Climate & Meteorology
II. How Coastal Terrain and Other Local Features Affect Meteorological Conditions
A. Effects on local sea-breeze circulation
The sea-breeze circulation, which accounts for the large diurnal changes in coastal weather, is strongly modified by the coastal mountains, the depth and stratification characteristics of the marine mixed layer, the cloud cover, and the large-scale wind direction (Banta et al. 1994). A typical daily cycle of winds, temperatures and other weather parameters is shown in Figure 3. The most notable features of the sea breeze is the relatively rapid increase in wind speed from 2.5 m/sec (5 knots) during the night and early morning hours to 8 m/sec (16 knots) at 20:30 UTC (1:30 PM PDT). This abrupt wind speed increase is accompanied by a wind shift from a southeasterly direction to a west or northwesterly direction and a pronounced decrease in the air temperature associated with the movement of cool marine air inland. The day depicted in Figure 3 is clear, as evident from the solar radiation curve, but many sea breeze days have considerable cloud cover associated with them. Round (1993) classified the Monterey Bay sea breeze into four distinct patterns of diurnal evolution ranging from gradual onset to abrupt onset as well as no sea breeze at all. Knapp (1994) found that these characteristic patterns of sea breeze onset are associated with different large-scale weather patterns.
In addition to variations in the wind cycle associated with the sea breeze, substantial variability occurs in the winds across the region as well. The sea breeze forcing tends to accelerate the air across the coast but due to the presence of the coastal mountains the actual wind direction relative to the coast is often not across the coast. The sea breeze winds tend to blow parallel to the coast in Santa Cruz due to interaction with the topography. Foster (1993) found that the distribution of winds around the Monterey Bay is such that convergence occurs in the north central portion of the bay as air rapidly accelerates down the Salinas Valley. This results in southwesterly winds at Pt. Pinos. The offshore extent of the sea breeze is not well known for the Monterey Bay region but extends at least 20 km out from the coast (Banta et al. 1994) and probably extends beyond the line connecting Pacific Grove to Santa Cruz.
B. Effects on winds outside Monterey Bay
The coastal mountains also play a significant role in shaping the winds along the coast outside the Monterey Bay. The very warm interior temperatures and cold upwelled water along the coast produce a large cross-coast thermal gradient and associated pressure gradient. This cross-coast pressure gradient results in a strong parallel low-level jet along the coast during the warm season (Burk and Thompson 1995, Zemba and Friehe 1987). This coastal jet is responsible for the 30-knot and greater wind speeds that often occur just off the coast.
Headlands and coast bends can generate substantial variations in the winds due to hydraulic effects that result from mixed layer altitude changes (Samelson 1992, Winant et al. 1988). These hydraulic effects occur when the altitude of the marine inversion is relatively low and the wind speeds are fairly high (20 knots or greater for typical inversion heights of 300-1000 m). The impact is to produce higher winds downstream from a bend in the coast when the coast curves to the left of the flow direction (expansion fan) and lower winds and an abrupt inversion rise when the coast curves into the flow (hydraulic jump). Detailed observations of these effects are lacking for the Monterey Bay National Marine Sanctuary but are likely to occur near Point Sur and points south. The sensitivity of these hydraulic effects to changes in the large scale wind direction and other factors is largely unknown.
C. Coastally-trapped disturbances
The most dramatic changes in meteorological conditions during the dry season are associated with the passage of coastally-trapped disturbances. These disturbances occur primarily in the low-levels of the atmosphere and account for the often abrupt transition to fog, strong southerly winds and much cooler temperatures following warm periods during the summer months (Bond et al. 1995, Dorman 1985, Mass and Albright 1987, Reason and Steyn 1992). These disturbances are referred to as coastally-trapped because the dynamic processes governing the flow in the coastal zone are constrained by the low-level inversion and the coastal mountains.
Although there is considerable debate about the exact dynamic mechanisms that force these disturbances (Dorman 1987, Dorman 1988, Mass and Albright 1988, Rogerson and Samelson 1995), coastally-trapped disturbances are routinely characterized by southerly winds and a narrow tongue of coastal stratus as shown in Figure 4. These features propagate from south to north at a speed of about 6-8 m/sec (3-4 knots) and bring about rapid changes in the coastal weather (Bond et al. 1995). These dry-season events occur with an average frequency of 1-2 per month, usually associated with a one-to three-day period of offshore winds and warm coastal temperatures. The associated stratus surges are important as they dramatically influence the visibility over the ocean and coastal areas. Meteorological conditions from the coast out to about 100-150 km offshore are influenced by these mesoscale disturbances.
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