AUGUST 2001 FARFAÂ N AND ZEHNDER 2073 An Analysis of the Landfall of Hurricane Nora (1997) LUIS M. FARFAÂ N Department of Geography, Arizona State University, Tempe, Arizona JOSEPH A. ZEHNDER Department of Geography and Department of Mathematics, Arizona State University, Tempe, Arizona (Manuscript received 5 May 2000, in ®nal form 16 February 2001) ABSTRACT The changes to the motion and structure of the eastern Paci®c Tropical Cyclone Nora (1997) during landfall are investigated. Nora developed south of the Mexican coast, intensi®ed into a hurricane, and made landfall over northwestern Mexico and the southwestern United States. In this study, the authors document the evolution of the structure of Nora from a variety of sources. Observations derived from satellite imagery show the approach of the hurricane to the Baja California peninsula and the development of a localized area of mesoscale convection that intensi®ed while moving over the Gulf of California. The propagation of this convective system became the dominant feature of the landfall and provided heavy precipitation over a region con®ned to western Arizona. Numerical simulations with the Pennsylvania State University±National Center for Atmospheric Research Mesoscale Model version 5 are used to examine the details of the evolution of the observed area of convection over the gulf. Simulations at different spatial resolutions show that the model is able to provide a reasonable prediction of the changes to the track, structure, and intensity of the hurricane at landfall. In addition, the model ®elds show that orographic modi®cation of the low-level ¯ow is involved in the development of a secondary circulation over the gulf. The simulated location and structure of this circulation are consistent with the obser- vations derived from satellite imagery. 1. Introduction tober. These systems tend to originate in a region that is located south of Mexico and west of Central America, The 1997 tropical cyclone season had a total of 19 with a motion that is typically parallel to the Mexican named systems in the eastern Paci®c Ocean. Most of coast. At low latitudes, 108±158N, most cyclones move these systems remained at sea; however, four made land- westward toward the central Paci®c, following the di- fall late in the season and produced a considerable rection of the large-scale ¯ow throughout the lower and amount of damage along the coast of Mexico. The pre- middle portions of the troposphere. For storms that sent study is focused on the landfall of Hurricane Nora, move toward higher latitudes, the average motion is which originated south of Mexico, tracked parallel to northwestward. However, some of these storms may the western coast, and eventually moved over the con- move close to the coastline and experience some degree tinent. The hurricane made a ®rst landfall by crossing of recurvature, as an eastward component of motion over the Baja California peninsula, and another landfall may develop (e.g., Zehnder 1993). occurred at the northern edge of the Gulf of California. In a study of the characteristics of motion associated The remnants of Nora moved along the Colorado River with tropical cyclones in the eastern Paci®c, Allard and Valley, with accompanying strong winds and heavy Peterson (1987) found that most systems start to recurve rainfall over the southwestern United States. in a region that is located south of the Baja California Our current understanding of the origin, intensi®ca- peninsula. They showed that the western coast of the tion, and dissipation of tropical cyclones in the eastern peninsula is a primary area of landfall and that this tends Paci®c has been increased during the last few decades. to occur during the month of September. Another region For example, several studies have documented that the of frequent landfall is the eastern coast of the Gulf of climatology of tropical cyclone activity is such that the California. In fact, many of the storms that move into majority of the systems develop from June through Oc- the gulf have a large component of northward motion, and eventually they make landfall over northwestern Corresponding author address: Luis M. FarfaÂn, Department of Mexico. Geography, Arizona State University, Tempe, AZ 85287-0104. Some of the storms that develop in the tropical and E-mail: [email protected] subtropical portions of the eastern Paci®c have a major q 2001 American Meteorological Society Unauthenticated | Downloaded 10/01/21 05:57 AM UTC 2074 MONTHLY WEATHER REVIEW VOLUME 129 effect on the weather of the southwestern United States. In a numerical study of the interaction of vortices For example, Smith (1986) states that nearly every year with idealized topography, Yeh and Elsberry (1993) dis- the remnants of tropical cyclones move into such a po- cussed the distortion of the circulation associated with sition that the upper-level winds transport moisture and an incident vortex that crosses over Taiwan. A low-level then produce showers and thunderstorms in the South- circulation organized downstream of the topography as west. In this region, heavy rains have been the major the primary vortex dissipated on the upstream side, and problem, and the states that are most frequently affected a new pressure center, separate from the primary vortex, are California, Arizona, New Mexico, and Texas. became the center around which a secondary circulation The study documented by Smith (1986) also found organized. They suggest that this discontinuous dis- that the presence of intense tropical cyclones in the placement of the system could be created by either a southwestern United States is a rare event that tends to downward extension of the upper-level circulation or by affect primarily southern California and western Ari- the upward building of the vortex that is created at low zona. Smith speculated that the region could experience levels. a hurricane if the sea surface temperatures along the The known effects of the western Paci®c islands on coast west of Baja California were to rise to at least the track and intensity of incident cyclones are not 248C and the storm were to move fast enough to avoid unique to that region. For example, Bender et al. (1987) signi®cant weakening while passing over land. In ad- performed numerical simulations to study the effect of dition, it is stated that a hurricane could move over the mountain ranges on islands located over the western Colorado River Valley if it ®rst were to pass through Atlantic Ocean. This included Cuba, Hispaniola, and the northern Gulf of California and not experience con- Puerto Rico. They found that these islands caused siderable dissipation upon reaching the northern coast changes to the track of the cyclones by de¯ecting them of the gulf. toward the north before reaching the eastern coast of In general, the approach of a tropical cyclone to a the islands. In some cases, a surface low continued to region with complex terrain brings associated changes move with the upper-level vortex as it transversed the to the structure and motion, due to modi®cation of the mountain range. Then, as the storms moved away from circulation by the terrain. The behavior of tropical cy- the island, they reintensi®ed rapidly due to the energy clones under the in¯uence of mountains has been doc- supplied by the warm sea surface. umented in several studies, most of which use obser- The goal of this paper is to discuss the observed vational data and model simulations. For example, changes that occurred during the landfall of Tropical Brand and Blelloch (1974) and Chang (1982) examined Cyclone Nora over northwestern Mexico. An aspect that the behavior and effect of typhoons that approached is the central focus in this study is the understanding of Taiwan. The mountain range that characterizes this is- the role played by topography in the structural changes land has horizontal scale of the order of 300 km and that Nora experienced during landfall over Baja Cali- maximum elevations above 2000 m. Among the most fornia. In this region, there is a mountain range that has signi®cant results derived from these studies is the doc- a cross-stream dimension comparable to those of the umentation of intensity and track ¯uctuations that occur western Paci®c and Caribbean islands discussed above. with the approach of the storm circulations to the land- As Nora approached the Gulf of California, a secondary mass. In general, there is weakening of the storm in- circulation developed over the gulf, and this feature tensity, acceleration of the translational motion, and a moved into the United States, producing heavy rain over track de¯ection that begins far upstream of the landfall a limited area in the Southwest. area. The de¯ection tends to occur toward the north and This paper is divided into ®ve sections. The sources to the right of the direction of storm motion. See Wu of observational and model data are discussed in section and Kuo (1999) for an additional discussion of studies 2. A description of the initial stages of development, associated with typhoons that have affected Taiwan. intensi®cation, and landfall of Nora is provided in sec- An interesting result from these studies is related to tion 3. A discussion of the model simulations that is the formation of secondary circulations on the down- focused on a study of the changes to the hurricane's stream side of the mountains. Chang (1982) used a prim- track and structure upon landfall is presented in section itive equation model to reproduce the changes to the 4. The ®fth and ®nal section summarizes our results and structure and motion of a simulated tropical cyclone, suggests areas of future research.