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Michael C. Johnston
,
Christopher E. Holloway
, and
Robert S. Plant

Abstract

Cloud trails are primarily thermally forced bands of cloud that extend downwind of small islands. A novel algorithm to classify conventional geostationary visible-channel satellite images as cloud trail (CT), nontrail (NT), or obscured (OB) is defined. The algorithm is then applied to the warm season months of five years at Bermuda comprising 16 400 images. Bermuda’s low elevation and location make this island ideal for isolating the role of the island thermal contrast on CT formation. CTs are found to occur at Bermuda with an annual cycle, peaking in July, and a diurnal cycle that peaks in midafternoon. Composites of radiosonde observations and ERA-Interim data suggest that a warm and humid low-level environment is conducive for CT development. From a Lagrangian perspective, wind direction modulates CT formation by maximizing low-level heating on local scales when winds are parallel to the long axis of the island. On larger scales, low-level wind direction also controls low-level humidity through advection.

Full access
Samantha Ferrett
,
John Methven
,
Steven J. Woolnough
,
Gui-Ying Yang
,
Christopher E. Holloway
, and
Gabriel Wolf

Abstract

Equatorial waves are a major driver of widespread convection in Southeast Asia and the tropics more widely, a region in which accurate heavy rainfall forecasts are still a challenge. Conditioning rainfall over land on local equatorial wave phases finds that heavy rainfall can be between 2 and 4 times more likely to occur in Indonesia, Malaysia, Vietnam, and the Philippines. Equatorial waves are identified in a global numerical weather prediction ensemble forecast [Met Office Global and Regional Ensemble Prediction System (MOGREPS-G)]. Skill in the ensemble forecast of wave activity is highly dependent on region and time of year, although generally forecasts of equatorial Rossby waves and westward-moving mixed Rossby–gravity waves are substantially more skillful than for the eastward-moving Kelvin wave. The observed statistical relationship between wave phases and rainfall is combined with ensemble forecasts of dynamical wave fields to construct hybrid dynamical–statistical forecasts of rainfall probability using a Bayesian approach. The Brier skill score is used to assess the skill of forecasts of rainfall probability. Skill in the hybrid forecasts can exceed that of probabilistic rainfall forecasts taken directly from MOGREPS-G and can be linked to both the skill in forecasts of wave activity and the relationship between equatorial waves and heavy rainfall in the relevant region. The results show that there is potential for improvements of forecasts of high-impact weather using this method as forecasts of large-scale waves improve.

Open access
Mohd Fadzil Firdzaus Mohd Nor
,
Christopher E. Holloway
, and
Peter M. Inness

Abstract

Severe rainfall events are common in western Peninsular Malaysia. They are usually short and intense, and occasionally cause flash floods and landslides. Forecasting these local events is difficult and understanding the mechanisms of the rainfall events is vital for the advancement of tropical weather forecasting. This study investigates the mechanisms responsible for a local heavy rainfall event on 2 May 2012 that caused flash floods and landslides using both observations and simulations with the limited-area high-resolution Met Office Unified Model (MetUM). Results suggest that previous day rainfalls over Peninsular Malaysia and Sumatra Island influenced the development of overnight rainfall over the Strait of Malacca by low-level flow convergence. Afternoon convection over the Titiwangsa Mountains over Peninsular Malaysia then induced rainfall development and the combination of these two events influenced the development of severe convective storm over western Peninsular Malaysia. Additionally, anomalously strong low-level northwesterlies also contributed to this event. Sensitivity studies were carried out to investigate the influence of the local orography on this event. Flattened Peninsular Malaysia orography causes a lack of rainfall over the central part of Peninsular Malaysia and Sumatra Island and produces a weaker overnight rainfall over the Strait of Malacca. By removing Sumatra Island in the final experiment, the western and inland parts of Peninsular Malaysia would receive more rainfall, as this region is more influenced by the westerly wind from the Indian Ocean. These results suggest the importance of the interaction between landmasses, orography, low-level flow, and the diurnal cycle on the development of heavy rainfall events.

Free access