local climate by contributing to the formation of low stratus clouds and 

 fog below the boundary layer inversion, which lies between 300 and 

 500 m above the surface. Farther seaward the cloud deck breaks up into 

 cumulus clouds (Neiburger 1960). 



DISTRIBUTION OF COASTAL STRATUS 

 IN RELATION TO COASTAL UPWELLING 



From May to September the coasts of California and Baja California 

 are influenced by an extensive deck of low-level stratus and stratocu- 

 mulus clouds. These clouds have a profound effect on the short-wave 

 and net long-wave radiation reaching the sea surface. Figure 15 dis- 

 plays a schematic pattern of this stratus region which was derived from 

 analysis of early satellite photographs by Gerst (1969). Typical surface 

 isobar and sea surface temperature patterns are included in the figure. 

 The wedge-shaped area of stratus clouds has its northern terminus near 

 lat. 40°N; Gerst found that it was most frequently located between lat. 

 35 °N and 4 1 °N . The boundaries of this stratus cloud deck appear to be 

 controlled by the direction of the air flow, the sea surface temperature 

 gradients, and the divergence of the wind field. The coastal boundary 

 of the stratus deck is known to undergo diurnal shifts in the onshore- 

 offshore direction due to the land-sea thermal contrast, which results in 



130° 



120° 



Figure 15. — Schematic diagram of the i\ pical summer pattern of low stratus 

 clouds and the corresponding surface pressure and sea surface temperature 

 patterns (after Gerst 1969). The region of the stratiform cloud mass is shaded. 

 Typical summer values of sea surface temperature (°C) and surface pressure 

 (mbar) are indicated on the chart. The contour intervals are 1.0 °C for temper- 

 ature and 2.5 mbar for pressure. 



the afternoon dissipation of clouds in a narrow zone adjacent to the. 

 coast. The diurnal variation of the stratus cloud cover along the coast 

 appears to be strongest in the Southern California Bight south of lat. 

 34.5°N. 



Neiourgeret al. (1945) distinguished between stratus formation 

 processes in southern California and those in central and northern 

 California. Abrupt turning of the coastline at Point Conception, the 

 presence of the Channel Islands, and the extensive area of shallow 

 water influencing the sea temperatures along the coast all contrib- 

 ute to the different nature of the stratus south of lat. 34.5 °N. In 

 northern and central California, the formation and dissipation of 

 the stratus was thought to be an advective phenomenon. Frequently 

 the subtropical high pressure center extends into the Pacific 

 Northwest and the winds along the coasts of Oregon and northern 

 California become northeasterly (Lane 1965). This is the situation 

 depicted by the surface isobar pattern in Figure 15. The resulting 

 offshore flow of dry, warm air is not favorable for stratus forma- 

 tion. 



The presence of cool, upwelled water in a narrow zone, of the order 

 of 50 km. adjacent to the coasts of Oregon and California, particularly 

 in the vicinity of prominent capes and headlands, is thought to enhance 

 the low level cloudiness and often promotes the formation of fog when 

 the prevailing air flow brings relatively warm, moist air over the cold 

 water. Tont (1975) described a relationship between low values of per- 

 cent possible sunshine measured at San Diego airport (lat. 32° 44'N, 

 long. 1 17°10'W) and high values of an upwelling index (Bakun 1973) 

 computed for a location 2° of longitude offshore. He attributed the low 

 values of percentage sunshine during May and June to relatively heavy 

 cloud cover along the coast. Relatively high surface salinity values 

 measured at the Scripps Institution pier during these months were 

 assumed to indicate strong upwelling. 



To investigate the relationship between coastal upwelling and low 

 level cloudiness we compared the annual cycles of monthly anomalies 

 of low level cloud amount at several locations known to be influenced 

 by coastal upwelling with the anomalies in 1° squares situated 10° of 

 longitude offshore (Fig. 16). In this figure the deviations of the mean 

 monthly low cloud amounts from the annual mean are plotted for the 

 1 ° coastal squares at lat. 40°N. 37°N, 33°N, 30°N, and 27°N and for 

 the offshore squares at the same latitudes. The offshore squares all 

 show positive anomalies during some or all of the months from May to 

 September, with seasonal changes of up to 0.2. At the coastal squares 

 between lat. 30°N and 37°N, the summer increase in low cloud 

 amount is the greatest and the amplitude of the seasonal change is as 

 large as 0.3. The larger amplitude of the seasonal change in low cloud 

 cover between Punta Baja. lat. 30°N. and near Monterey Bay. lat. 

 37°N. may be attributed to the effects of coastal upwelling. However, 

 the region near Cape Mendocino, lat. 40°N. does not show any signifi- 

 cant changes in low cloud amount during summer, although this area 

 experiences the coldest sea surface temperatures during this season 

 (Fig. 13). This lack of an increase in the mean stratus cloud cover may 

 be due to the frequent offshore flow of air (Lane 1965). The monthly 

 statistics of low cloud cover at lat. 40°N showed that approximately 

 40% of the reports during May through September were coded as clear 

 (i.e.. no clouds visible). A large number of clear-sky reports could 

 result from a predominance of reports taken in the afternoon if there 

 were strong diurnal variation in the cloud cover. However, the long- 

 term composite low cloud amounts for the 1 ° square at lat. 40°N did 

 not show statistically significant differences between the morning 

 (1000 PST) and afternoon ( 1600 PST) synoptic observation times. The 



5 Lee. T. F. 1979. Diurnal variations of coastal stratus. 

 Center. TP-80-02. Point Mugu. Calif.. 51 p. 



Pacific Missile Test 



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