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Peter T. May and Greg J. Holland

Abstract

The implied heating and potential vorticity generation in tropical cyclone rainbands is derived from observed vertical motion profiles. High levels of potential vorticity generation are found in the stratiform rain regions, sufficient to generate substantial wind maxima along the bands within a couple of hours. Such generation may represent a significant source of potential vorticity for the system as a whole and may have implications for cyclone intensity.

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Vickal V. Kumar, Alain Protat, Christian Jakob, and Peter T. May

Abstract

Some cumulus clouds with tops between 3 and 7 km (Cu3km–7km) remain in this height region throughout their lifetime (congestus) while others develop into deeper clouds (cumulonimbus). This study describes two techniques to identify the congestus and cumulonimbus cloud types using data from scanning weather radar and identifies the atmospheric conditions that regulate these two modes. A two-wet-season cumulus cloud database of the Darwin C-band polarimetric radar is analyzed and the two modes are identified by examining the 0-dBZ cloud-top height (CTH) of the Cu3km–7km cells over a sequence of radar scans. It is found that ~26% of the classified Cu3km–7km population grow into cumulonimbus clouds. The cumulonimbus cells exhibit reflectivities, rain rates, and drop sizes larger than the congestus cells. The occurrence frequency of cumulonimbus cells peak in the afternoon at ~1500 local time—a few hours after the peak in congestus cells. The analysis of Darwin International Airport radiosonde profiles associated with the two types of cells shows no noticeable difference in the thermal stability rates, but a significant difference in midtropospheric (5–10 km) relative humidity. Moister conditions are found in the hours preceding the cumulonimbus cells when compared with the congestus cells. Using a moisture budget dataset derived for the Darwin region, it is shown that the existence of cumulonimbus cells, and hence deep convection, is mainly determined by the presence of the midtroposphere large-scale upward motion and not merely by the presence of congestus clouds prior to deep convection. This contradicts the thermodynamic viewpoint that the midtroposphere moistening prior to deep convection is solely due to the preceding cumulus congestus cells.

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Alison W. Grimsdell, M. Joan Alexander, Peter T. May, and Lars Hoffmann

Abstract

Atmospheric gravity waves have a major effect on atmospheric circulation, structure, and stability on a global scale. Gravity waves can be generated by convection, but in many cases it is difficult to link convection directly to a specific wave event. In this research, the authors examine an event on 12 January 2003 when convective waves were clearly generated by a period of extremely intense rainfall in the region of Darwin, Australia, during the early morning. The waves were observed by the Atmospheric Infrared Sounder (AIRS) instrument on board the Aqua satellite, and a dry version of a nonlinear, three-dimensional mesoscale cloud-resolving model is used to generate a comparable wave field. The model is forced by a spatially and temporally varying heating field obtained from a scanning radar located north of Darwin at Gunn Point. With typical cloud-resolving model studies it is generally not possible to compare the model results feature-for-feature with observations since although the model precipitation and small-scale heating may be similar to observations, they will occur at different locations and times. In this case the comparison is possible since the model is forced by the observed heating pattern. It is shown that the model output wave pattern corresponds well to the wave pattern observed by the AIRS instrument at the time of the AIRS overpass.

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Vickal V. Kumar, Christian Jakob, Alain Protat, Christopher R. Williams, and Peter T. May

Abstract

Cumulus parameterizations in weather and climate models frequently apply mass-flux schemes in their description of tropical convection. Mass flux constitutes the product of the fractional area covered by convection in a model grid box and the vertical velocity in cumulus clouds. However, vertical velocities are difficult to observe on GCM scales, making the evaluation of mass-flux schemes difficult. Here, the authors combine high-temporal-resolution observations of in-cloud vertical velocities derived from a pair of wind profilers over two wet seasons at Darwin with physical properties of precipitating clouds [cloud-top heights (CTH), convective–stratiform classification] derived from the Darwin C-band polarimetric radar to provide estimates of cumulus mass flux and its constituents. The length of this dataset allows for investigations of the contributions from different cumulus cloud types—namely, congestus, deep, and overshooting convection—to the overall mass flux and of the influence of large-scale conditions on mass flux. The authors found that mass flux was dominated by updrafts and, in particular, the updraft area fraction, with updraft vertical velocity playing a secondary role. The updraft vertical velocities peaked above 10 km where both the updraft area fractions and air densities were small, resulting in a marginal effect on mass-flux values. Downdraft area fractions are much smaller and velocities are much weaker than those in updrafts. The area fraction responded strongly to changes in midlevel large-scale vertical motion and convective inhibition (CIN). In contrast, changes in the lower-tropospheric relative humidity and convective available potential energy (CAPE) strongly modulate in-cloud vertical velocities but have moderate impacts on area fractions. Although average mass flux is found to increase with increasing CTH, it is the environmental conditions that seem to dictate the magnitude of mass flux produced by convection through a combination of effects on area fraction and velocity.

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Robert Schafer, Peter T. May, Thomas D. Keenan, Kendal McGuffie, Warner L. Ecklund, Paul E. Johnston, and Kenneth S. Gage

Abstract

Data collected during the Maritime Continent Thunderstorm Experiment (MCTEX) (10 November–10 December 1995) have been used to analyze boundary layer development and circulations over two almost flat, tropical islands. The two adjacent islands have a combined length of about 170 km from east to west and 70 km from north to south. Intense thunderstorms formed over these islands every day of the field campaign. The boundary layer depth, temperature, and circulation over the island have been measured over the full diurnal cycle using a multiple radar analysis combined with surface and radiosonde measurements. On average, the island boundary layer depth reaches 1.5 km by early to midafternoon coinciding with the development of the deep convection. Thus, the island boundary layer is significantly deeper than the typical tropical oceanic boundary layer. In the midafternoon, thunderstorm outflows and their associated cold pool stabilize the lower boundary layer, suppressing late convection. This is followed by a period of partial boundary layer recovery for 1–2 h. After sunset, cooling leads to a deepening ground-based inversion below a residual mixed layer. Near the island center, the residual mixed layer of island-modified air is replaced by air of oceanic origin by about 2300 LST (local standard time) that then persists until sunrise the next day. The advection of boundary layer air of oceanic origin over the islands every evening resets the boundary layer development cycle. It is shown that much of the variation in the diurnal temperature profile is a result of thunderstorm activity, radiative processes, and the advection of island and oceanic boundary layer air.

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