the relation between brightband processes and precipitation intensity and kinematics (mesoscale downdraft in particular) below the melting layer. The profiler and Doppler radar analyses, designed to maximize vertical resolution of flows within the melting layer, disclose a striking convergence–divergence couplet through the melting layer that implies a prominent cooling-induced finescale circulation. Melting-driven cooling initiates midlevel convergence in the upper part of the melting region, while weak convergence to positive divergence is analyzed within the lower melting layer. A melting-layer parameter study indicates the significance of the level of maximum reflectivity that separates convergence above from divergence below and also reveals a mixture of aggregation and breakup of ice particles, with aggregation being dominant. In this vigorous rainband case, the presence of strong mesoscale downdrafts cannot be ignored for accurate retrievals of raindrop size distribution and precipitation parameters from the Sans Air Motion model. When downdrafts are included, retrieved rainfall estimates were much higher than those under the zero vertical air motion assumption and were slightly less than those from a power-law Z–R relation. The rainfall estimates show a positive correlation with reflectivity factor and brightband intensity (i.e., aggregation degree) but less dependence on brightband height.

Abstract

Using NOAA’s S-band High-Power Snow-Level Radar (HPSLR), a technique for estimating the rain drop size distribution (DSD) above the radar is presented. This technique assumes the DSD can be described by a four parameter, generalized gamma distribution (GGD). Using the radar’s measured average Doppler velocity spectrum and a value (assumed, measured, or estimated) of the vertical air motion w, an estimate of the GGD is obtained. Four different methods can be used to obtain w. One method that estimates a mean mass-weighted raindrop diameter D_m from the measured reflectivity Z produces realistic DSDs compared to prior literature examples. These estimated DSDs provide evidence that the radar can retrieve the smaller drop sizes constituting the “drizzle” mode part of the DSD. This estimation technique was applied to 19 h of observations from Hankins, North Carolina. Results support the concept that DSDs can be modeled using GGDs with a limited range of parameters. Further work is needed to validate the described technique for estimating DSDs in more varied precipitation types and to verify the vertical air motion estimates.

Abstract

A unique set of Doppler and polarimetric radar observations were made of a microburst-producing storm in Amazonia during the Tropical Rainfall Measuring Mission (TRMM) Large-Scale Biosphere–Atmosphere (LBA) field experiment. The key features are high reflectivity (50 dBZ) and modest size hail (up to 0.8 mm) in high liquid water concentrations (>4 g m⁻³) at the 5-km 0°C level, melting near the 3-km level as evidenced by the Doppler spectrum width on the profiler radar (PR), by differential polarization on the S-band dual-polarized radar (S-POL), and a sharp downward acceleration from 2.8 to 1.6 km to a peak downdraft of 11 m s⁻¹, followed by a weak microburst of 15 m s⁻¹ at the surface. The latter features closely match the initial conditions and results of the Srivastava numerical model of a microburst produced by melting hail. It is suggested that only modest size hail in large concentrations that melt aloft can produce wet microbursts. The narrower the distribution of hail particle sizes, the more confined will be the layer of melting and negative buoyancy, and the more intense the microburst. It is hypothesized that the timing of the conditions leading to the microburst is determined by the occurrence of an updraft of proper magnitude in the layer in which supercooled water accounts for the growth of hail or graupel.

Abstract

A model of rain shaft microphysics that solves the stochastic advection–coalescence–breakup equation in an atmospheric column was used to simulate the evolution of a stratiform rainfall event during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) in Darwin, Australia. For the first time, a dynamic simulation of the evolution of the drop spectra within a one-dimensional rain shaft is performed using realistic boundary conditions retrieved from real rain events. Droplet size distribution (DSD) retrieved from vertically pointing radar (VPR) measurements are sequentially imposed at the top of the rain shaft as boundary conditions to emulate a realistic rain event. Time series of model profiles of integral parameters such as reflectivity, rain rate, and liquid water content were subsequently compared with estimates retrieved from vertically pointing radars and Joss–Waldvogel disdrometer (JWD) observations. Results obtained are within the VPR retrieval uncertainty estimates. Besides evaluating the model’s ability to capture the dynamical evolution of the DSD within the rain shaft, a case study was conducted to assess the potential use of the model as a physically based interpolator to improve radar retrieval at low levels in the atmosphere. Numerical results showed that relative improvements on the order of 90% in the estimation of rain rate and liquid water content can be achieved close to the ground where the VPR estimates are less reliable. These findings raise important questions with regard to the importance of bin resolution and the lack of sensitivity for small raindrop size (<0.03 cm) in the interpretation of JWD data, and the implications of using disdrometer data to calibrate radar algorithms.