An Investigation of the Catalina Eddy

JUNE 1997 THOMPSON ET AL. 1135 An Investigation of the Catalina Eddy WILLIAM T. T HOMPSON AND STEPHEN D. BURK Marine Meteorology Division, Naval Research Laboratory, Monterey, California J. ROSENTHAL Geophysics Branch, Naval Air Warfare Center, Weapons Division, Point Mugu, California (Manuscript received 5 March 1996, in ®nal form 29 August 1996) ABSTRACT The Catalina eddy event of 21 July 1992 is simulated using a mesoscale data assimilation system featuring an optimum interpolation analysis, incremental update, and second-order closure physics. The results are con- trasted with other recent modeling studies of the Catalina eddy. Genesis of the eddy occurs when changes on the synoptic scale lead to an intensi®cation of the east±west pressure gradient near the coast, resulting in enhanced northwesterly ¯ow along the coast and over the mountains east of Point Conception. Lee troughing results in an alongshore pressure gradient at the coast with higher pressure to the south. Topographically trapped, ageostrophic southerly ¯ow is then initiated. The combination of southerly ¯ow along the coast with strong northwesterly ¯ow to the west results in formation of a cyclonic eddy in the bight. The zone of southerly ¯ow is characterized by a deep, cool, cloud-topped boundary layer that can considerably alter coastal weather and impact activities involved with aviation, air quality, ®re weather, and microwave refractivity. While other recent modeling studies have failed to properly represent boundary layer structure, the data assimilation system used in the present study reproduces these features. Results show that the model forecast eddy is in relatively good agreement with surface wind observations. The data assimilation system, which consists of the analysis±initialization scheme and the forecast model, retains much of the mesoscale structure of the forecast, while adjusting the position of the eddy to better ®t the observations. Within the zone of southerly ¯ow, rapid deepening of the boundary layer is accompanied by the formation of stratus clouds. Through the use of sensitivity studies, the authors demonstrate that the deepening of the boundary layer results from convergence and upward motion forced by the topographic barrier along the coast and that the interaction between clouds and radiation plays a signi®cant role. 1. Introduction pressure to the south. There is a topographically trapped ageostrophic response to the alongshore pressure gra- In their recent paper, Ulrickson et al. (1995) present dient, giving rise to southerly ¯ow near the coast. Within an interesting series of simulations of the Catalina eddy one Rossby radius of deformation of the coastal moun- event described by Mass and Albright (1989). Ueyoshi and Roads (1993) have also recently published a mod- tains, the wind cannot achieve geostrophic balance and eling study of this event. The most comprehensive study ¯ows ageostrophically down the pressure gradient (i.e., of the Catalina eddy to date is that of Mass and Albright toward the north) since eastward turning due to the Cor- (1989) in which a 50-event composite and a detailed iolis torque is impeded by the coastal mountains [this case study were used to document the dynamics of the process is treated in more detail by Gill (1982)]. With Catalina eddy. They ®nd that synoptic-scale changes southerly ¯ow along the coast and strong northerlies to lead to an intensi®cation of the east±west pressure gra- the west, strong cyclonic vorticity is produced in the dient at the coast, which in turn strengthens the north- bight, leading to the formation of a Catalina eddy. westerly ¯ow along the coast and over the San Rafael In a well-developed eddy, the marine boundary layer and Santa Ynez Mountains east of Point Conception (see (MBL) is well mixed and deepens in the region of south- Fig. 1). Lee troughing results in an alongshore pressure erly ¯ow near the coast on the eastern side of the eddy. gradient in the Southern California bight with higher The MBL in this region is characterized by cooling temperatures and topped by persistent coastal stratus (Rosenthal 1972). The deep, cool MBL in the southerly ¯ow zone near the coast creates a pressure ridge along Corresponding author address: Dr. William T. Thompson, Naval the coast with lower pressure in the eddy (the Catalina Research Laboratory, Marine Meteorology Division, 7 Grace Hopper Ave., Monterey, CA 93943-5502. eddy low pressure center) in especially strong events. E-mail: [email protected] Mass and Albright (1989) (hereinafter, MA89) explain Unauthenticated | Downloaded 09/29/21 10:54 PM UTC 1136 MONTHLY WEATHER REVIEW VOLUME 125 both Ulrickson et al. (1995 hereafter UHRV) and Ue- yoshi and Roads (1993, hereafter UR). In all three in- vestigations, strong cyclonic circulations are simulated in the Southern California bight resulting from strong northwesterly ¯ow over the San Rafael and Santa Ynez Mountains and to the west of the bight with southerly ¯ow along the coast. In the present study and that of UHRV, similar sensitivity studies are performed in order to examine the mesoscale dynamics of the eddy. There are similarities also between the model used in the present study and that of UR in terms of initialization and treatment of lateral boundary conditions. In the inves- tigations of both UHRV and UR, however, important boundary layer physical processes, including the interaction between low-level clouds and radiation, are omit- FIG. 1. Map of study area showing the cross-sectional plane ted or crudely parameterized. Another serious failing of (dashed) and location of points A, B, and C. both of these simulations is that the boundary layer in the zone of southerly ¯ow is stable rather than deep and well mixed as documented by MA89. Also, model in- MBL deepening due to two effects: 1) damming of the itialization in UHRV and UR is de®cient. In the present southerly ¯ow by coastal topography as eastward turn- study, we demonstrate the importance of boundary layer ing due to the Coriolis torque is inhibited, and 2) the processes and of model initialization in order to further MBL near the coast is shallow as a result of offshore the work begun by UHRV and UR and to provide a ¯ow of warm, dry air; southerly ¯ow brings cool, moist more complete understanding of boundary layer pro- air above the top of the MBL, causing it to deepen. cesses in the genesis and maintenance of the Catalina As this zone of southerly ¯ow proceeds up the coast, eddy. fog and low stratus move inland. As the eddy circulation Onset of the southerly ¯ow along the coast impacts intensi®es, the MBL depth continues to increase along a number of activities, particularly those involved with the coast. If the eddy persists for several days, the MBL aviation, air quality, and microwave refractivity. Con- may extend upward to 1500 m or more and move into ditions leading to the onset of the eddy are typically the desert regions to the east. In the early phase, a west- preceded by a frontal passage with advancing high pres- erly sea breeze may overwhelm the eddy, as in the study by Clark and Dembek (1991). The sea breeze may also sure over the Paci®c Northwest while troughing con- result in translation of the eddy to the southeast. Once ditions aloft prevail offshore of Southern California. The the eddy matures, it can persist for several days, oc- widespread, deep low cloudiness results in strong cool- casionally expanding into a subsynoptic-scale feature. ing near the coast, bringing relief from the hot sum- During the morning hours when the stratus and stra- mertime weather in this region. This regime also aids tocumulus are deepest, light drizzle is common. Satellite ®re®ghters in gaining control of brush ®res in rugged imagery reveals that there is usually a very distinct terrain. Low cloudiness reduces insolation which, in boundary between the thicker stratus and stratocumulus combination with cooler temperatures, limits ozone pro- moving along the coast and the lower stratus or clear duction in the polluted air of the Los Angeles basin conditions to the west (Rosenthal and Posson 1977). As (Wakimoto 1987; Bosart 1983). Pollution concentra- the cloud cover invades the center of the eddy circu- tions are also reduced by the deepening boundary layer lation, there often remains a narrow clear slot at the and by advection of polluted air out of the basin toward western edge of the eddy separating the thicker low the northwest. However, this results in lower visibilities clouds from the lower and more uniform clouds to the and higher concentrations along the coast to the northwest. west. Fog and low visibility may also be a hindrance In the present study, we describe an investigation of to aviation. Naval operations are also impacted by mod- a Catalina eddy that was observed on 21 July 1992 using i®cation in radio and radar propagation. Climatologi- a coastal mesoscale data assimilation system. This sys- cally, refractive layers are higher and stronger to the tem features high vertical and horizontal resolution and west where the MBL structure is better de®ned and the second-order closure physics. In addition to a control inversion is higher. This changes rapidly with the es- simulation, sensitivity studies are performed in order to tablishment of the eddy. Signi®cant height increases oc- understand processes important in the evolution of the cur in the southerly convergent ¯ow near the coast while boundary layer structure and the mechanisms discussed the height of refractive layers to the west decreases, above (damming and advection of cold air above the reversing the typical height gradient. MBL) are critically examined. All of these impacts are a direct consequence of mod- In general, our results are consistent with those of i®cations in boundary layer structure and the extent of Unauthenticated | Downloaded 09/29/21 10:54 PM UTC JUNE 1997 THOMPSON ET AL.

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