Tropical Extratropical Cloud Bands Research Articles

CloudbandPy 1.0: an automated algorithm for the detection of tropical–extratropical cloud bands

Abstract. Persistent and organized convective cloud systems that arise in convergence zones can lead to the formation of synoptic cloud bands extending from the tropics to the extratropics. These cloud bands are responsible for heavy precipitation and are often a combination of tropical intrusions of extratropical Rossby waves and processes originating from the tropics. Detecting these cloud bands presents a valuable opportunity to enhance our understanding of the variability of these systems and the underlying processes that govern their behavior and that connect the tropics and the extratropics. This paper presents a new atmospheric cloud band detection method based on outgoing longwave radiation using computer vision techniques, which offers enhanced capabilities to identify long cloud bands across diverse gridded datasets and variables. The method is specifically designed to detect extended tropical–extratropical convective cloud bands, ensuring accurate identification and analysis of these dynamic atmospheric features in convergence zones. The code allows for easy configuration and adaptation of the algorithm to meet specific research needs. The method handles cloud band merging and splitting, which allows for an understanding of the life cycle of cloud bands and their climatology. This algorithm lays the groundwork for improving our understanding of the large-scale processes that are involved in the formation and life cycle of cloud bands and the connections between tropical and extratropical regions as well as evaluating the differences in cloud band types between different ocean basins.

Extreme rainfall events over the Okavango river basin, southern Africa

Characteristics of extreme rainfall events in the highly biodiverse Okavango River Basin (ORB) in west central southern Africa are not well understood. Hence, this study analyses the frequencies, intensities, spatial distributions, variability and trends of extreme rainfall events for the main rainy season, January–April (JFMA). The events are accumulated over 1-day (DP1) and 3-days (DP3). To identify these events, a method that accounts for both event intensity and spatial extent is used. On average, DP1 (DP3) events contribute ∼10% (∼17%) rainfall totals but in some years their contributions exceed 30%. Tropical-extratropical cloud bands are responsible for most of the events with tropical lows also important. The considerable interannual variability in extreme events appears related to El Niño-Southern Oscillation (ENSO) and the Botswana High.Although ENSO influences the extreme events as well as rainfall totals more generally over southern Africa, by far the wettest season over the iconic Okavango Delta region, in Ngamiland, Botswana, occurred during neutral JFMA 2017. These heavy rains resulted from a deeper Angola Low, weaker mid-level Botswana High which favoured convection in the region together with anomalous westerly moisture fluxes from the tropical South East Atlantic and enhanced low level moisture convergence over most of the region during January-early March. A dry period from mid-March was broken by the second most intense rainfall event on April 22nd, resulting from a cut-off low.Significant increasing trends were found in DP1 frequencies, as well as in rain-days and rain totals over many areas. These trends have important implications for water and agricultural management, and wildlife conservation in the highly biodiverse ORB.

Rainfall onset and cessation over Mozambique and relationship with sea surface temperature and atmospheric patterns

AbstractMozambique is highly dependent on subsistence agriculture. Thus a climatology of rainfall onset and cessation is derived together with their interannual variability and trends using gridded daily data and station data. Mean onset is found to occur from early to mid‐November over southernmost regions, mid‐ to late November over central regions and late November to early December over the northern and coastal regions while cessation is from mid to late March over southern and late March to early April over most of central and northern Mozambique. The two gridded datasets showed differences (~30 days) in rainfall onset and cessation dates over southern Mozambique with standard deviations of 20–35 days. Timing of early onset over northern and central Mozambique appears associated with both zonal and meridional moisture convergence and tropical extratropical cloud bands, while timing of late onset is associated with southeast moisture fluxes from the subtropical South Indian Ocean and uplift over the landmass. Early (late) onset appears to be linked to La Niña (El Niño), particularly over central Mozambique. Rainfall cessation over most of Mozambique tends to be early (late) during El Niño (La Niña) summers. The relationship of ENSO with onset and cessation over large areas in Mozambique may help farmers and other user groups in planning their activities for the rainy season.

Characteristics of tropical–extratropical cloud bands over tropical and subtropical South America simulated by BAM‐1.2 and HadGEM3‐GC3.1

AbstractTropical–extratropical cloud bands are common in South America (SAm), contributing significantly to the total rainy season precipitation. Thus, it is fundamental that climate and weather forecast models correctly represent them and their associated dynamic aspects. Adopting an event‐based framework, we evaluate the performance of two global models in simulating the observed cloud bands over SAm: the Brazilian Global Atmospheric Model version 1.2 (BAM‐1.2) and the Hadley Centre Global Environment Model in the Global Coupled configuration 3.1 (HadGEM3‐GC3.1). Both models reproduce the main characteristics of cloud bands and the dynamical aspects leading to their development and persistence. Nonetheless, the biases in precipitation during simulated cloud bands contribute more than 50% of the bias in total precipitation in some regions. BAM‐1.2 simulates fewer but more persistent cloud bands than observed; HadGEM3‐GC3.1 simulates weaker cloud band activity during early summer and more persistent events after January than observed. In all models, the biases in cloud band events arise from the interaction between biases in the basic state and the synoptic‐scale regional circulation. In the basic state, stronger upper level westerlies over the midlatitude South Pacific support the propagation of longer and slower Rossby waves towards subtropical SAm, increasing the duration of the cloud band events. This bias interacts with negative biases in the upper level westerlies over subtropical SAm, increasing the wind shear, hindering the propagation of synoptic‐scale Rossby waves into lower latitudes, and resulting in biases in the cloud band location, intensity, and seasonality. The application in this study of an event‐based framework robust to differences in model resolution and complexity enables the identification of small but critical biases in circulation. These biases are linked to synoptic‐scale rainfall system biases and help to explain the season total rainfall model biases.

Rossby Wave Dynamics over South America Explored with Automatic Tropical–Extratropical Cloud Band Identification Framework

AbstractDuring austral summer, persistent tropical–extratropical cloud bands, such as the South Atlantic convergence zone (SACZ) over South America (SAm), link the tropical humid areas to the subtropics. In this study, we utilize an automatic object-based methodology to identify synoptic cloud band events occurring over SAm that are responsible for almost 60% of the precipitation during the rainy season (November–March). In addition to identifying SACZ events as cloud bands persisting 4 or more days, the framework also highlights the relevance of transient events (i.e., events persisting for 3 days or less) to the climatology. The location and persistence of the cloud band events are modulated by the propagation of synoptic-scale extratropical disturbances interacting with intraseasonal variability in the basic-state upper-level zonal wind. During persistent events (i.e., lasting 4 or more days), upper-level westerly anomalies over the subtropics favor the propagation extratropical disturbances deeper into the tropics. Conversely, transient events occur when the Bolivian high is displaced/expanded southeastward, bringing upper-level easterly winds into subtropical latitudes and blocking the propagation of Rossby waves into lower latitudes. Subsequent anomalous subtropical convection from the cloud bands result in sources of Rossby waves that interact with the basic flow, resulting in downwind enhancement or damping of the extratropical disturbances. The adopted methodology proved to be a powerful framework in demonstrating this interaction between scales, with the basic state influencing and being modified by the synoptic disturbances.

Coupled Climate Model Simulation of Tropical–Extratropical Cloud Bands over Southern Africa

AbstractThere are increasing efforts to use climate model output for adaptation planning, but meanwhile there is often limited understanding of how models represent regional climate. Here we analyze the simulation in global coupled climate models of a key rainfall-generating mechanism over southern Africa: tropical temperate troughs (TTTs). An image-processing algorithm is applied to outgoing longwave radiation data from satellites and models to create TTT event sets. All models investigated produce TTTs with similar circulation features to observed. However, there are large differences among models in the number, intensity, and preferred longitude of events. Five groups of models are identified. The first group generates too few TTTs, and relatively dry conditions over southern Africa compared to other models. A second group generates more TTTs and wet biases. The contrast between these two groups suggests that the number of TTTs could explain intermodel variations in climatological rainfall. However, there is a third group of models that simulate up to 92% more TTTs than observed, but do not have large rainfall biases, as each TTT event is relatively weak. Finally, there are a further two groups that concentrate TTTs over the subcontinent or the ocean, respectively. These distinctions between models are associated with the amount of convective activity in the Congo Basin, the magnitude of moisture fluxes into southern Africa, and the degree of zonal asymmetry in upper-level westerly flow. Model development focused on tropical convection and the representation of orography is needed for improved simulation of TTTs, and therefore southern African rainfall.

Variability in the Mozambique Channel Trough and Impacts on Southeast African Rainfall

AbstractThe Mozambique Channel trough (MCT) is a cyclonic region prominent in austral summer in the central and southern Mozambique Channel. It first becomes evident in December with a peak in strength in February when the Mozambique Channel is warmest and the Mascarene high (MH) is located farthest southeast in the Indian Ocean basin. The strength and the timing of the mean MCT are linked to that of the cross-equatorial northeasterly monsoon in the tropical western Indian Ocean, which curves as northwesterlies toward northern Madagascar. The interannual variability in the MCT is associated with moist convection over the Mozambique Channel and is modulated by the location of the warm sea surface temperatures in the south Indian Ocean. Variability of the MCT shows a strong relationship with the equatorial westerlies north of Madagascar and the latitudinal extension of the MH. Summers with strong MCT activity are characterized by a prominent cyclonic circulation over the Mozambique Channel, extending to the midlatitudes. These are favorable for the development of tropical–extratropical cloud bands over the southwestern Indian Ocean and trigger an increase in rainfall over the ocean but a decrease over the southern African mainland. Most years with a weak MCT are associated with strong positive south Indian Ocean subtropical dipole events, during which the subcontinent tends to receive more rainfall whereas Madagascar and northern Mozambique are anomalously dry.

A classification of synoptic weather patterns linked to extreme rainfall over the Limpopo River Basin in southern Africa

In the last few decades, the Limpopo River Basin (LRB) has experienced a number of extreme rainfall events which were responsible for considerable socio-economic and environmental impacts. Most of the population here is poor and dependent on rain-fed agriculture. In order to better understand these events over the LRB, CHIRPS, 0.05° gridded rainfall data are used to identify the daily extreme events, analyse their interannual variability and examine relationships with large scale climate modes over the 1981–2016 period. Analysis of the top 20 events suggests a pattern with rainfall generally decreasing from the eastern to western parts of the basin. Typically, the highest rainfall amounts occur over the regions where there are steep topographical gradients between the mountainous regions of northeastern South Africa and the Mozambican floodplains. Almost half of the top 200 extreme events are associated with tropical extra-tropical cloud bands (48%), with tropical low-pressure systems (28%), Mesoscale Convective Systems (14%), and cut-off lows (10%) in the mid-upper atmosphere, also making sizeable contributions. The monthly distribution of the events showed that most of the events occurred during the late summer months (January–March) when tropical lows and cloud bands are more common. On interannual time-scales, most of the summers with above average number of events coincide with La Nina conditions and, to lesser extent, a positive subtropical South Indian Ocean Dipole.

On the Likelihood of Tropical–Extratropical Cloud Bands in the South Indian Convergence Zone during ENSO Events

The Southern Hemisphere subtropical convergence zones are important regions of rainfall in the subtropics. The south Indian Ocean convergence zone (SICZ) has the strongest seasonality and exhibits substantial interannual variability in strength and position during austral summer. On synoptic time scales, the SICZ is a preferred region for the formation of tropical–extratropical (TE) cloud bands with local maxima over the southern African mainland and Madagascar. This study investigates how the seasonality in satellite-observed cloud band frequency emerges from the interplay between the asynchronous seasonal cycles in convective instability and upper-level flow, as represented by reanalysis data. These atmospheric mean states are diagnosed with a gross convective instability metric and a method to distinguish between subtropical and eddy-driven jet axes. Month-by-month analysis of these diagnostics elucidates how mean-state perturbations during ENSO events modify cloud band likelihood. Typically, 150%–200% more cloud bands develop during La Niña seasons supported by 5°–10° latitudinal separation between the local subtropical and eddy-driven jets and higher values of convective instability, especially in semiarid parts of mainland southern Africa. During El Niño events, fewer cloud bands develop over southern Africa in a more convectively stable environment without a distinct subtropical jet. However, east of Madagascar cloud bands are 150% more likely. Plausible teleconnection pathways based on these ENSO-related perturbations are discussed. The paper concludes with a conceptual framing of the seasonal cycle in the mean-state pertinent to TE cloud band likelihood.

Variability in the Botswana High and its relationships with rainfall and temperature characteristics over southern Africa

ABSTRACTThe seasonal and interannual variability of the mid‐tropospheric Botswana High and its relationships with various characteristics of rainfall and temperature over southern Africa are examined. This High typically forms in August and then strengthens and moves southward over southern Africa during the spring and summer. It also expands in zonal extent so that by March it is part of an anticyclonic ridge extending from the South Atlantic Convergence Zone eastwards over this ocean, across southern Africa and to the western Indian Ocean, with its greatest intensity in February. Its position is always to the south and southeast of the region of heavy precipitation across tropical southern Africa associated with the Inter Tropical Convergence Zone (ITCZ) and its meridional arm that passes through the eastern Congo Basin. Substantial interannual variability exists in the strength of the Botswana High. While the High is typically stronger (weaker) during El Niño (La Niña) events, there are also a number of neutral El Niño Southern Oscillation (ENSO) summers with large anomalies in the Botswana High. Neutral summers with an anomalously weak Botswana High are also characterized by a cyclonic anomaly extending from Angola south to the Antarctic and an anticyclonic anomaly over the South West Indian Ocean. These circulation patterns favour the development of tropical extratropical cloud bands, the main synoptic summer rainfall producing system, and the inflow of more moisture from the Indian Ocean towards the source of the cloud bands and hence increased rainfall. Roughly, the reverse circulation anomalies occur during neutral summers with anomalously large values of the Botswana High. Relationships are also found between the Botswana High and dry spell frequency, maximum temperature, diurnal temperature range and the number of days with extreme temperatures during the summer.

Intraseasonal Teleconnections between South America and South Africa

Abstract Teleconnection of climate anomalies between various parts of the tropics and extratropics is a well-established feature of the climate system. Building on previous work showing that a teleconnection exists between the South American monsoon system and interannual summer rainfall variability over southern Africa, this study considers intraseasonal variability over these landmasses. It is shown that strong teleconnections exist between South African daily rainfall and that over various areas of South America, with the latter leading by 4–5 days, for both winter and summer, involving regions with strong rainfall in these seasons. During the summer, the mechanisms involve both a modulation of the local Walker cell as well as extratropical Rossby wave trains. For winter, the latter mechanism is more important. While in summer tropical convective anomalies over South America play an important role, in winter the subtropics become more important. In both cases, these modulations lead to regional changes in circulation over southern Africa that are favorable for the dominant synoptic rainfall-producing weather systems such as cutoff lows and tropical extratropical cloud bands.

Role of Cross-Equatorial Waves in Maintaining Long Periods of Low Convective Activity over Southern Africa

Abstract Periods of low convective activity over southern Africa during the peak rainy season from December to February are known to be due to the northeastward displacement of the tropical temperate trough (TTT) systems from the landmass. In this study, using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, the authors show that the displacement of the TTT systems during long periods of low convective activity has origins in the Northern Hemisphere. Using standardized area-averaged outgoing longwave radiation (OLR) daily anomalies over southern Africa, long periods of low convective activity are defined as periods of positive OLR anomalies lasting consecutively for 5 or more days with a standard deviation of 1 or more. An eddy streamfunction anomaly composite of the periods of low convective activity shows an upper-level anomalous wave originating in the Northern Hemisphere and extending to southern Africa from the eastern Pacific and displacing the tropical–extratropical cloud bands from the southern African landmass into the southwestern Indian Ocean. The wave train is also seen to generate an anticyclonic anomaly over southern Africa, resulting in suppressed convective activity. Understanding the causes of the long periods of low convective activity will help in improving their predictability and also the predictability of seasonal rainfall over southern Africa.

Dry spell frequencies and their variability over southern Africa

Marked interannual fluctuations in rainfall are a fundamental aspect of southern African climate. This study uses dry spell frequency (DSF) to assess spatial and temporal patterns in the con- sistency of rainfall during the mid-summer (DJF) season and their relationships to interannual drought occurrences in southern Africa. The Climate Prediction Center Merged Analysis of Precipi- tation (CMAP) pentad data are used to identify dry spells, which are defined here as a pentad with mean daily rainfall less than 1 mm. It was found that DSFs over most of southern Africa are highest (lowest) during El Nino (La Nina) events and that their occurrence is associated with shifts in the loca- tion of the tropical-temperate-trough (TTT) systems that are the dominant rain-producing systems over much of southern Africa. The latter are tropical-extratropical cloudbands that link a tropical low over low latitude southern Africa with a westerly disturbance passing south of the landmass. A pre- ferred zone of occurrence of high DSFs, within which the interannual range is also highest, lies across the 20 to 25° S band in southern Africa and is identified as the drought corridor. This region often experiences half or more of the season under dry spells. In addition, a tendency during 1979 to 2002 towards an increasing frequency of heavier rainfall events was observed over Angola/Namibia in the west and Tanzania/Mozambique in the east. Only a small area in central southern Africa showed the reverse tendency.

Tropical–extratropical cloudbands and Australian rainfall: I. climatology

This paper examines the contribution of tropical–extratropical cloudbands, and of interactions between tropical cloud and mid-latitude systems, to cool season (April–October) rainfall in agriculturally marginal areas of Australia. A following paper describes inter- and intra-annual variability of these features. A classification scheme for these tropical influences based on GMS satellite imagery is described, and used to compile a 15-year archive of events. It is shown that cloudbands extending from the tropical oceans bordering Australia (‘Oceanic’ Cloudbands) are most frequent and influential between April and July, but decrease sharply after August, at which time bands originating over the continental interior (‘Continental’ Cloudbands) increase. The contribution of these systems to rainfall at stations representing agriculturally marginal areas is assessed. Oceanic Cloudbands originating west of 120°E contribute 70–90 per cent of cool-season rain in north-western Australia, with the contribution decreasing to the south and east. North-eastern Australia receives a significant portion of its rain from Cloudbands originating east of 120°E. Tropical–mid-latitude interactions are more important over eastern than western Australia, and produce some 30–40 per cent of rain over much of inland eastern Australia. The overall tropical influence (Cloudbands plus interactions) on rainfall is least in South Australia and western Victoria, but still amounts to some 35–40 per cent of cool-season rain in those areas. The proportion of events producing significant rainfall (>10 mm) is also examined: almost two-thirds of the Oceanic Cloudbands to affect western Australia produce significant rain, and about half of those affecting eastern Australia. © 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 807–829 (1997) (No. of Figures: 10. No. of Tables: 4. No. of

Tropical–extratropical cloudbands and Australian rainfall: I. climatology

This paper examines the contribution of tropical–extratropical cloudbands, and of interactions between tropical cloud and mid-latitude systems, to cool season (April–October) rainfall in agriculturally marginal areas of Australia. A following paper describes inter- and intra-annual variability of these features. A classification scheme for these tropical influences based on GMS satellite imagery is described, and used to compile a 15-year archive of events. It is shown that cloudbands extending from the tropical oceans bordering Australia (‘Oceanic’ Cloudbands) are most frequent and influential between April and July, but decrease sharply after August, at which time bands originating over the continental interior (‘Continental’ Cloudbands) increase. The contribution of these systems to rainfall at stations representing agriculturally marginal areas is assessed. Oceanic Cloudbands originating west of 120°E contribute 70–90 per cent of cool-season rain in north-western Australia, with the contribution decreasing to the south and east. North-eastern Australia receives a significant portion of its rain from Cloudbands originating east of 120°E. Tropical–mid-latitude interactions are more important over eastern than western Australia, and produce some 30–40 per cent of rain over much of inland eastern Australia. The overall tropical influence (Cloudbands plus interactions) on rainfall is least in South Australia and western Victoria, but still amounts to some 35–40 per cent of cool-season rain in those areas. The proportion of events producing significant rainfall (>10 mm) is also examined: almost two-thirds of the Oceanic Cloudbands to affect western Australia produce significant rain, and about half of those affecting eastern Australia. © 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 807–829 (1997) (No. of Figures: 10. No. of Tables: 4. No. of

Mixed layer modelling with respect to ocean-atmosphere interactions in the eastern Indian Ocean

As a precursor to the embedding of a thermal stratification model of the ocean into an Ocean General Circulation Model (OGCM) we have compared the performance of an integral (or bulk) (Kraus-Turner) and a differential (Eddy Diffusion Dimension 1) thermal stratification model for the eastern Indian Ocean. Forced with the Comprehensive Ocean-Atmosphere Data Set (COADS) and Subjectively Estimated Cloudiness in the Australian Region (SECAR) data sets, both models successfully stimulated the annual cycle in sea surface temperature. On the other hand, their predictions of the mixed layer depth differed substantially. Whereas the differential model slightly underestimated the winter mixed layer depth, the winter mixed layer depths calculated by the bulk model were substantially larger than the observed values. An additional analysis of the simulated heat storage cycle of the eastern Indian Ocean with respect to the activity of tropical-extratropical cloudbands in the Australian region has reveaved a strong relationship between these two parameters. The results have shown that a stronger activity of the tropical-extratropical cloudbands coincides with periods of heat transfer oriented from the ocean to the atmosphere.

Tropical‐extratropical cloudbands in the Australian region

AbstractThe climatology of the tropical‐extratropical cloudbands (TECBs) in the Australian region is examined, using a subdivision of the cloudband phenomena, according to their convective sources of origin. In addition to previously described parameters (e.g. amplitude of mid‐latitude troughs, cross‐equatorial flow, jet stream), the 1979–1983 TECB activity is analysed with respect to other indices, such as global pressure and sea‐surface temperature, the Southern Oscillation Index (SOI), 40–50‐day oscillation, Hadley cell circulation or cloudiness anomalies. It is shown that during a period marked by a significant decrease in the TECB activity, 5–6 months prior to the lowest value of the SOI, the TECB tends to develop in 40–50‐day intervals, whereas otherwise, its development can be associated with higher frequency (18–21 day) oscillations. Similar fluctuations have also been observed in tropical cloudiness in the TECB area of origin. On a large‐scale, the TECB activity seems to be closely linked to large‐scale pressure fluctuations in the Indian and northern Pacific Oceans.