Abstract

This paper, the first of a series, examines the synoptic-scale mechanisms involved in the initiation, structure, and evolution of a mesoscale convective system observed during TOGA COARE. This study relies upon the use of the Japanese Geosynchronous Meteorological Satellite-4 imagery and ECMWF model outputs, from which diagnostic parameters are derived and interpreted. This mesoscale convective system consists initially of two groups of convective entities that progressively move toward each other and merge. It is shown that the synoptic-scale flow creates a favorable environment for the formation of this large convective system through the production of convective available potential energy (CAPE) by horizontal advection and the enhancement of low-level convergence in the region where the convective system formed. Moreover, the general evolution of the system is found to be governed by the synoptic-scale circulation and, more precisely, by the temporal evolution of CAPE and low-level convergence. The mechanisms leading to the initiation and general evolution of the system are examined. The easterly equatorial jet at 500 hPa triggered positive potential vorticity areas that propagated westward and generated an anticyclonic circulation. This anticyclonic circulation was enhanced during the development phase of the convective system through vortex stretching and tilting, which accelerated the low-level westerlies (corresponding to the southern branch of this circulation) and enhanced the low-level convergence associated with the studied convective system.

In a companion paper, the mesoscale and convective-scale processes involved in the internal organization of this mesoscale convective system are examined using the airborne Doppler radar dataset collected within the system from 1700 to 2100 UTC. The downscale interactions (i.e., from synoptic scale to mesoscale and convective scale) are scrutinized using both the synoptic-scale context described in this paper and the mesoscale and convective-scale characteristics derived from the airborne radar observations.

Abstract

This paper, the second of a series, documents the precipitation, and kinematic and thermodynamic structure of a tropical mesoscale convective system observed by instrumented aircraft on 15 December 1992 during TOGA COARE. Radar-derived precipitation fields indicate that the studied system consists of two subsystems, S1 and S2, characterized by distinct internal dynamics and morphological structures. The retrieved kinematic and thermodynamic structures are compared in detail with the synoptic-scale characteristics described in Part I of this paper, so as to evaluate the scale interactions involved in the internal organization of this convective system. It is shown essentially that the synoptic-scale circulation governs the mesoscale and convective-scale motions and, therefore, determines the internal organization of the selected system. In particular, this study highlights the major role played by the synoptic-scale vertical wind shear in the internal structure of this tropical mesoscale system. Momentum flux calculations show that upward transport of horizontal momentum on the mesoscale is large and mostly carried out at a scale of motion larger than the mesoscale (i.e., by the mean component of the total momentum flux). The westerly rear inflow exhibits characteristics consistent with the density current theory. However, specific mesoscale and convective-scale processes linked to the presence of precipitation modulate this dominant synoptic-scale forcing. A mesoscale interaction between the two distinct subsystems S1 and S2 is identified. The apparition of a shallow density current resulting from the downward spreading of air at the ground within S1 is suspected to be the triggering mechanism for S2.

Abstract

Interpolation of ground-based radar measurements is required when mapping data from their native spherical coordinates to a Cartesian grid. For reflectivity the question arises as to whether this processing should be performed in units of Z (mm⁶ m⁻³) or dBZ. This study addresses this question using one year of data from three radars, operating in diverse climates across Australia. For each radar, a subset of 800 volume scans is processed to identify “triads”—groups of three consecutive gates with valid data—in each of the three coordinate directions: range, azimuth, and elevation. For every triad, the reflectivity at the central gate is estimated by linearly interpolating between the outer two gates in both Z and dBZ. The resulting values are then compared with the true reflectivity at the central gate to quantify the interpolation errors. For all three sites and in all three coordinate directions, we find that interpolation in Z is more accurate on average, especially in regions of high reflectivity and strong reflectivity gradient (i.e., convective cores). However, interpolation in dBZ is better in regions of low and monotonically increasing/decreasing reflectivity. It is therefore recommended that reflectivities be converted from dBZ to Z prior to interpolation except when identifying echo-top height or other low-reflectivity boundaries.

Abstract

The present study is devoted to new analyses of single-beam airborne Doppler radar data referred to as SAVAD (single-beam airborne velocity–azimuth display) and double SAVAD. These techniques permit processing of Doppler radial velocities from circular trajectories performed by the aircraft. As in the VAD and double VAD analyses for ground-based radars, these SAVAD and double SAVAD analyses permit diagnosis of crucial mesoscale kinematic and thermodynamic parameters such as the horizontal wind, its horizontal divergence, its stretching and shearing deformations, the vertical air motion, the mean fall velocity of the hydrometeors, the mesoscale vertical vorticity, and horizontal gradients of pressure and temperature perturbations. These analyses are mathematically described. An application of the method to simulated data and to a real dataset extracted from the TOGA COARE database is also presented.

Abstract

The present paper describes the vertical structure of the wind field obtained by analysis of “purls,” that is, circular trajectories regularly performed by airborne dual-beam Doppler radars within the FASTEX frontal cyclones. Kinematic information on these systems are obtained using a new analysis scheme named DAVAD (Dual-Beam Antenna Velocity Azimuth Display). Using this scheme, it is possible to obtain the mesoscale wind field and its first-order derivatives, that is, the horizontal divergence (thus the vertical velocity), the stretching and shearing deformations, and the vertical component of vorticity. A unique advantage of this analysis is that it also provides a direct estimate of the terminal fall velocity of the hydrometeors. All these parameters are crucial for validation and initialization of mesoscale and large-scale models. The capabilities of this method and the best conditions for its application are assessed through simulations. Finally, an example of application of the scheme on the secondary low observed during the Fronts and Atlantic Storm Track Experiment (FASTEX) Intensive Observation Period (IOP) 12 is discussed. Results of purl processing using DAVAD are being included in the FASTEX database.

Abstract

The effect of ship motion on shipborne polarimetric radar measurements is considered at C band. Calculations are carried out by (i) varying the “effective” mean canting angle and (ii) separately examining the elevation dependence. Scattering from a single oblate hydrometeor is considered at first. Equations are derived (i) to convert the measured differential reflectivity for nonzero mean canting angles to those for zero mean canting angle and (ii) to do the corresponding corrections for nonzero elevation angles. Scattering calculations are also performed using the T-matrix method with measured drop size distributions as input. Dependence on mean volume diameter is examined as well as variations of the four main polarimetric parameters. The results show that as long as the ship movement is limited to a roll of less than about 10°–15°, the effects are tolerable. Furthermore, the results from the scattering simulations have been used to provide equations for correction factors that can be applied to compensate for the “apparent” nonzero canting angles and nonzero elevation angles, so that drop size distribution parameters and rainfall rates can be estimated without any bias.

Abstract

The objective of this paper is to assess the performances of the proposed ice water content (IWC)–radar reflectivity Z and IWC–Z–temperature T relationships for accurate retrievals of IWC from radar in space or at ground-based sites, in the framework of the forthcoming CloudSat spaceborne radar, and of the European CloudNET and U.S. Atmospheric Radiation Measurement Program projects. For this purpose, a large airborne in situ microphysical database is used to perform a detailed error analysis of the IWC–Z and IWC–Z–T methods. This error analysis does not include the error resulting from the mass–dimension relationship assumed in these methods, although the expected magnitude of this error is bounded in the paper. First, this study reveals that the use of a single IWC–Z relationship to estimate IWC at global scale would be feasible up to −15 dBZ, but for larger reflectivities (and therefore larger IWCs) different sets of relationships would have to be used for midlatitude and tropical ice clouds. New IWC–Z and IWC–Z–T relationships are then developed from the large aircraft database and by splitting this database into midlatitude and tropical subsets, and an error analysis is performed. For the IWC–Z relationships, errors decrease roughly linearly from +210%/−70% for IWC = 10⁻⁴ g m⁻³ to +75%/−45% for IWC = 10⁻² g m⁻³, are nearly constant (+50%/−33%) for the intermediate IWCs (0.03–1 g m⁻³), and then linearly increase up to +210%/−70% for the largest IWCs. The error curves have the same shape for the IWC–Z–T relationships, with a general reduction of errors with respect to the IWC–Z relationships. Comparisons with radar–lidar retrievals confirm these findings. The main improvement brought by the use of temperature as an additional constraint to the IWC retrieval is to reduce both the systematic overestimation and rms differences of the small IWCs (IWC < 0.01 g m⁻³). For the large IWCs, the use of temperature also results in a slight reduction of the rms differences but in a substantial reduction (by a factor of 2) of the systematic underestimation of the large IWCs, probably owing to a better account of the Mie effect when IWC–Z relationships are stratified by temperature.

Abstract

In this paper, statistical properties of rainfall are derived from 14 years of Tropical Rainfall Measuring Mission data to optimize the use of flight hours for the upcoming High Altitude Ice Crystals (HAIC)/High Ice Water Content (HIWC) program. This program aims to investigate the convective processes responsible for the generation of the high ice water content that has been recognized as a threat to civil aviation. The probability that convective cells are conducive to HIWC is also further investigated using three years of C-band polarimetric radar data. Further insights into the variability of convective rainfall and favorable conditions for HIWC are also gained using two different methods to characterize the large-scale atmospheric conditions around Darwin, Australia (the Madden–Julian oscillation and the Darwin atmospheric regimes), and the underlying surface type (oceanic vs continental). The main results from the climatology relevant to flight-plan decision making are (i) convective cells conducive to HIWC should be found close to Darwin, (ii) at least 90% of convective cells are conducive to HIWC at 10- and 12-km flight levels, (iii) multiple flights per day in favorable large-scale conditions will be needed so as to utilize the 150 project flight hours, (iv) the largest numbers of HIWC radar pixels are found around 0300 and 1500 local time, and (v) to fulfill the requirement to fly 90 h in oceanic convection and 60 h in or around continental convection, a minimum “acceptable” size of the convective area has been derived and should serve as a guideline for flight-decision purposes.

Abstract

Best estimates of the bulk microphysical and radiative properties (ice water content, visible extinction, effective radius, and total concentration) are derived for three case studies of tropical ice clouds sampled during the Tropical Warm Pool International Cloud Experiment (TWP-ICE). Two case studies are aged cirrus clouds produced by deep convection (the so-called 27/01 and 29/01 cases), and the third (“02/02”) is a fresh anvil produced by deep convective activity over the Tiwi Islands. Using crystal images obtained by a Cloud Particle Imager (CPI), it is observed that small ice particles (with maximum dimension D < 50–100 μm) were predominantly quasi spherical, with the degree of nonsphericity increasing rapidly in the 50 < D < 100-μm range. For D > 100 μm, the aged cirrus clouds were predominantly characterized by bullet rosettes and aggregates of bullet rosettes, plates, and columns. In contrast, the fresh anvil had more frequent occurrences of plates, columns, aggregates of plates, and occasionally capped columns. The impact of shattering of large ice crystals on probe tips and the overall quality of the TWP-ICE in situ microphysical measurements are assessed. It is suggested that shattering has a relatively small impact on the CPI and cloud droplet probe (CDP) TWP-ICE data and a large impact on the Cloud Aerosol Spectrometer data, as already documented by others. It is also shown that the CPI size distributions must be multiplied by a factor of 4 to match those of the cloud imaging probe (CIP) for maximum dimension larger than 100 μm (taken as a reference). A technique [named Best Estimate of Area and Density (BEAD)] to minimize errors associated with the density (ρ)–D and projected area (A)–D assumptions in bulk microphysics calculation is introduced and applied to the TWP-ICE data. The method makes direct use of the frequency of occurrence of each particle habit as classified from the CPI data and prescribed ρ–D and A–D relationships from the literature. This approach produces ice water content (IWC) estimates that are virtually unbiased relative to bulk measures obtained from a counterflow spectrometer and impactor (CSI) probe. In contrast, the use of ρ–D and A–D relationships for single habits does produce large biases relative to the CSI observations: from −50% for bullet rosettes to +70%–80% for aggregates. The so-called width, length, area, and perimeter (WLAP) technique, which also makes use of individual CPI images, is found to produce positively biased IWCs (by 40% or so), and has a standard deviation of the errors similar to the BEAD technique. The impact of the large variability of the size distributions measured by different probe combinations on the bulk microphysical properties is characterized. The mean fractional differences with respect to the CSI measurements are small for the CPI + CIP, CPI, and CDP + CIP combinations (2.2%, −0.8%, and −1.1%, respectively), with standard deviations of the fractional differences ranging from 7% to 9%. This result provides an independent validation of the CPI scaling factor. The fractional differences produced between the CPI + CIP, CPI, and CDP + CIP combinations for extinction, effective radius, and total concentration are 33%, 10%–20%, and 90%, respectively, with relatively small standard deviations of 5%–8%. The fractional difference on total concentration varies greatly over the concentration range though, with values being larger than a factor of 2 for total concentrations smaller than 40 L⁻¹, but reducing to 10%–20% for concentrations larger than 100 L⁻¹. Therefore, caution should be exercised when using total concentrations smaller than 60–80 L⁻¹ as references for radar–lidar retrieval evaluation.

Abstract

Current climate models cannot resolve individual convective clouds, and hence parameterizations are needed. The primary goal of convective parameterization is to represent the bulk impact of convection on the gridbox-scale variables. Spectral convective parameterizations also aim to represent the key features of the subgrid-scale convective cloud field such as cloud-top-height distribution and in-cloud vertical velocities in addition to precipitation rates. Ground-based radar retrievals of these quantities have been made available at Darwin, Australia, permitting direct comparisons of internal parameterization variables and providing new observational references for further model development.

A spectral convective parameterization [the convective cloud field model (CCFM)] is discussed, and its internal equation of motion is improved. Results from the ECHAM–HAM model in single-column mode using the CCFM and the bulk mass flux Tiedtke–Nordeng scheme are compared with the radar retrievals at Darwin. The CCFM is found to outperform the Tiedtke–Nordeng scheme for cloud-top-height and precipitation-rate distributions. Radar observations are further used to propose a modified CCFM configuration with an aerodynamic drag and reduced entrainment parameter, further improving both the convective cloud-top-height distribution (important for large-scale impact of convection) and the in-cloud vertical velocities (important for aerosol activation).

This study provides a new development in the CCFM, improving the representation of convective cloud spectrum characteristics observed in Darwin. This is a step toward an improved representation of convection and ultimately of aerosol effects on convection. It also shows how long-term radar observations of convective cloud properties can help constrain parameters of convective parameterization schemes.