November Rain

The Wind River old-growth forest, in the southern Cascade Range of Washington State, is a cool (average annual temperature, 8.7°C), moist (average annual precipitation, 2223 mm), 500-year-old Douglas-fir–western hemlock forest of moderate to low productivity at 371-m elevation on a less than 10% slope. There is a seasonal snowpack (November–March), and rain-on-snow and freezing-rain events are common in winter. Local geology is characterized by volcanic rocks and deposits of Micocene/Oligocene Micocene-Oligocene (mixed) Micocene and Quaternary age, as well as intrusive rocks of Miocene age. Soils are medial, mesic, Entic Vitrands that are deep (2–3 m), well drained, loams and silt loams, generally stone free, and derived from volcanic tephra. The vegetation is transitional, between the Western Hemlock Zone and the Pacific Silver Fir Zone, and the understory is dominated by vine maple, salal, and Oregon grape. Stand structural parameters have been measured on a 4-ha plot. There are eight species of conifers, with a stand density of 427 trees ha−1 and basal area of 82.9 m2 ha−1. Dominant conifers include Douglas-fir (35 trees ha−1), western hemlock (224 trees ha−1), Pacific yew (86 trees ha−1), western red cedar (30 trees ha−1), and Pacific silver fir (47 trees ha−1). The average height of Douglas-fir is 52.0 m (tallest tree, 64.6 m), whereas western hemlock averages 19.0 m (tallest tree, 55.7 m). The regional disturbance regime is dominated by high-severity to moderate-severity fire, from which this forest is thought to have originated. There is no evidence that fire has occurred in the forest after establishment. Primary agents of stand disturbance, which act at the individual to small groups of trees scale, are wind, snow loads, and drought, in combination and interacting with root-rot and butt-rot fungi, heart-rot fungi, dwarf mistletoe, and bark beetles. The forest composition is slowly shifting from dominance by Douglas-fir, a shade-intolerant species, to western hemlock, western red cedar, Pacific yew, and Pacific silver fir, all shade-tolerant species. The Wind River old-growth forest fits the regional definition of Douglas-fir “old growth” on western hemlock sites.

Strong winds and heavy rains pummeled the North African coast over the period from 9 November 10 to November 2001 in association with the movement of a deep upper level trough over the region from the northwest. Heavy rain fell in the city of Algiers beginning late Friday November 9 and ending near noon on Saturday November 10. Over 100mm of rain fell in a 6-hour period early that Saturday. The rains were accom- panied with strong winds. The heavy rains led to massive flooding killing 737 people in Algeria and leaving 23,000 homeless. This amounted to the worst flooding and second worst natural disaster to hit Algeria since independence in 1962. The unusual weather continued for nearly 48 hours and was associated with strong winds along the Mediterranean coast, and yellow clouds producing heavy dust laden rains in Rome. CNR lidar measurements confirmed the heavy dust presence on 12 and 14 November. In this paper, a high-resolution numerical simulation of the storm is presented. Results show that the storm was preceded by a southward penetration of the jet stream into northern Africa associated with an unusually deep tropopause fold. With the fold was a strong potential vorticity maximum aloft that spawned an intense low-level response producing a strong surge of low-level flow from the north in the western Mediterranean and a strong low-level surge from the south into the south-central Mediterranean north of Libya. It was the northerly flow surge that picked up moisture from the Mediter- ranean and produced the orographic rains and wind affecting Algiers. The southerly surge built northward, producing the strong winds over Rome. The two wind streams together created strong low-level vorticity and together with the moisture drawn from the Mediterranean and convection resulting created conditions favorable of the trop- ical (warm-core) storm genesis. The result was the growth of an intense low-level cyclone with unusually strong winds and tropical characteristics north of Algiers and west of Sardinia that moved northward slowly and persisted into November 11. The event was interesting not only because of the damage suffered, but because of the se- ries of mesoscale and synoptic scale interactions that led to its development including the effects of several mountain ranges, the fluxes of moisture from the water surface and associated deep moist convection, and thermal fluxes from the floor of the Sa- hara desert. Together these events came together to produce 2001Ss most significant weather event. Modeling results depicting how these mesoscale structures evolved will be discussed at the oral presentation.

On 29-30 November 2006, heavy rains from Supertyphoon Durian remobilized volcanic debris on the southern and eastern slopes of Mount Mayon, generating major lahars that caused severe loss of life and property in downstream communities. The nearby Legaspi City weather station recorded 495.8 mm of rainfall over 1.5 days at rates as high as 47.5 mm/h, far exceeding the initiation threshold for Mayon lahars. For about 18 h, floods and lahars from the intense and prolonged rainfall overtopped river bends, breaching six dikes through which they created new paths, buried downstream communities in thick, widespread deposits, and caused most of the 1,266 fatalities. In order to mitigate damage from future lahars, the deposits were described and analyzed for clues to their generation and impact on structures and people. Post-disaster maps were generated from raw ASTER and SPOT images, using automated density slicing to characterize lahar deposits, flooded areas, croplands, and urbanized areas. Fieldwork was undertaken to check the accuracy of the maps, especially at the edges of the lahar deposits, and to measure the deposit thicknesses. The Durian event was exceptional in terms of rainfall intensity, but the dikes eventually failed because they were designed and built according to flood specifications, not to withstand major lahars.

Three juvenile Cory's Shearwaters Calonectris diomedea diomedea were fitted with back-mounted satellite tags and tracked during post-fledging migration. The birds spent several weeks in the central Mediterranean before migrating westwards. Two tags stopped transmitting after 21 and 35 days; the third bird passed through the Strait of Gibraltar and along the West African coast until transmitting ceased after day 43, by which time it was 114.6 km off the coast and 4,390 km from Malta. Cory's Shearwaters from other Mediterranean islands winter further south in equatorial waters, in the eastern South Atlantic or in the northeast tropical Atlantic associated with the Canary current, and further research is needed to define the wintering areas of Maltese Cory's Shearwaters.

October to November (ON) rains provide critical moisture for the growing period of the main rice cultivation season in Sri Lanka that lasts from October to March. Decisions on rice cultivation are made at a seasonal conference convened each year in September. Such decisions are presently based on climatological rainfall in the past 30 years, water levels in irrigation reservoirs and farmers' indigenous knowledge related to historical analogues of wind-direction in September. Past studies documented the skill in seasonal climate predictability in tropical regions in the boreal fall. In recent years there has been a proliferation of seasonal climate forecasts from Global Circulation Models (GCMs). Given the above facts, and the long record of precipitation observations at hundreds of rain gauges scattered across Sri Lanka, it is useful to examine whether statistical downscaling of precipitation could provide additional climate information that could be used for decision-making in agriculture and water resources management This paper analyzes the skill in ON precipitation totals over Sri Lanka by downscaling regional atmospheric variables, identified as affecting ON precipitation, from GCMs. A diagnostic analysis using historical precipitation observations at 145 rain gauges from 1961-2005 and reanalysis climate data reveals that ON precipitation is significantly correlated with September mean sea level pressure (MSLP) over the domain 40°E-270°E and 30°S-20°N and contemporaneous geopotential height anomalies at 200hPa and 850hPa over the domain 40°-270°E and 30°S-45°N. The Model Output Statistics (MOS) approach is utilized to develop seasonal predictions from hindcasts of September MSLP and October-November geopotential height anomalies at 200hPa and 850hPa from the ECHAM4.5 GCM (two versions: forced with constructed analogue SSTs; and persisted anomalies) and the fully-coupled NCEP-CFS GCM. ON precipitation forecasts are derived using cross validated Canonical Correlation Analysis (CCA). Precipitation skill assessments are made by computing Hit Skill Scores based on downscaled tercile - i.e. whether above-normal, near-normal or below-normal - precipitation.

This dissertation develops an operational tool for predicting and monitoring drought applicable to the humid tropics. Using Sri Lanka as a case example, it examines whether droughts in the humid tropics are predictable on an operational basis, and investigates how moisture stress may be monitored as a season unfurls. Droughts in Sri Lanka occur when rainfall during the main cultivation season -- the Maha (October-March) -- fails. Such droughts profoundly impact rice production. From 1951-2008, there were 4 extreme [Standardized Precipitation Index (SPI) <-2.0], 1 severe (-1.9<SPI<-1.5), 5 moderate (-1.49<SPI<-1.0) and 4 mild (-0.99<SPI<-0.5) droughts. Maha droughts can be operationally predicted by forecasting the failure of the two rainfall regimes during the season. The contemporaneous westerly zonal wind at 850hPa (U850), over 60°E-105°E and 5°S-15°N, controls the strength of the October-November convective rains -- with rain failure associated with anomalously strong U850. The contemporaneous northerly vertical shear of the mean meridional wind (V S ), over 80°E-90°E and 0°N-20°N, controls the strength of the December-February northeast monsoon rains -- with rain failure associated with an anomalously weak VS. Drought forecast skill was assessed for 1981-2002 using predicted fields of U850 issued in September, and VS, issued in November, from the NCEP Climate Forecast System and the ECHAM4.5 forced with two scenarios of prescribed sea surface temperature (SST) anomalies. October-November rain failure can be predicted with good skill over the rice cultivation areas in the central and southeastern regions using forecast U850 from the two versions of the ECHAM4.5. December-February rain failure can be predicted with good skill in the rice cultivation areas in the eastern, central and north central regions with forecast VS from the ECHAM4.5 forced with constructed analogues of SST anomalies. The utility of the Vegetation Temperature Condition Index (VTCI) -- calculated with Terra-MODIS Land Surface Temperature and Normalized Difference Vegetation Index products -- as an indicator of abnormally wet or dry conditions was tested for the Maha season. Results show that the VTCI is a suitable metric for the near-real time monitoring of Maha drought because it captures the onset and progression of moisture stress as the season unfurls and complements the seasonal rainfall forecast.

On 29–30 November 2006, heavy rains from Supertyphoon Durian remobilized volcanic debris on the southern and eastern slopes of Mount Mayon, generating major lahars that caused severe loss of life and property in downstream communities. The nearby Legaspi City weather station recorded 495.8 mm of rainfall over 1.5 days at rates as high as 47.5 mm/h, far exceeding the initiation threshold for Mayon lahars. For about 18 h, floods and lahars from the intense and prolonged rainfall overtopped river bends, breaching six dikes through which they created new paths, buried downstream communities in thick, widespread deposits, and caused most of the 1,266 fatalities. In order to mitigate damage from future lahars, the deposits were described and analyzed for clues to their generation and impact on structures and people. Post-disaster maps were generated from raw ASTER and SPOT images, using automated density slicing to characterize lahar deposits, flooded areas, croplands, and urbanized areas. Fieldwork was undertaken to check the accuracy of the maps, especially at the edges of the lahar deposits, and to measure the deposit thicknesses. The Durian event was exceptional in terms of rainfall intensity, but the dikes eventually failed because they were designed and built according to flood specifications, not to withstand major lahars.