Cut-off lows in the Southern Hemisphere and their extension to the surface

Many cut-off low (COL) climatologies have been done throughout the Southern Hemisphere. Few have focused on COL vertical depth and their link to surface cyclones that often accompany these systems. Here we extend these climatologies in order to gain an understanding of the spatial, mobility, temporal, and seasonal variability of COL extensions towards the surface. Deep COLs (dCOLs), with extension all the way to the surface, are most frequent in the autumn months, are longer lasting, are more mobile and found most frequently situated in the high latitudes. They are usually collocated with Rossby wave breaking (RWB) on multiple isentropic surfaces. These RWB events drive high potential vorticity air into the upper troposphere. The depths of these intrusions are also shown to be critical to the development of COL extensions with dCOLs associated with deeper intrusions into the mid-troposphere. Upper-level PV features are collocated with warm surface potential temperature anomalies which can play a critical role in surface cyclogenesis. The warm surface potential temperature features, when out of phase with coupled upper tropospheric processes (surface features lagging behind upper level processes), can inhibit surfaceward extension and result in shallow COL (sCOL) development. Composite analysis shows that dCOLs that drive their own surface low development result in the simultaneous amplification of troughs throughout the troposphere, with the surface cyclone developing within a day of the COL.

     

Interpretation of Localized Deficits in Coronal Emission

Coronal images recorded above the limb in Fexiv (530.3 nm) and Fex (637.5 nm) sometimes have localized regions of anomalously low emission, with the appearance of an abrupt gap in the background corona. These dark spaces have been previously described in the literature in the case of the 530.3 nm line and tentatively explained by reduced coronal plasma density and/or a decrease in the line intensity due to temperatures above or below the optimal ionization temperature for Fexiv. However, loops are sometimes observed spanning gaps, with diminished loop brightness over the region of the gap. It is concluded that at least some of these regions of reduced brightness are caused by absorption of the coronal emission. An analysis reveals that absorption by coronal ions is inadequate as a mechanism to explain the phenomenon. Absorption by neutral hydrogen is, however, consistent with the observations in terms of the reduced brightness of the gaps. The concentration of cool material in the coronal environment associated with large magnetic fields on the disk could explain the gaps. Hence, neutral hydrogen continuum absorption appears to provide a plausible interpretation of, at least, some coronal gaps. Based on this result and from measured intensities, the electron density in the region of a gap is derived and found to be consistent with estimates derived elsewhere.     

Proton collisional excitation in the ground configuration of Fe+12

Values of the proton collisional excitation and deexcitation cross sections for all transitions between the Fe⁺¹² ground configuration levels are calculated using semi-classical Coulomb excitation theory. Rate constants for these processes are then derived for coronal temperatures and are shown to be comparable in all cases to the corresponding electron rate constants.     

A study of the He λ 10830 line emission from quiescent prominences

Observations of the He 10830 line emission in a number of quiescent prominences are presented. The line shapes are analyzed to obtain values for the Doppler widths and optical thicknesses, leading to a determination of the 2³ P – 2³ S excitation temperature of 4020 K. The results are compared with those of previous observational and theoretical research.     

Climatological and structural differences between wet and dry troughs over southern Africa in the early summer

Summary Differences in the convective potential of troughs passing over the plateau of southern Africa in the early summer are assessed using operational synoptic weather data and radiosonde time-height sections. Wet and dry trough cases are chosen on the basis of the intensity and distribution of rainfall, sharp thermodynamic changes across the plateau and the passage of a geopotential wave. Composite differences are computed and indicate a high-low geopotential anomaly in the east-west direction, and a threefold increase in precipi-table water from 15 mm in dry cases to 28 mm in wet cases. The 500 hPa structure obtained by differencing wet and dry composites is dominated by low geopotentials and cyclonic vorticity over the plateau near 25°S, 25°E, and high geopotentials and anticyclonic vorticity to the south over the oceans near 40°S, 30°E. The dipole anomaly suggests a diffluent tilted baroclinic wave in the subtropical jet stream in convective cases. A double jet streak structure in the wet events enhances upper divergence which contributes to widespread uplift in the mid-troposphere. A case study comparison highlights the importance of pre-frontal moist influx, the kinematic trigger and thermodynamic instability.With 13 Figures     

Simulations of an isolated two-dimensional thunderstorm: Sensitivity to cloud droplet size and the presence of graupel

Cloud Resolving Models (CRMs) which are used increasingly to make operational forecasts, employ Bulk Microphysics Schemes (BMSs) to describe cloud microphysical processes. In this study two BMSs are employed in a new Nonhydrostatic σ-coordinate Model to perform two hour simulations of convection initiated by a warm bubble, using a horizontal grid resolution of 500 m. Different configurations of the two BMSs are applied, to test the effects of the presence of graupel with one scheme (2-configurations) and of changing the cloud droplet sizes in the second scheme (4-configurations), on the simulation of idealised thunderstorms. Maximum updrafts in all the simulations are similar over the first 40 minutes, but start to differ beyond this point. The first scheme simulates the development of a second convective cell that is triggered by the cold pool that develops from the outflow of the first storm. The cold pool is more intense in the simulation with graupel because of melting of graupel particles, which results in relatively large raindrops, decreases the temperature through latent heat absorption, causing stronger downdrafts, which all contribute to the formation of a more intense cold pool. The second scheme simulates the development of a second cell in two of its configurations, while two other configurations do not simulate the redevelopment. Two configurations that simulate the secondary redevelopment produce a slightly stronger cold pool just before redevelopment. Our results show that small differences in the microphysics formulations result in simulations of storm dynamics that diverge, possibly due nonlinearities in the model.     

Dynamical seasonal prediction of Southern African summer precipitation

Prediction skill for southern African (16°–33°E, 22°–35°S) summer precipitation in the Scale Interaction Experiment-Frontier coupled model is assessed for the period of 1982–2008. Using three different observation datasets, deterministic forecasts are evaluated by anomaly correlation coefficients, whereas scores of relative operating characteristic and relative operating level are used to evaluate probabilistic forecasts. We have found that these scores for December–February precipitation forecasts initialized on October 1st are significant at 95 % confidence level. On a local scale, the level of prediction skill in the northwestern and central parts of southern Africa is higher than that in northeastern South Africa. El Niño/Southern Oscillation (ENSO) provides the major source of predictability, but the relationship with ENSO is too strong in the model. The Benguela Niño, the basin mode in the tropical Indian Ocean, the subtropical dipole modes in the South Atlantic and the southern Indian Oceans and ENSO Modoki may provide additional sources of predictability. Within the wet season from October to the following April, the precipitation anomalies in December-February are the most predictable. This study presents promising results for seasonal prediction of precipitation anomaly in the extratropics, where seasonal prediction has been considered a difficult task.