Reconstructing Historical Hurricanes

The frequency of hurricanes and the life span of trees are such that the long-term impacts of hurricanes on forests can be understood only at a scale of centuries. For much of the North Atlantic basin, the historical record provides evidence of past hurricanes over the last 300 to 500 years since European settlement. At the Harvard Forest we developed a method for interpreting this historical record using a combination of wind damage assessment and meteorological modeling. This historical-modeling method and its application to hurricanes in New England and Puerto Rico are outlined here (for more details, see Boose et al. 2001, in press). The computer models and historical data used in our analyses are available on the Harvard Forest web page (http://harvardforest.fas.harvard.edu).

The first task was to create a list of hurricanes in each study region for which there was historical evidence of wind damage. We relied on the works of other scholars to identify significant hurricanes during the early period (e.g., Salivia 1950; Ludlum 1963; Millas 1968). However, our assessment of the impacts of each storm was based, wherever possible, on contemporary accounts, mostly newspapers for the later period and letters, diaries, and government documents for the earlier period. As expected, the number of historical reports was greater for recent and/or severe hurricanes. Efforts focused on obtaining good regional coverage for each storm.

Actual wind damage in each hurricane was classified using Fujita's system (1971) for assessing wind damage in tornadoes and hurricanes. Fujita's damage classes extend from F0, minor damage caused by gale or storm force winds, to F5, extreme damage in the most severe tornadoes. Each F-scale (Fujita scale) class is defined by specified levels of damage to common cultural and biological features of the landscape. For New England we used a slightly modified version of Fujita's original system, whereas for Puerto Rico we made additional changes to account for different building practices and higher wind speeds (table 2.1). Though wind damage to exposed forests is strongly dependent on composition and structure (Foster and Boose 1992), as a general rule, F0 = loss of leaves and branches, F1 =

Table 2.1 The Fujita scale of wind damage, modified for application to New England and Puerto Rico.

F0 Damage

F1 Damage

F2 Damage

F3 Damage

Sustained wind speed (m/s)a Trees

18-25

Leaves and fruit off, branches broken, trees damaged

26-35

Trees blown down

36-47

Extensive blowdowns

48-62

Most trees down

Crops

Damaged or blown down

Masonry buildings

Minor damage

Roof peeled, windows broken, chimneys down

Unroofed

Blown down or destroyed

Wood housesb

Minor damage

Roof peeled, windows broken, chimneys down

Unroofed or destroyed

3+ blown down or destroyed in same town

Unspecified buildings, wood-zinc housesc

Minor damage

Unroofed or damaged

Blown down or destroyed

50% or more blown down or destroyed in same townd

Barns, churches, town halls, cottagese

Minor damage Unroofed, steeple Blown down or blown down, destroyed damaged

Shacks, sheds, outbuildings, warehouses

Minor damage

Unroofed, blown down, or destroyed

Hutsf

Damaged

Blown down or destroyed

Furniture, bedding, clothes

Not moved

Blown out of building

Masonry walls, radio towers, traffic lights

No damage

Blown down

Utility poles

Wires down

Poles damaged or blown down, hightension wires down

Signs, fences Autos

Damaged No damage

Blown down Moving autos pushed off road

Stationary autos Heavy autos moved or pushed lifted and over thrown

(continued)

30 Short-Term Climate Events Table 2.1 Continued

F0 Damage

F1 Damage

F2 Damage F3 Damage

Trains

No damage

Pushed along tracks

Boxcars pushed over

Trains overturned

Marinas, small airplanes

Minor damage Destroyed

Small boats

Blown off mooring

Sunk

Missiles

None

None

Light objects, metal roofs

Notes: PR = Puerto Rico. Corresponding sustained wind speed values are derived from Fujita's equations (1971), assuming a wind gust factor of 1.5 over land. bDescribed as well constructed or owned by a wealthy person (PR); also municipal buildings (PR). cConstructed with light wood frame and metal roof (PR). dF2 assigned if buildings described as rural or poor (PR). eAlso schools, sugar mills, commercial buildings, and military buildings (PR). fConstructed of palm leaves or similar materials (PR). Adapted from Boose et al. 2001, in press.

scattered blowdowns (small gaps), F2 = extensive blowdowns (large gaps), and F3 = most trees down.

Reports of wind damage were collected and indexed by town to create a database for each hurricane. Each report that contained sufficient information was assigned an F-scale value based on the highest level of damage reported. Care was taken to exclude coastal damage caused by the storm surge, valley damage caused by river flooding, and (in Puerto Rico) local damage caused by landslides. Regional maps of actual damage were then created for each hurricane using the maximum F-scale value assigned for each town. These maps provided a quantitative, spatial assessment of actual damage for each storm.

Meteorological modeling complemented the asssessment of actual wind damage by providing informed estimates for sites that lacked data as well as a complete regional picture of the impacts of each storm. The range and quantity of meteorological data for New England and Puerto Rico have, of course, increased dramatically since European settlement as a result of more widely distributed populations, better historical records, and steady improvements in technology (Ludlum 1963; Neumann et al. 1987). For hurricanes since 1851, our main source of meteorological data was the HURDAT (Hurricane Data) database maintained by the U.S. National Hurricane Center (NHC), which provides estimates of hurricane position and maximum sustained wind speed every 6 hours. HURDAT is available on the NHC web page (http://www.nhc.noaa.gov) (see Landsea et al. 2001 for information on current revisions). HURDAT values were modified in a few cases (see Boose et al. 2001, in press), including simulated weakening as hurricanes passed over the island of Puerto Rico (where actual damage maps showed a consistent pattern of storm weakening over the interior mountains). For New England, track and wind speed data for hurricanes before 1851 were reconstructed from contemporary accounts and from analyses by Ludlum (1963). Though actual measurements of wind speed are not available for the early period, observers often left careful records of wind speed (in qualitative terms) and direction (eight points of the compass) and noted the times of peak wind, wind shift, lulls, and changes in cloud cover and precipitation intensity. For Puerto Rico, reconstructions of hurricanes before 1851 were not attempted because of the lack of reliable estimates of hurricane tracks.

A simple meteorological model (HURRECON), based on published empirical studies of many hurricanes, was used to reconstruct the impacts of each storm (Boose et al. 2001, in press). HURRECON uses information on the track, size, and intensity of a hurricane, as well as the cover type (land or water), to estimate surface wind speed and direction. The model also estimates wind damage on the Fu-jita scale by using the correlation between maximum wind velocity and wind damage proposed by Fujita (1971; table 2.1). The model was parameterized and tested by comparing maps of actual and reconstructed F-scale wind damage for recent hurricanes, where the meteorological data used as input to the model were independent of the maps of actual damage created from historical accounts. The model was then used to reconstruct earlier storms. The resulting maps of reconstructed wind damage for each hurricane were compiled to create regional maps showing the number of storms at different damage levels (F0-F3) during the period of study. Model estimates were also compiled for individual sites to create time lines of hurricane damage and plots of wind damage as a function of peak wind direction. The latter were used in combination with a simple topographic exposure model (EXPOS) to create landscape-level maps of exposed and protected areas for individual hurricanes (Boose et al. 1994).

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