The April 1982 "Big SurVtorm: An Example of Extreme Rainfall in California

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1 The April 1982 "Big SurVtorm: An Example of Extreme Rainfall in California Richard Grotjahn and Su-Tzai Soong Dept. of Land, Air, and Water Resources Univ. of California, Davis, modumon During the wi.nter of 1982 there were two periods of unusually heavy rainfall in California. In January a major storm dumped a lot of snow in the central Sierra Nevada mountains. This storm closed interstate highway 80 for nearly 3 days! In April, the second stormy period was punctuated by an intense low of subtropical origin which brought heavy rain along a diagonal band from the central California coast to the northern Sierras. This talk focusses upon this second system. We have referred to the April storm as the "Big Sur" storm because the heavy rainfall it brought (on top of saturated soils) led to numerous landslides along the California coast. A particularly massive slide closed California highway 1, along the big sur coast, for more than a year. Two-day rainfall totals for some representative California stations are shown in figure 1. The diagonal orientation of the precipitation, southwest to northeast, is apparent in figure 1. This is also consistent with the orientation during some other recent heavy rainfalls, most notably the february 1986 storm. The 1986 storm is the subject of the next talk, and its diagonal band of precipitation was located about 100 km to the north. What can cause the high amounts of precipitation? It is widely believed that higher topography causes higher rainfall amounts, but that is only partly true. One might make that case for Sierra City, which sits at 1290 m elevation in a broad mountain valley. However, the higher rainfall amounts reported at Ben Lommond (elevation 137 m) and Pfeiffer Big Sur State Park (elevation 72 m) are not caused by these stations being at higher elevation. However, there are steep-sided mountains to the lee of these stations which rise to km elevation. Such high ridgelines may lead to convergence on their windward side which enhances the precipitation there. The presentation by Soong and Kim may address this issue of upstream convergence. In this case, and most cases of widespread heavy rainfall, the topography acts to'modulate the rainfall distribution. What creates the larger scale, widespread nature of the precipitation is the focus of this talk. We shall see that several rather uncommon events must come together in just the right way in order for an event like the big sur storm to happen. This is probably obvious, since such events are rare. What is interesting is to identify just what those events are. To identify those events, we use a good set of observational analyses prepared by the European Centre for Medium-range Weather Forecasts (ECMWF). These data are provided at equally spaced latitude and longitude grid intervals of degrees. The period we show is 25 March through 14 April. During these three weeks we find two storms which form in same part of the subtropics. The former storm does not develop much strength. The latter storm becomes intense. Rainfall amounts from the latter storm are about 10 times those of the first storm. Hence, these two storms help highlight what is needed to produce a storm capable of heavy rainfall.

2 Richarson Grove S. P. 3.43" I* Eureka 1.14" * Redding 0.63" I Sierra City 7.82" Fort Bragg.0.92" * 1 Echo Summit 2.45" Davis 3.35" Santa Rosa 2.05" * * *\ Calaverras Big Trees S. P. 4.76" San Francisco 1.50" ft Stockton 1.26" \ Half h4oon Bay 2.15" City 1.38" Ben Lommond " b Santa Cruz 3.04" Monferey 1.65" 4 * Fresno 0.71" I 3-4 Pfeiffer Big Sur S. P. 6.37" g Coalinga 0.84" \ \ * Morro Bay 2.62" Bakersfield 0.32" I * San Luis Obispo U " I Santa Barbara 1.77" (Los Angeles 0.7") Figure 1. Two-day rainfall totals on 10 and 11 April for selected California stations. The heaviest rainfall occurs mainly on 11 April.

3 SYNOPTIC SUMMARY An 11 minute film has been prepared which shows these fields at these levels: Segment 1: 850 mb level, relative humidity > 85% (shaded), isobaric trajectories (arrows), geopotential height (contours, with 1470 m dashed). Segment 2: 850 mb level, isotherms (contours, with dashed line for 273 K), isobaric trajectories (arrows). Segment 3: 850 mb level, isotherms (contours), eddy horizontal heat flux (arrows). Segment 4: 300 mb level, relative humidity > 60% (shaded), isobaric trajectories (arrows), and geopotential height (contours, 9120 m dashed). Similar format as segment 1. Segment 5: 300 mb level, geopotential height (contours), eddy horizontal momentum fluxes (arrows). Segment 6: 1000 mb level, specific humidity > 10 gmkg (shaded) and > 15 gmkg (double shading), isobaric trajectories (arrows), and geopotential height (contours, 9120 m dashed). There are many features that one can examine in the segments, but we highlight the following uncommon events which happened prior to and during the big sur storm. 1. The subtropical high is well to the west of its normal location. This has several implications upon the atmospheric flow. (a) On the west side of this ridge is warm advection which strengthens the ridge at upper levels and helps to block flow from the west. By blocking the flow, the big sur low is able to stall just off the California coast (for an extra day or so) which increased the total rainfall for the storm. (b) On the east side of this ridge occurred cold advection which allowed an upper level low to develop as well as a strengthening of the meridional temperature gradient upstream of the low. Both provide support to the developing low. Also, the baroclinic energy conversion is fostered this way, unlike the first low which never develops. (c) The presence of the long wave ridge fosters the development of the deep upper level low which in turn steers the big sur low in an unusual track: from southwest to northeast. The track therefore parallels the orientation of the cold front. Hence the line of precipitation is dragged across California in the direction of its longest dimension. 2. A short wave travels across the subtropics well to the south of the ridge. (a) This short wave provides the initial boost that forms a cyclonic circulation in the subtropics at lower levels. It continues to propagate past the lower level features and does not play a role in the final intensification of the big sur storm. (b) By initiating a low in the subtropics, very moist air in a deep layer is "capturedn by the developing system. The earlier storm forms in the same location as the big sur storm. Hence, the upper and lower level troughs (in height patterns) look superficially similar during the early stages. The first storm also taps into the subtropical moisture, though perhaps not quite as much. Two things are most notably different for the first storm: the ridge in the west side of the domain is much weaker and the meridional gradient of temperature near the surface low is much weaker. Both of these differences foster the development of the low by baroclinic means. The presence of the strong ridge also allows the big sur low to stall, extending the time period of rain. (Both lows follow similar tracks.) ' Illustrative figures (2-4) follow which are drawn from the film.

4 60' N JOOmb 60 Z and above 9120m dashed - 50 m/s JOOmb 60 X and above 912Om dashad - 50 m/a 50. N 40. N SO' N 20. N lwomb 8Om dashad MIXING RATIO (DOTS) $hadlnp o bw 10 ond 15 pm/kg 40 m sonlour In1~nol loodmb 8Om d0sh.d MIXING RATIO (DOTS) shadlng abwa 10 ond 15 gm/kg 40 m contour Intawal 850mb 273K dashad EODY HEAT FLUX (ARROWS) TEMPERATURE (CONTOURS) mk/a 4 K sonlour lntawal 60 ' 85Omb 273K doshad EDDY HEAT FLUX (ARROWS) TEMPERATURE (CONTOURS)- 200 mk/a 4 K contour inlarval 50 ' 40 ' SO. 20 Figure 2. Left column shows film segments for the first storm, which has weak development. The right column shows the big sur storm. The patterns of height and moisture at 300 mb (top charts) and 1000 mb (middle charts) are quite similar. The temperature pattern (850 mb, bottom charts) has much stronger meridional gradient for the big sur storm.

5 SOOmb 60 Z and abc 9lZOm dashad - 50 m/s ond above - 25 m/s April SOOmb 9120m dashmd 60 X and abo,.- 50 m/s 850mb 85 X and obore 1470m dashed - 25 m/s April ' N 300mb 60 X and above 912Om dashed - SO m/s REUTIVE HUMIDITY (SHADED) 850mb 85 Z and above 1470m dashed - 25 m/s 50- N 40. N SO. N 20. N Figure 3..Left column shows time sequence of 300 mb height contours. The corresponding height contours at 850 mb are shown in the right column. Upper trough (labelled "Aw) has entered the domain at the southwest corner 4 days earlier; it has initiated a low at 850 on 3 April (top charts). In the middle charts, upper trough "A" is now east of the 850 mb trough. Trough "B" eventually intensifies and provides the support that helps intensify the big sur low.

6 1000mb 80m dashed MIXING RATIO (DOTS) shading above 10 and 15 gm/kg 40 m contour Interval 242 April April Figure 4. Time sequence showing the big sur storm over 3 days. The storm moves from the. subtropics in a direction parallel to the orientation'of the cold front, and therefore parallel to the 'band of precipitation. The band of precipitation would lie approximately where to the shaded area is near California.

DEPARTMENT OF GEOSCIENCES SAN FRANCISCO STATE UNIVERSITY. Metr Fall 2012 Test #1 200 pts. Part I. Surface Chart Interpretation.

DEPARTMENT OF GEOSCIENCES SAN FRANCISCO STATE UNIVERSITY NAME Metr 356.01 Fall 2012 Test #1 200 pts Part I. Surface Chart Interpretation. Figure 1. Surface Chart for 1500Z 7 September 2007 1 1. Pressure