The first Forecasters Handbook for West Africa

Bridging the gap between rapidly moving scientific research and specific forecasting tools, Meteorology of Tropical West Africa: The Forecasters' Handbook’, gives unprecedented access to the latest science and combines this with pragmatic approaches to forecasting. It is set to change the way forecasters, researchers and students learn about tropical meteorology and will serve to drive demand for new forecasting tools. The Handbook builds upon the legacy of the AMMA project, making the latest science applicable to forecasting in the region. By bringing together, at the outset, researchers and forecasters from across the region, and linking to applications, user communities and decision-makers, the Forecasters’ Handbook provides a template for finding much needed solutions to critical issues such as building resilience to climate change in West Africa

Genesis of Twin Tropical Cyclones as Revealed by a Global Mesoscale Model: The Role of Mixed Rossby Gravity Waves

In this study, it is proposed that twin tropical cyclones (TCs), Kesiny and 01A, in May 2002 formed in association with the scale interactions of three gyres that appeared as a convectively-coupled mixed Rossby gravity (ccMRG) wave during an active phase of the Madden-Julian Oscillation (MJO). This is shown by analyzing observational data and performing simulations using a global mesoscale model. A 10-day control run is initialized at 0000 UTC 1 May 2002 with grid-scale condensation but no cumulus parameterizations. The ccMRG wave was identified as encompassing two developing and one non-developing gyres, the first two of which intensified and evolved into the twin TCs. The control run is able to reproduce the evolution of the ccMRG wave and the formation of the twin TCs about two and five days in advance as well as their subsequent intensity evolution and movement within an 8-10 day period. Five additional 10-day sensitivity experiments with different model configurations are conducted to help understand the interaction of the three gyres. These experiments suggest the improved lead time in the control run may be attributed to the realistic simulation of the ccMRG wave with the following processes: (I) wave deepening associated with wave shortening and/or the intensification of individual gyres, (2) poleward movement of gyres that may be associated with bOlll1dary layer processes, (3) realistic simulation of moist processes at regional scales in association with each of the gyres, and (4) the vertical phasing of low- and mid-level cyclonic circulations associated with a specific gyre

Cycles and Propagation of Deep Convection over Equatorial Africa

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

Predicting Tephra Dispersion with a Mesoscale Atmospheric Model and a Particle Fall Model: Application to Cerro Negro Volcano

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

The water cycle across scales

Despite the importance of water and a tremendous body of work researching its processes, critical questions regarding the global water cycle remain. These questions range from fundamental details of the spectroscopy of water, to the forecasting of precipitation, to closing the budget of the hydrological cycle on many scales. This paper discusses integrating themes and strategic opportunities identified during the first National Center for Atmospheric Research (NCAR) Early Career Scientist Association Junior Faculty Forum on Future Scientific Directions in June 2003. These include: transport and transformations (atmospheric convection as prototype issue); developing collaborative research; fostering operational-research linkages; the role of observations in water-cycle research; and improved metrics for evaluating theories and models