SELECTED CHARACTERISTICS OF PRECIPITATION
AT LONG BEACH， CALIFORNIA
John C． KIMURA and Gary L． PETERS
INTRODUCTION
Long Beach， California， is included with the Mediterranean Dry−Summer Subtropica1
（Csa）climatic type． The city is situated at 33°46 North Latitude and l l 8°12 West Longi・
tude． Despite its coastal position， the region is included within the warm summer phase of the Mediterranean climatic type because of the east・west coastal trend．Accordingly， the sea−
breeze does not counteract the effects of solar heat substantially to cool the area in summer．
Similar conditions are noted for other regions with an east・west coastal orientation．
Since Long Beach is situated in the southern extremity of the climatic region， average annual precipitation is on the low range for this climate type． Although there are variations in the average annual rainfall because of topography， coastal Southern California receives between 10 and 15 inches（250−375 mm）． This rain is heav皿y concentrated during the winter half of the year．
Summers are dry at Long Beach because of the subsidence of air that is associated with the Pacific Subtropical Anticyclone． This high pressure cell has strengthened and has mi・
grated to a more northerly position and dominates the weather along the entire Pacific coast of the United States by effectively steering Pacific storms away from the California coast．
Summer precipitation is primarily of convectional origin． This type is rare because of the dominance of the Pacific Subtropical High． However， when this High weakens or becomes displaced， moist tropical air from the Gulf of Mexico， Gulf of California， or the tropical eastern Pacific Ocean may invade this region． Thunderstorms may then result． Tropical storms are even more rare than convectional storms because of the dominance of the afore−
mentioned Pacific High， and， in addition， because of the effect of the cold water of the California Current．
Winter storms that affect coastal Southern California are frontal． These storms advance from the northwest as cold fronts as the Pacific High weakens and migrates to the south．
Along this coast 90％of the annual precipitation occurs from November through April， and over 50％is recorded in the three winter months of December，January and February．
Six years ago California State University， Long Beach， received a National Science Foun・
dation matching grant fbr the purpose of establishing a weather station． The station has been in operation fbr about five years and is maintained by the Geography Department． The sta・
tion is about 75 feet（23 meters）above sea・level and is on the roof of a three−story building．
This paper deals primarily with various aspects ofprecipitation as recorded by the automatic rain gauge at the station， and in particular with rainfall intensity．Observations from the rain・
fall year（July−June）1971−72 through 1975−76 were analysed． During this period the equipment was under repair for four of the months， unfbrtunately during the rain season．
Because the station has been in operation for only a few years， Long Beach Airport means
and extremes as reported in the Department ofCommerce publication，Climatography of the
United States No．60−4（1970）， were utilized where necessary． Long Beach Airport is situ−
ated apProximately 31／2 miles（6 kilometers）from the university．
AVERAGE ANNUAL PRECIPITATION
The average annual precipitation at the Long Beach Airport is 9．85 inches． de Violini
（1974）suggests that the average annual amount is misleading as a measure of estimating the annual expected rain in a low rainfall climate like that of Southern California，and that the figure is biased toward the high side． He advocates the use of the median value in place of the average annual figure．To obtain the median for the 30・year period 1931−1960，Depart・
ment of Commerce publications， Climatography of the United States nos．11−4（1953）and 86−4（1964）were consulted． Because data for the Long Beach Airport were not available，
figures fbr Downtown Long Beach were used． The median fbr this period for Downtown Long Beach is 11．53 inches compared with a long−term average of 13．00 inches． The median is 1．47 inches（about l l％）less than the mean． This implies that the annual expected precipi・
tation for the Long Beach Airport is 8．74 inches．
In regions where the average annual rainfall is low， the average number of rainy days 1ike・
wise tends to be low． Futhermore， the pbrcentage of the mean annual amount occurring in short periods then tends to be higher． For instance，the Imperial Valley receives over 40％of its annual precipitation on the wettest single day． In the state of California， Department of Water Resources publication（1972）， an interesting series of maps depicts short−period rain−
fall as a ratio of the annual average values． The publication shows Long Beach as receiving 2．6％of its precipitation in the wettest oneくluarter hour；6．1％of the annual precipitation is recorded during the wettest one hour；in the wettest 6・hour period，11．5％of the annual normal is received；and， nearly 20％of the annual rain is received in the wettest 24−hour period．
MONTHLY MEANS AND EXTREMES
Monthly rainfall normals fbr Long Beach Airport were used in lieu of those for the University weather station in analysing seasonality of precipitation（Table 1）． The findings revealed that the months of June， July and August have an average of O．07 inches． This represents less than 1％of the average annual precipitation． The values increase f6r the months of September， October and November to 1．26 inches and nearly 13％． However，
November alone is the greatest contributor to this period． November has an average rainfall of 1．03 inches， which represents about 10％． This means that the percentage of rainfall for September and October combined is just slightly rnore than 2％． The winter months of December， January and February are the rainiest．During this period the total average is 6．27 inches， and in these three months nearly 64％of the annual rain is recorded at the Long Beach Airport． Diminished amounts are noted fbr March， April and May． However， these three months have 2．25 inches of rain， which represents approximately 23％of the annual tota1． March alone has 1．37 inches，which represents about 14％of the yearly tota1．The fbre・
going shows that the rainiest months are from November through March， when approxi−
mately 88％of the total annual precipitation is recorded at the Long Beach Airport．Conroy
（1933）made a similar study of the precipitation distribution by decade periods fbr Los
Angeles and San Diego from 1887 to 1927．
Table l Frequencies（percentages）of observed rainfall at California S tate University，
Long Beach，1）y categories of intensity＊（July，1971−June，1976）・
Rainfall Intensity
Month
Average
@ Monthly
orecipitation＊＊ Light（％） Medium（％） Heavy（％）
July
̀ugust reptember nctober movember cecember ianuary eebruary larch
̀pril
lay iune
sOTAL
0．Ol n．03 O．06 O．17 P．03 P．97 P．99 Q．31 P．37 O．77 O．ll n．03 X．85
0
@0
U（100）
Q0（77）
P3（45）
Q7（54）
P6（52）
V9（65）
T9（54）
R5（66）
R（75）
V（100）
0 P（100）
@0 U（23）
撃戟i38）
P9（38）
P2（39）
R8（31）
S7（43）
P7（32）
P（25）
@0
＊Rainfall intensity categories are as foUows：Light＝0．01 to O．04 inches， Medium＝0．05 to O．20inches， and Heavy＝0．210r more inches．0．Ol inches＝0．254 mm；lmm＝0．039 inches
＊＊Long Beach Airport
Measurable rain（one−onehundredth of an inch or greater）occurs on an average of only 27days annually．Highest frequencies are for the three winter months of December， January and February with 13 days， nearly half of the total annual rainy days；however， compared with the total number of days， only a few have precipitation． March， likewise， has a high frequency with 5 days， and November shows 3．Therefore，November through March has 21 rainy days compared to the average annual number of 27 precipitation days．
Precipitation variabiity from the annual average is high， and monthly variation likewise has extrenies． While the average monthly values show at least some precipitation for every month of the year， at the same time no recorded rain occurred for each and every month sometime during the existence 6f the Long Beach Airport weather station． The rainy winter months are no exceptions． On the other hand， far greater values than normal f6r individual months， too， have been recorded． While the deviation in measurable amount is small for the summer months（0．52 inches was recorded in June，1963）， the absolute percentage variation is high． The June，1963， amount represents a positive variation of over 1，700％」n Septem・
ber，1961，1．31 inches of rain were recorded at the Long Beach Airport． This figure repre−
sents a positive deviation that is greater than 2，200％． The greatest deviation in terms of
レ amount occurred in January，1969， when 11．24 inches were recorded． This is a positive deviation of nearly 600％compared with the normal of 1．99 inches． In January，1969， a series of frontal storms that originated at relatively low latitudes advanced on the California coast．
CHARACTERISTICS OF RAINFALL
Table l illustrates two characteristics of precipitation in Long Beach． First， the seasonal
distribution of precipitation， from a minimum in July to a maximum in February， is clearly
discernible， though the data give frequency of rainfall intensities rather than actual amounts．
This patterh is perceived despite the fact that in the five years that the University weather station has been in operation the rain gauge was under repair once during November， twice in December， and on another occasion in January． This means that f6ur of the sixty data・
base months have been lost．Second， the data show that，for any season， most recorded rain一 飴11is in the light category， while very little、rain occurs in the heavy category． Most of the precipitation results from frontal storms． These storms advance from the northwest， and usually only the trailing edge of the cold front affects Southern California． Accordingly，
these fronts pass through with extremely light rain， or in the dry state． The rainfall fre・
quencies in the light and medium categories both vary from month to month， but it is particularly interesting to note the changes in frequency of medium rain as a percentage of total rainfall observations．
Table l also provides an indication of the changing contribution of medium rainfall to the total rain as we progress from October to January． Of these data， only February does not fit the overall pattern． Further data collection may show that February is not actually an anomaly but， rather， the small sample data failed to accurately demonstrate the pattern．
The overall seasonal pattern of rainfall frequencies， as well as monthly variations in the frequency categories， raises a broader question about whether the seasonal patterns of observed rainfall intensities differ significantly in a statistical sense． Data here are aggregated into three・month seasons as fbllows：1）June， July and August（JJA）；2）September，
October and November（SON）；3）December， January and February（DJF）；and 4）March，
April and May（MAM）． For comparative purposes， the June， July and August period is not used because of the low frequency in which rain was recorded．
Chi・squared tests are used to determine whether there are statistically different patterns of observed rainfall intensities between pairs of season categories as follows：1）SON and DJF，2）DJF and MAM，and 3）SON and MAM．
Data fbr the three chi−squared tests appear in Tables 2，3and 4． The associated chi・
Table 2 RainfaU frequencies for September， Octo・
ber， November versus December， January，
February．
Rainfall Intensity Season
Light Medium Heavy
SON cJF
39 P22
17 U9
512
Chi−squared＝2．98
Table 3 RainfaU frequencies for December， Janua− Table 4 Rainfall frequencies for September， Octo−
ry， February versus March， April， May． 1）er， November versus March， April， May．
Rainfall Intensity Season
Light Medium Heavy DJF
lAM
122 X7
69 U5
12 S
Chi−squared＝2．16
Rainfall Intensity Season
Light Medium Heavy
SON
lAM
39 X7
17 U5
54
