Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Research Articles

Tropical large-scale circulations: asymptotically non-divergent?

Although the large-scale tropical atmospheric circulations are often considered as primarily divergent, a simple scale analysis, originally presented by Charney (1963), suggests otherwise—a dominance of vorticity over divergence.The present paper quantitatively documents the asymptotic non-divergence of the large-scale tropical atmosphere, in association with Madden—Julian oscillations, with use of the Tropical Ocean Global Atmosphere—Coupled OceanAtmosphere Response Experiment, Large-Scale Array (TOGA—COARE LSA) data set. The vorticity is larger than the divergence at the majority (70%–80%) of points at any instant for the levels between 850 and 250 hPa, and the vorticity is more than 10 times stronger than the divergence both at 850 and 500 hPa more than half of the time. The root mean square (rms) ratio between the transient components of divergence and vorticity, which is defined as the deviation from the mean for the whole data period, decreases substantially with increasing horizontal scales from 100 to 2000 km, over an intraseasonal timescale (20–100 d). The analysis suggests that the Madden—Julian oscillations are dominated more by vorticity than divergence and more so than at the smaller scales.The analysis as a whole suggests the feasibility of constructing an asymptotically non-divergent theory for large-scale tropical circulations. A brief sketch of the formulation is presented.

Reply to comment by John H. Helsdon Jr. on “On the roles of deep convective clouds in tropospheric chemistry”

Reply to comment by John H. Helsdon Jr. on “On the roles of deep convective clouds in tropospheric chemistry”

Comment on “Partitioning tropical oceanic convective and stratiform rains by draft strength” by David Atlas et al.

[1] The crux of this comment is to clarify attribution. If someone looks at a published graph and has a different interpretation from that of the original authors, that new interpretation should not be attributed to the original authors. [2] The references given by Atlas et al. [2000] regarding distinct convective and stratiform reflectivity Z to rain rate R relations attributed to Yuter and Houze [1997] are incorrect. The convective and stratiform Z-R relations labeled as those of Yuter and Houze in Table 3 (incorrectly referred to there as 1996) and mentioned in the text on pp. 2262, 2264, and 2265 of Atlas et al. [2000] do not appear anywhere in the Yuter and Houze [1997] paper. The Yuter and Houze [1997] study used two-dimensional (2-D) particle probe drop spectra collected by the National Center for Atmospheric Research Electra aircraft during the Tropical Ocean-Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) to compute Z and R values over 6 s intervals of flight track ( 1400 L sample volumes). These data were classified into convective and stratiform subsets using radar data obtained by the National Oceanic and Atmospheric Administration WP-3D airborne radar. Segments of the Electra flight track were categorized by noting when the Electra flew within regions of convective and stratiform precipitation classified according to a radar reflectivity texture algorithm based on the paper by Steiner et al. [1995]. The main conclusion of the Yuter and Houze [1997] study was that the populations of radarclassified convective and stratiform Z-R points overlapped in dbZ-log R space and did not form two statistically distinct populations. [3] The previous statements do not rule out the possibility that different physical processes can yield different drop size distributions (DSD) [e.g., Braun and Houze, 1994, pp. 2749–2750]. However, in the large areas of radar-classified convective and stratiform precipitation examined in this study, the physical processes mixed, exhibited natural variation, and were subject to sampling error to such a degree that the Z and Rvalues associated with the DSD samples in the convective and stratiform populations did not form distinct populations. A classification method that can truly distinguish between dominant precipitation growth by vapor deposition versus accretion (riming and collection/coalescence) would have the property of yielding distinct populations in lowrain-rate regions of dbZ-log R space where both processes are known to occur. 2. Follow-up

The microphysical properties of tropical convective anvil cirrus: A comparison of models and observations

This paper describes the use of two-dimensional (2-D) cloud-resolving numerical model simulations of a period of active convection from the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment to generate statistics of the distribution of total ice water content (IWC) and of the relative contribution to IWC of small (sub-200 μm) particles with temperature. The model data are sampled in a way which excludes contributions from active convective updraught regions, so that the model results may be compared with similar such distributions derived from in situ sampling of a number of tropical anvil ice clouds in the vicinity of Kwajalein in the west Pacific. The model gives a good description of the mean and mode IWC in each of seven temperature ranges and, for temperatures warmer than –40°C, correctly predicts the dominant contribution of large particles to the IWC. At lower temperatures, the model retains an excessive fraction of its IWC in large particles. Estimates of the maximum crystal length (MCL) that would be sampled by a 2-D Optical Array Probe exceed the observed values in this temperature range but are otherwise in good agreement. Tests of the sensitivity of model results to the cloud ice bulk density, and the absence of a homogeneous freezing source of ice crystals, suggest that the excessive MCL values are due in part to excessive rates of autoconversion and aggregation of cloud ice.

An approach to parameterization of the oceanic turbulent boundary layer in the western Pacific warm pool

Vertical profiles of zonal velocity and the dissipation rate ε of the turbulent kinetic energy obtained during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (COARE) are analyzed in the context of planetary boundary layer relationships previously derived from atmospheric measurements. The presence of a barrier layer and the striking effect of increased dimensionless shear and ε at the bottom of the surface mixed layer of the ocean, features often observed in the western Pacific warm pool area, are consistent with the boundary layer laws. The gradient Richardson number Ri is found to be a convenient parameter for scaling the nonstationary and horizontally heterogeneous mixed layer in the warm pool area. The approach to parameterization of the turbulent eddy coefficient within the mixed layer as a function of Ri is tested as part of a one‐dimensional model. A comparison between the observed and modeled upper ocean temperatures for two COARE examples shows a reasonable agreement over a wide range of wind speed conditions.

TOGA-COARE IOP期間中に熱帯対流圏下層で観測された周期1～2時間の擾乱

Mesoscale disturbances in the tropical lower troposphere having periods of 1-2 hours were detected in the wind profiler data during the intensive observing period of the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Seven cases were detected with average amplitudes of the vertical velocity greater than 0.1ms-1.One case that was observed at Kapingamarangi (1.1°N, 154.8°E) on January 7, 1993 was examined in detail. This disturbance had a period of 60 minutes and a duration of 6 hours. Assuming that this disturbance was a gravity wave, the vertical and horizontal wavelengths were estimated as 5.4-7.6km and 24-37km, respectively. Using satellite and sounding data, it was inferred that the disturbance was excited by a mesoscale cloud line lying about 400km ESE from Kapingamarangi, and then propagated in a wave duct in the troposphere.

Bayesian estimation of precipitating cloud parameters from combined measurements of spaceborne microwave radiometer and radar

The objective of this paper is to evaluate the potential of a Bayesian inversion algorithm using microwave multisensor data for the retrieval of surface rainfall rate and cloud parameters. The retrieval scheme is based on the maximum a posteriori probability (MAP) method, extended for the use of both spaceborne passive and active microwave data. The MAP technique for precipitation profiling is also proposed to approach the problem of the radar-swath synthetic broadening; that is, the capability to exploit the combined information also where only radiometric data are available. In order to show an application to airborne data, two case studies are selected within the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). They refer to a stratiform storm region and an intense squall line of two mesoscale convective systems, which occurred over the ocean on February 20 and 22, 1993, respectively. The estimated rainfall rates and columnar hydrometeor contents derived from the proposed algorithms are compared to each other and to radar estimates based on reflectivity-rainrate (Z-R) relationships. Results in terms of reflectivity profiles and upwelling brightness temperatures, reconstructed from the estimated cloud structures, are also discussed. A database of combined measurements acquired at nadir during various TOGA-COARE flights, is used for applying the radar-swath synthetic broadening technique in the case of an along-track radar-failure countermeasure. A simulated test of the latter technique is performed using the case studies of February 20 and 22, 1993.

On the Determination of the Neutral Drag Coefficient in the Convective Boundary Layer

Based on the idea that free convection can be considered as a particular case of forced convection, where the gusts driven by the large-scale eddies are scaled with the Deardorff convective velocity scale, a new formulation for the neutral drag coefficient, CDn, in the convective boundary layer (CBL) is derived. It is shown that (i) a concept of CDn can still be used under strongly unstable conditions including a pure free-convection regime even when no logarithmic portion in the velocity profile exists; (ii) gustiness corrections must be applied for rational calculations of CDn; and (iii) the stratification Ψ function used in the derivation of CDn should satisfy the theoretical free-convection limit. The new formulation is compared with the traditional relationship for CDn, and data collected over the sea (during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) and the San Clemente Ocean Probing Experiment (SCOPE)) and over land (during the BOREX-95 experiment) are used to illustrate the difference between the new and traditional formulations. Compared to the new approach, the traditional formulation strongly overestimates CDn and zo in the CBL for mean wind speed less than about 2 m s-1. The new approach also clarifies several contradictory results from earlier works. Some aspects related to an alternate definition of the neutral drag coefficient and the wind speed and the stress averaging procedure are considered.