Abstract

The environments associated with three episodes of heavy precipitation and flash floods in the Caribbean are diagnosed. Analysis of the hydrometeorological conditions leading up to flash floods on 3–4 January 1998, 5–6 January 1992, and 4 March 1998 are focused on the synoptic features as well as the surface conditions. Subsequent flood mitigation efforts are briefly discussed.

In the first case, deep convection and heavy precipitation were associated with a surface trough that developed in the wake of a quasi-stationary front. Warm, moist unstable air conveyed by a low-level jet and impinging on steep terrain created a quasi-stationary cloud cluster that produced more than 400 mm of rainfall in 2 days. Upper-level divergence and weak midtropospheric vorticity advection enhanced ascending motion. Antecedent precipitation from the front, the steep terrain, river basin topography, and human encroachment in the flood plain aggravated the flood hazard. The second case had similar conditions, with additional lift induced by an upper-level trough. The third case was weaker than the first two primarily because its low-level airflow was northerly and weaker. In all cases, the orography, the low-level wind velocity, a deep layer of moisture, and potential instability played important roles. These findings agree with other studies of heavy orographic precipitation from convection. Interestingly, these cases of heavy precipitation occurred during what is normally the dry season in the Caribbean. However, during El Niño years, midlatitude systems track well south of their normal tracks. Fronts, prefrontal troughs, and upper-level low pressure systems can then contribute to the development of deep convection and heavy precipitation in the Caribbean.

Abstract

Cloud-to-ground (CG) lightning flashes from the National Lightning Detection Network are analyzed to determine if the El Niño–Southern Oscillation (ENSO) cycle influences lighting activity along the Gulf Coast region. First, an updated climatology of lightning was developed for the region. Flash density maps are constructed from an 8-yr dataset (1995–2002) and compared with past lightning climatologies. Second, lightning variability is compared with the phases of ENSO. Winter lightning distributions are compared with one published study of ENSO and lightning days in the Southeast.

Flash density patterns are, overall, consistent with past U.S. lightning climatology. However, the peak flash density for the annual mean was less than observed in previous climatologies, which could be due to the disproportionately large percentage of cool ENSO periods compared to previous lightning climatologies.

The highest annual lightning counts were observed in 1997, which consisted of mostly warm ENSO seasons; the 1997–98 El Niño was one of the strongest on record. The lowest lightning counts were observed in 2000, which had mostly cool or neutral phases of ENSO including the lowest Niño-3.4 anomaly of the study period. Analysis of winter season lightning flash densities substantiated the role of the ENSO cycle in winter season lightning fluctuations. Winter lightning activity increased dramatically during the 1997–98 El Niño. The lowest winter flash densities are associated with cool ENSO phases. Although 8 yr is inadequate to establish a long-term pattern, results indicate that ENSO influences lightning and that further study is warranted. As more years of lightning data are acquired, a more complete climatology can be developed.

Abstract

Digitized full-disk infrared satellite imagery from the European geostationary satellite (Meteosat) for 1986 and 1987 was used to construct a climatology of mesoscale convective complexes (MCCs) in Africa One hundred ninety-five systems formed over Africa and its near vicinity during the two-year study period. From this database, characteristics of African MCCs were calculated. The results indicate that these MCCs display many of the same characteristics as those found in the Americas, the Indian subcontinent, and the western Pacific region. The systems are predominantly nocturnal and tend to form over or in the immediate vicinity of land. Much of the activity occurs over the African Sahel. while comparatively little occurs over the equatorial rain forest. The average lifetime of African MCCs is about 11.5 h, whereas systems in the western Pacific region and the Americas last about 11 and 10 h, respectively. The size distributions of the African systems are also extremely similar to those of the Americas, the Indian subcontinent, and the western Pacific region, with most systems exhibiting areas between 2 × 10⁵ and 3 × 10¹ km². The monthly frequency distribution of African systems indicates that peak activity tends to occur during the period of most intense insulation. Like the MCCs in the western Pacific region and the Americas, the African MCCs tend to propagate toward the low-level high-θ _e air that feeds the convective systems. Systems over northern Africa moved toward the west-southwest, with a few developing into tropical cyclones over the Atlantic. Systems over southeastern Africa generally moved toward the northeast and east.

It is concluded that the satellite-observed systems over Africa are essentially the same phenomena as the MCC populations observed over the Americas, the Indian monsoon region, and the western Pacific region. In addition, the large number of MCCs found worldwide (approximately 300–400 per year) indicate that they may be significant contributors to the global tropospheric energy budget and hydrological cycle.

Abstract

The mean genesis environment was constructed for each of five mesoscale convective complex (MCC) population centers around the world: Africa, Australia, China, South America, and the United States. It is found that the environments are very similar and exhibit many of the same dynamic and thermodynamic structures that are present with systems in the United States. In particular, MCCs initiate within prominent baroclinic zones characterized by locally large values of lower-tropospheric vertical wind shear and convective available potential energy (CAPE). Typically, a low-level jet of air with low static stability, high equivalent potential temperature, oriented nearly perpendicular to the baroclinic zone, intrudes into the genesis region and is forced to ascend over a relatively shallow, surface-based layer of relatively cool air. Pronounced warm advection accompanied by strong lower-tropospheric veering overlays the surface-based cool layer. A local maximum in absolute humidity and a local minimum in static stability mark the favored region for formation of the convective system. Low-level convergence, upper-level divergence, and an approaching midlevel vorticity maximum associated with a weak short-wave trough are also typical of the mean genesis environment.

Abstract

The El Niño–Southern Oscillation (ENSO) cycle is known to influence weather and climate along the Gulf Coast region, causing anomalously high precipitation during El Niño winters. This region is also known for having the highest lightning flash density in the United States. An 8-yr dataset (1995–2002) of cloud-to-ground (CG) lightning flashes was analyzed to determine if the ENSO cycle influences lighting activity along the Gulf Coast region. Simple Pearson’s correlations were computed between concurrent monthly pairings of Niño-3.4 sea surface temperature (SST) and CG lightning flash deviation values from the study area. The correlation results are mapped and analyzed for links to meteorological features.

Statistically significant correlation values greater than 0.8 were noted over large swaths of the study area during each winter month. The highest correlations were arranged in banded swaths and associated with regions of low flash densities during December and February. In January, areas of high correlation were spatially coincident with areas of enhanced flash density. Both the enhanced CG flash regions and high correlation values and patterns are indicative of a southerly shift in the midlatitude storm track known to occur during warm ENSO events. During the spring and summer, most of the region has weak correlation with ENSO except for August, which has a large area of negative correlations. These findings indicate that lightning increases during La Niña summers. Correlation patterns in late fall are similar to those of winter. The ENSO–lightning relationship has implications for hazard assessment and can be a useful tool for long-term seasonal planning.

Abstract

The West African monsoon (WAM) is a vital source of rainfall for the African Sahel. In addition to the agricultural benefit of its rains, it benefits public health because bacterial meningitis outbreaks end with the monsoon onset. Outbreaks occur between December and May, a period of low humidity. Knowledge of the onset of high humidity could aid in predicting where the outbreaks will cease. Therefore, this study investigates the variability of atmospheric moisture during the spring over West Africa, characterizing the sources of moisture, as well as circulation patterns and relative influences of tropical and midlatitude systems. A conceptual model of the evolution of the premonsoon period is presented.

The meridional and temporal variability of surface moisture during the spring is modulated by multiscale interactions, as illustrated for the period from mid-April to early May 2009. As westward-propagating, synoptic disturbances move across West Africa, a corresponding peak occurs in the surface relative humidity. With the passage of each disturbance a new and more humid regime is established. Filtered anomalies of outgoing longwave radiation (OLR) indicate that Kelvin waves, equatorial Rossby waves, and possibly the MJO contributed to the initiation and intensification of the synoptic disturbances. During the last of the disturbances, whose passage raised the relative humidity above 40%, a critical threshold for meningitis, an extratropical cyclone also contributed to moisture influx over the Sahel. Analysis of the period 2000–09 shows the relative influences of synoptic and subseasonal circulations on the onset of high relative humidity over the Sahel during the spring.

Abstract

Models of volcanic ash (tephra) fallout are increasingly used to assess volcanic hazards in advance of eruptions and in near–real time. These models often approximate the wind field using simplistic assumptions of the atmosphere that do not account for four-dimensional variations in wind velocity. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) is used to improve forecasts of tephra dispersion. MM5 is a 3D model that can predict circulations in areas with sparse meteorological observations and complex terrain, such as volcanic plateaus. MM5 is applied to the 1995 eruption of Cerro Negro, Nicaragua. Validation of MM5 is achieved by comparing the simulated winds with rawinsonde observations. Estimates of diffusivity and particle settling velocities are used in conjunction with MM5-generated wind fields to forecast the major axes of the tephra dispersion. The predicted axes of dispersion derived from the MM5 winds approximate very closely the observed bilobate tephra accumulation and tephra plumes observed in satellite images. MM5 winds provide far more accurate spatial and temporal forecasts than do the wind assumptions that had been used previously to assess Cerro Negro tephra hazard.

Abstract

The contribution of mesoscale convective complexes to the July–September rainfall in Sahelian Africa is assessed using passive microwave data from the Special Sensor Microwave Image and infrared (IR) data from the European Geostationary Meteorological Satellite (Meteosat). A simple precipitation-estimation procedure, which takes advantage of the good time resolution of the IR and the strong relationship between the microwave radiance and rainfall, is developed and applied. The microwave technique uses the 37- and 86-GHz brightness temperatures to define the rain areas and the 86-GHz ice scattering signal to determine the rainfall intensity. The IR cloud shield areas are defined by the 219 K threshold.

Regression analyses are used to relate the microwave-derived precipitation characteristics of the system and the IR data closest to the time of the SSM/I observation. These relationships are used to compute the precipitation characteristics of the total set of systems and to determine their monthly rainfall contribution.

Results indicate that these systems have precipitation characteristics, such as rain area and volume, which are of the same order of magnitude as systems in the United States. In addition, they provide a significant fraction of the rainfall in Sahelian Africa.

Abstract

Long-term statistics of organized convection are vital to improved understanding of the hydrologic cycle at various scales. Satellite observations are used to understand the timing, duration, and frequency of deep convection in equatorial Africa, a region with some of the most intense thunderstorms. Yet little has been published about the propagation characteristics of mesoscale convection in that region. Diurnal, subseasonal, and seasonal cycles of cold cloud (proxy for convective precipitation) are examined on a continental scale. Organized deep convection consists of coherent structures that are characteristic of systems propagating under a broad range of atmospheric conditions. Convection is triggered by heating of elevated terrain, sea/land breezes, and lake breezes. Coherent episodes of convection result from regeneration of convection through multiple diurnal cycles while propagating westward. They have an average 17.6-h duration and 673-km span; most have zonal phase speeds of 8–16 m s⁻¹.

Propagating convection occurs in the presence of moderate low-level shear that is associated with the southwesterly monsoonal flow and midlevel easterly jets. Convection is also modulated by eastward-moving equatorially trapped Kelvin waves, which have phase speeds of 12–22 m s⁻¹ over equatorial Africa. Westward propagation of mesoscale convection is interrupted by the dry phase of convectively coupled Kelvin waves. During the wet phase, daily initiation and westward propagation continues within the Kelvin wave and the cold cloud shields are larger. Mesoscale convection is more widespread during the active phase of the Madden–Julian oscillation (MJO) but with limited westward propagation. The study highlights multiscale interaction as a major source of variability in convective precipitation during the critical rainy seasons in equatorial Africa.

Abstract

A large-domain convection-permitting numerical model is used to simulate episodes of deep convection, which are generated during the day over the Ethiopian Highlands and then propagate westward over the eastern and central Sahel region (5°–20°N) of northern Africa. The simulation comprises 12.5 days within the African Monsoon Multidisciplinary Analysis (AMMA) field campaign in 2006. During this period, long-lived precipitation episodes that survived beyond a single diurnal cycle occurred in the lee of the Ethiopian Highlands only every 2–3 days in both the simulation and observations. This contrasts with some other latitudinal corridors in the lee of major topography, such as the central United States, where long-lived heavy precipitation episodes frequently occur on successive nights.

The intermittency of long-lived events for the current case occurs despite regular daily triggering of convection along the upstream orography, and is linked to strong lower-tropospheric stabilization and reduction of daytime surface sensible heat flux due to residual cloudiness in the wake of long-lived precipitation events during the previous diurnal cycle. The vertical shear that helps organize deep convection is also weakened in the wake of the long-lived events by temporary disruptions of the midtropospheric African easterly jet.

The environments of mesoscale convection are presented for the eastern Sahel, a region where most Sahelian convection originates, but about which little is known at the mesoscale. The study describes the potential for early identification of long-lived convection episodes that are likely to have high impact on the central Sahel and West Africa.