1160 JOURNAL OF APPLIED METEOROLOGY VOLUME 36

Effects of Prior Precipitation and Source Area Characteristics on Threshold Wind

Velocities for Blowing Dust Episodes, Sonoran Desert 1948–78

TROY LEON HOLCOMBE

NOAA/National Geophysical Data Center, Boulder, Colorado

TREVOR LEY AND DALE A. GILLETTE

NOAA/Atmospheric Resources Laboratory, Boulder, Colorado

(Manuscript received 3 June 1996, in final form 18 December 1996)

ABSTRACT

A better understanding of the effects of precipitation and source area on blowing dust in the Sonoran Desert

has been sought through the study of 1190 dust episodes occurring during the 1948–78 time period at Blythe,

California, and Yuma, Arizona. Threshold mean hourly wind speeds (MHWSs) increase directly with prior

precipitation in proportion to the inhibiting effect of the vegetation canopy, which blooms following periods of

increased precipitation. Because of the time required for the vegetation canopy to fully develop and the persistence

of the vegetation canopy once developed, correlation between the threshold MHWS and precipitation is highest

for 4–6-month windows of total precipitation prior to each dust event at both stations. Many dust events associated

with unusually low MHWSs are clustered in time, and these events can be correlated with interstate highway

construction and soil preparation for new irrigation projects. Since threshold MHWSs for blowing dust lie well

below the recorded MHWSs during most dust events at most times, it is possible to predict that mean annual

precipitation could in the future increase to about 8 cm per 6 months without significantly reducing the occurrence

of blowing dust episodes. On the other hand, increases in future mean annual precipitation to 10–12 cm per 6

months would raise the threshold MHWS to the point that conditions for blowing dust would be substantially

reduced. Many of the infrequently occurring periods of elevated precipitation correlate in time with El Nin ̃o–

Southern Oscillation events, which typically repeat every 2–8 yr. Average MHWSs, and threshold MHWSs for

blowing dust, vary with wind direction at Blythe and Yuma. These variations can be related to variations in the

susceptibility of upwind source areas in most instances, but in one or more instances this variation may be

related to storm type.

1. Introduction

Correlation between susceptibility to blowing dust,

antecedent precipitation, and other factors is known and

documented in the Sonoran Desert and elsewhere although explaining this correlation precisely in terms of

source area, threshold wind speeds, vegetation, soil condition, storm type, topography, and other factors has

presented problems (Chepil 1956; Gillette and Passi

1988; Brazel 1989; MacKinnon et al. 1990). Soil condition (soil moisture, crusting, soil disturbance) and vegetation are controlling factors for potential dust emission, but the relative contribution of each is not easily

understood and quantified. In this paper we have sought

a better understanding of the conditions for blowing dust

in the Sonoran Desert by examining 1190 dust episodes

occurring during the 1948–78 time period at Blythe,

Corresponding author address: Dr. Troy Leon Holcombe, NOAA/

National Geophysical Data Center, 325 Broadway, Boulder, CO

80303-3328.

E-mail: info@ngdc.noaa.gov

California, and Yuma, Arizona. This area is a good natural laboratory for such study because blowing dust

episodes occur frequently enough to yield a large statistical sample, and other studies in this area provide a

prior basis of understanding to build on. We have examined wind speeds as a function of prior precipitation

and wind direction, and derived threshold wind speeds,

in order to test the following: 1) threshold wind speed

increases with increased antecedent precipitation; 2)

growth and decay of vegetation canopies in the source

area are the dominant controlling factors for threshold

wind speed; 3) response of the vegetation canopy to

amount of precipitation will be approximately the same

regardless of storm type or duration, wind direction, or

time of year; and 4) threshold wind speed varies with

natural susceptibility of upwind source areas.

2. The data

Records of blowing dust are contained in the hourly

weather observations from the Blythe and Yuma airport weather stations. For his study of dust climatol-
SEPTEMBER 1997 HOLCOMBE ET AL. 1161

ogy in the western United States, Changery (1983)

extracted from these hourly weather observations

(forms MF-10A) a record of the duration and intensity

of blowing dust episodes in the period 1948–78.

Changery defined dust events as times of reduced visibility. Specifically, he recorded the beginning and

ending times of every incidence of reduced visibility

due to blowing dust through four visibility thresholds:

11 300, 4800, 1600, and 1000 m. Starting with Changery’s log sheets of visibility data, we went back to

the original MF-10A hourly weather observations for

stations Blythe and Yuma and extracted mean hourly

wind speed and wind direction during each dust episode as defined by visibility falling through the

11 300-m threshold. Other functions of wind speed,

namely, highest hourly wind speed, lowest hourly

wind speed, mean extreme wind speed, highest extreme wind speed, and wind direction, were also extracted for each dust episode during 1948–78. Monthly totals of precipitation were extracted for both stations from the Historical Climatology Network database compiled by the Carbon Dioxide Information

Analysis Center of the Oak Ridge National Laboratory. Two spreadsheets were created containing all

this information for the Blythe and Yuma stations.

These spreadsheets contain one entry line for each

dust episode, containing the following columnar entries for each event: year; month; day; time of day;

begin times and end times when visibility dropped

below 11 300, 4800, 1600, and 1000 m, respectively;

mean hourly wind speed; wind direction; highest

hourly wind speed; lowest hourly wind speed; mean

extreme wind speed; highest extreme wind speed;

wind direction; and total monthly precipitation for the

current month. In all there were 379 dust episodes for

Blythe and 811 dust episodes for Yuma. Results of

this study were derived from analyses performed on

information contained in these spreadsheets.

3. The sites

Blythe and Yuma lie within the most arid part of

the Sonoran Desert. Mean annual precipitation is

about 7 cm at Yuma and 9 cm at Blythe, with this

precipitation being unevenly distributed through time;

the area experiences periods of months with almost

no rainfall.

Vegetation consists of ephemerals, which bloom after

precipitation events, and larger brush-type plants, which

manage to survive by growth, decay, and dormancy cycles in response to times and amounts of precipitation.

The larger scrub-type perennial plants are the dominant

features of the vegetation canopy and they consist predominantly of creosote (Larrea tridentata), white bursage (Ambrosia dumosa) or delta-leaf bursage (Ambrosia deltoidea), and big galleta (Hilaria rigida) (Crosswhite and Crosswhite 1982).

Features of the surficial geology are summarized in

Fig. 1. Geologically, the area is a mountain-and-basin

region characterized by northwest-trending faultblock terrane. The uplifted fault blocks of mostly Mesozoic volcanics and metamorphics are in an advanced stage of erosion, and their erosion products

are the predominant source of sediments making up

the intervening bolson, or basin-filling, deposits,

which at the surface are mostly of Quaternary age.

Surficial sediments, which are sources of dust, are

predominantly these bolson deposits, together with

alluvial deposits along the rivers and washes, and

evaporite deposits in dry lake beds. The areas having

the greatest susceptibility to blowing dust are several

dry lake beds west of Blythe, and noncultivated

regions lying west and south of Yuma (East Mesa and

Gran Desierto), which are underlain by Colorado River deltaic deposits.

Agriculture is possible only along the Colorado and

Gila Rivers and in low-lying areas of the Imperial Valley

where surface water is available (Fig. 1). Agricultural

activities, which rely on year-round irrigation, do not at

present contribute significantly to the supply of dust in

the area (MacKinnon et al. 1990).

For both Blythe and Yuma temporal distribution

throughout the year of dust episodes over the 30-yr

period of study is trimodal, with peak frequencies

occurring in February–April, July–August, and November (Fig. 2). Dust episodes have occurred on the

average at a rate of 13 per year at Blythe and a rate

of 27 per year at Yuma. Whereas the events are unevenly distributed with respect to time of year, substantial numbers of events have occurred during all

months of the year.

Rainfall (Fig. 3) occurs sporadically in the late fall

to early spring and also in the late summer to early fall

at both localities. Rainfall is negligible during the late

spring to early summer. Whereas the temporal distribution of dust episodes and rainfall differ significantly

from one another, both are associated with frontal

events, which move through the area during the winter

season, and with monsoonal convective storms, which

occur during the late summer.

Frequency distribution of wind speeds for all events

is shown in Fig. 4. The mean of all mean hourly wind

speeds is 929 cm s21 at Blythe and 850 cm s21 at Yuma;

the median is 923 cm s21 at Blythe and 894 cm s21 at

Yuma. The highest mean hourly wind speed recorded

was 2240 cm s21 at Blythe and 1967 cm s21 at Yuma;

95% of all events recorded lie between the extremes of

380 cm s21 and 1568 cm s21 at Blythe and 290 cm s21

and 1296 cm s21 at Yuma.

Frequencies of dust events as a function of wind

direction are illustrated in Fig. 5 for both sites. The

principal correlation is between frequency of dust

events and frequency of high wind events, but there

are also variations in mean wind speed for dust events

as a function of wind direction, which are probably

related to susceptibility of upwind sources and pos-
1162 JOURNAL OF APPLIED METEOROLOGY VOLUME 36

FIG. 1. Index map of the Blythe, California–Yuma, Arizona area showing the location of physiographic and cultural

features and containing a classification of land surface areas according to probable susceptibility to blowing dust.

sibly also to fetch. High winds at Blythe occur with

greatest frequency from the northwest to north and

from the southwest to west. High winds at Yuma occur

with greatest frequency from the north, from the

southeast to south, and from the west to west-northwest. To some degree, direction of prevailing winds

is influenced by topography, as examination of Fig.

1 reveals.

4. Mean hourly wind speeds, threshold mean

hourly wind speeds, and prior precipitation

Distribution of mean hourly wind speed (MHWS) as

a function of 1–12-month periods of prior precipitation,

for all 1948–78 dust episodes, is presented in expanded

form (Fig. 6, Blythe; Fig. 7, Yuma). In both of these

expansions the MHWSs of all dust events are plotted
SEPTEMBER 1997 HOLCOMBE ET AL. 1163

FIG. 2. Number of low-visibility events (visibility less than 11 300

m) due to blowing dust at (a) Blythe, California, and (b) Yuma,

Arizona, for the period of study 1948–78, as a function of month of

the year. Months numbered 1 through 12 are January through December, respectively.

FIG. 3. Mean rainfall for the period of study 1948–78 at (a) Blythe,

California, and (b) Yuma, Arizona, as a function of month of the

year. Months numbered 1 through 12 are January through December,

respectively.

on 12 successive x–y plots, but each plot depicts the

MHWSs plotted against different time period functions

of prior precipitation. In this paper threshold MHWS is

defined as the minimal MHWS necessary to entrain dust

into the atmosphere in quantities sufficient to reduce

visibility below 11 300 m. By definition, all MHWSs

associated with dust events exceed threshold MHWS.

Most of the MHWSs shown in Figs. 6 and 7 are randomly distributed, having wind speeds well in excess

of the threshold wind speed. The lower edge of the

envelope for each x–y plot is defined by a few dust

events, the MHWSs of which barely exceed threshold

MHWS. Increase of threshold MHWS with increasing

prior precipitation is expected and is shown on most of

the x–y plots.

Threshold MHWSs as a function of 1–12-month periods of prior precipitation were obtained by deriving

the lower envelope for each x–y distribution depicted

in Figs. 6 and 7; these are shown in a complementary

series of x–y plots (Figs. 8 and 9). The regression curve

using exponential best fit is shown on each plot. Correlation (R2

) of each fit is plotted as a function of prior

precipitation for Blythe and Yuma, respectively, in Fig.

10. Also shown in Fig. 10 are the zero intercepts for

each regression analysis. These illustrations highlight

several important relationships between threshold

MHWS for blowing dust and prior precipitation: 1) the

threshold MHWS increases exponentially with increasing prior precipitation; 2) the response of the threshold

MHWS to increased precipitation is not immediate, and

correlation, which is poor between threshold MHWS

and precipitation for the prior 1–2 months, increases to

a maximum correlation in the prior 4–8-month period

and deteriorates in the prior 9–12-month period; 3) maximum correlation occurs at 5 months at Blythe and 6

months at Yuma, corresponding to R2 values of 0.766

(R 5 0.875) and 0.788 (R 5 0.888), respectively; and

4) zero intercepts are in the range of 150–180 cm s21

at both stations in the 4–8 months prior precipitation

windows.

Corresponding to the high correlation associated with

the 5-month window of prior precipitation at Blythe and

the 6-month window of prior precipitation at Yuma, the

regression equations are as follows.

U 5 21 t e[5.0425 1 (p5 3 0.1622788)] cm s for Blythe

and
1164 JOURNAL OF APPLIED METEOROLOGY VOLUME 36

FIG. 4. Histogram of mean hourly wind speeds (MHWSs) recorded during low-visibility events due to blowing dust at (a)

Blythe, California, and (b) Yuma, Arizona, during the period of

study 1948–78.

FIG. 5. Number of dust events occurring at (a) Blythe and (b)

Yuma during the period of study 1948–78, as a function of wind

direction.

U 5 21 e[5.166 1 (p 3 0.1158)] cm s for Yuma. t 6

Using the above regression equations, threshold

MHWS is plotted against time at Blythe and Yuma for

the 1948–78 time period (Figs. 11 and 12). The MHWSs

of each dust episode plotted against time also appear on

the plots.

The few events defining the lower limits of the envelope have MHWSs that are well below the main body

of MHWSs recorded in most events (Figs. 11 and 12).

To more accurately characterize lower limits of the

MHWSs of the main body of dust events, a second set

of regression analyses for both stations were done as

previously described except that the fifth percentile instead of lowest value from each 1-cm bin was used to

define the lower envelope. Correlation, which is poor

between threshold wind speeds and precipitation for the

prior 1–2 months, increases to a maximum correlation

in the prior 3–6-month period and deteriorates in the

prior 7–12-month period; the maximum correlation,

which occurs at 4 months at Blythe and 6 months at

Yuma, corresponds to R2 values of 0.834 (R 5 0.913)

and 0.781 (R 5 0.883), respectively. Zero intercepts in

the prior 3–6-month period are in the range of 400 cm

s21

.

These regression equations can be used to compute

a threshold MHWS at any time, and they have the attribute of allowing such computation from monthly precipitation alone. However, many of the dust events having the lowest MHWSs apparently result from unusually

high susceptibility such as would occur following maninduced soil disturbances. Caution is therefore advised

in applying the above regression equations to long precipitation records and precipitation proxies in order to

hindcast naturally occurring threshold wind speeds; use

of a regression derived from the lower envelope, as

defined by the fifth percentiles of MHWS, is probably

a better simulation of naturally occurring conditions.

Those dust events having the unusually low

MHWSs are clustered in time. Clusters occurred during 1950–52, 1957–58, 1960, 1965–66, 1969, 1971,

and 1974 at Yuma; and during 1959–62, 1969, and

1971–72 at Blythe. It is possible that one or two of

these events may have been mistakenly associated

with fog or smoke rather than blowing dust. Many

of these events may be associated with intense summer convective storms, which are characterized by
SEPTEMBER 1997 HOLCOMBE ET AL. 1165

FIG. 6. MHWS (cm s21) for each dust episode at Blythe, California, occurring during the

period 1948–78, plotted as a function of antecedent monthly precipitation totals (pp) in

centimeters, for 1- through 12-month windows of antecedent precipitation. For each dust

event MHWS was paired with prior-month precipitation for 12 time windows ranging from

the prior month to the prior 12 months, MHWS 5 f ( pp); pp 5 S pi, i 5 m 2 1, m 2 w and

w 5 1, 2, 3, . . . , 12, where pp is the prior precipitation, m is the current month during which

each particular dust episode occurred, pi is the total monthly precipitation for a given month

as designated, and w is the length of time of the prior precipitation window in months.

high winds in the source areas but only moderate

winds at the weather station. Many of these events

are probably also associated with local man-made

disturbances of the land surface, which for a period

of time result in conditions of extraordinarily high

susceptibility to dust entrainment. Note that during

the late 1950s and early 1960s, there was a large

project to bring new land into irrigation in the Palo

Verde Valley south of Blythe (G. Davis, Manager,

Palo Verde Irrigation District, 1992, personal communication), which would have created a major disturbance of a dry land surface. During 1969 through
1166 JOURNAL OF APPLIED METEOROLOGY VOLUME 36

FIG. 7. MHWS (cm s21) for each dust episode at Yuma, Arizona, occurring during the

period 1948–78, plotted as a function of antecedent monthly precipitation totals (pp) in

centimeters, for 1- through 12-month windows of antecedent precipitation. See Fig. 6 for

further explanation.

1972, there was heavy construction on Interstate

Highway 10 through the Blythe area. In the Yuma

area irrigation development occurred progressively

in the Wellton–Mohawk area east of Yuma during

1948–58; irrigation development in the Yuma–Mesa

area in the immediate vicinity of the Yuma Marine

Corps Air Station occurred during 1957–58 [B. Stevenson, district conservationist, U.S. Department of

Agriculture (USDA) Soil Conservation Service

1992, personal communication]. The All-American

Canal was constructed in the 1950s. Construction of

Interstate Highway 8 was carried out in the Yuma

area in the time frame 1965–78 (Yuma Daily Sun

1978). It is reported that some dust occurrences in

the 1970s and 1980s were associated with simulated

dust conditions (‘‘dust courses’’) and other militaryrelated activities at the U.S. Army Yuma Proving

Ground northeast of Yuma and the Luke Air Force

Range southeast of Yuma (B. Stevenson 1992, personal communication).
SEPTEMBER 1997 HOLCOMBE ET AL. 1167

FIG. 8. Lower edge of the envelope of MHWSs (cm s21) at Blythe, California, plotted as

a function of antecedent monthly precipitation totals (pp) in centimeters, for 1- through

12-month windows of antecedent precipitation. For all x–y distributions, the lower edge of

the envelope was obtained by binning 1 cm s21 intervals of prior precipitation, frequency

of MHWS 5 frequency of pp between pp 5 x 2 1 and pp 5 x, where x represents the upper

limit of precipitation of each successive bin in centimeters. The lowest MHWS within each

bin was selected to define the lower edge of the envelope. An exponential regression curve

has been fitted to each set of plotted points.

5. Influence of the vegetation canopy on the

correlation between threshold mean hourly

wind speeds and prior precipitation

Low correlation between the threshold MHWSs for

blowing dust and prior 1–2-month precipitation totals

(1–3 months at Yuma; Fig. 10) is very likely because

the response of the vegetation canopy to increased or

decreased precipitation is not immediate; time is required to develop a new vegetation canopy following

precipitation, and once a canopy of vegetation is in

place, it can survive for several months without precipitation before it dies, decays, and its effect on threshold
1168 JOURNAL OF APPLIED METEOROLOGY VOLUME 36

FIG. 9. Lower edge of the envelope of MHWSs (cm s21) at Yuma, Arizona, plotted as a function

of antecedent monthly precipitation totals (pp) in centimeters, for 1- through 12-month windows

of antecedent precipitation. See Fig. 8 for further explanation.

MHWS diminishes. Diminished correlation in the 9–

12-month window of prior precipitation can also be attributed to the fact (see Figs. 6 and 7) that there are few

intervals as long as 9–12 months without significant

precipitation; that is, wet–dry cycles are beginning to

repeat and cancel each other out. Highest correlation

between threshold MHWS and prior precipitation in the

4–6-month window and deteriorating correlation in the

7–8-month window is therefore, we suspect, a function

of the response time to precipitation necessary to grow

a vegetation canopy and the response time to drought

that is necessary for a vegetation canopy to die and

waste away. The shorter response time at Blythe as compared to Yuma may be a function of a relatively greater

contribution to the vegetation canopy of grasses and

ephemerals at Blythe. At Yuma, the effect of slowergrowing and more drought-resistant shrubform vegetation may be the dominant factor affecting the response

time of the vegetation canopy.

The threshold MHWS for entrainment of blowing

dust lies well below the MHWS of a majority of recorded dust episodes at both Blythe and Yuma (Figs.
SEPTEMBER 1997 HOLCOMBE ET AL. 1169

FIG. 10. Results of the exponential regression analyses performed

on each dataset shown in Figs. 8 and 9: R2 (a) and zero intercept (b)

are plotted for each of the 1- through 12-month windows of antecedent

precipitation.

FIG. 11. Threshold MHWS Ut at Blythe derived from the regression analysis of the 5-month window of antecedent precipitation, plotted as a function of time for the 1948–78 period (solid

line). Peaks labeled ‘‘E’’ are those associated with ENSO events.

MHWS u for each dust episode at Blythe is also plotted as a

function of time.

FIG. 12. Threshold MHWS Ut at Yuma, Arizona, derived from

the regression analysis of the 6-month window of antecedent precipitation, plotted as a function of time for the 1948–78 period

(solid line). Peaks labeled ‘‘E’’ are those associated with ENSO

events. MHWS u for each dust episode at Blythe is also plotted

as a function of time.

11 and 12), even if the higher-threshold MHWS of 400

cm s21 (ignoring unusually low speed events) is assumed. Only at infrequent intervals is precipitation sufficient so that the resulting vegetation canopy significantly impedes dust entrainment. In effect, drought is

the norm and significant precipitation the exception, at

both stations. Therefore, the significantly larger number

of events occurring at Yuma compared to Blythe must

be a result of a higher number of dust-producing wind

events per unit time at Yuma. Statistics of the distribution of MHWSs for all events show that both stations

are similar in terms of means and medians for event

MHWSs, although those for Blythe are slightly higher

(Fig. 4). Brazel (1989) shows that for 17 stations in the

arid southwest there is considerable scatter in the frequency of dust events for the most arid sites.

We can now speculate regarding vegetation-related

changes in susceptibility to blowing dust, which would

likely accompany future precipitation changes in the

area. It seems evident that increased drought will only

result in marginal increases in the overall frequency of

dust episodes, since threshold MHWSs are already well

below the MHWSs encountered in most dust episodes.

Modest increases in precipitation would for the same

reason not be expected to significantly alter the incidence of blowing dust episodes per unit time; precipitation could increase to 8 cm over a 6-month period

before the vegetation canopy would begin to significantly impede entrainment of blowing dust. On the other

hand, an increase in precipitation to 10–12 cm over a

6-month period would be expected to result in a vegetation canopy sufficient to significantly reduce the

number of dust events, and an increase to 16–18 cm

over a 6-month period would be expected to largely

eliminate the occurrence of blowing dust. Comparing

frequency of dust episodes versus precipitation at 17

stations in Arizona, California, and Nevada, Brazel

(1989) showed that dust episodes are rare at sites where

mean annual precipitation exceeds 30 cm; this result

and the overall inverse relationship between dust storm

frequency and mean annual precipitation as illustrated

by Brazel (1989) agree generally with our predictions.
1170 JOURNAL OF APPLIED METEOROLOGY VOLUME 36

6. Correlation between periods of high

precipitation and El Nin ̃o–Southern Oscillation

For the period 1948 through 1978, threshold wind

speed as shown in Figs 11 and 12 is a proxy for a moving

5-month (Blythe) or 6-month (Yuma) window of total

precipitation. This time series of precipitation shows

that precipitation at both stations is characterized by

long intervals of relative drought, which is the norm,

interspersed with infrequent intervals of anomalously

high precipitation. Many of these high-precipitation

events correlate with historic El Nin ̃o–Southern Oscillation (ENSO) events (see Quinn et al. 1987; Cole et

al. 1992). The correlation is particularly impressive at

Blythe, where seven of nine high-precipitation events,

and all the highest precipitation events, correlate with

ENSO events. At Yuma the correlation is much lower,

only 5 of 15 precipitation events, and none of the highest

precipitation events, correlate with ENSO events. This

significant lack of synchroneity between two stations

only 80 km apart is surprising and suggests local

rain-shadow effects resulting from Yuma’s proximity to

the mountain ranges of Baja California, which lie to the

southwest. Some of the Yuma high-precipitation events

occur in the warmer season and may be related to local

monsoon conditions deriving from Yuma’s proximity to

the Gulf of California.

7. Effect of source area on threshold mean hourly

wind speeds

Source areas for blowing dust have been characterized

for the Blythe–Yuma region in Fig. 1. As previously

stated, mountains, wetlands, irrigated agricultural

regions, water-covered areas, and urban areas are not

significant source areas for dust. Mountain basins underlain by pediments and bolson deposits are significant

sources for dust. As we have seen, susceptibility of the

bolson areas varies with the condition of the vegetation

canopy as influenced by prior precipitation. Susceptibility also varies locally. Near the mountains, desert

pavement is more ubiquitous and it provides some protection against deflation of the underlying dust. Farther

from the mountains, desert pavement is less continuous

and it affords less protection for the underlying soil.

Susceptibility is greatly increased throughout the bolsons by any disturbance of the desert pavement, for

example a passing vehicle. A major increase in susceptibility occurs as a result of any land surface modification such as road construction, canal digging, or bench

leveling. The areas most susceptible to blowing dust are

dry lake beds and nonirrigated portions of the Colorado

River delta. These areas have the least vegetation (or

no vegetation at all) and much of the underlying sediment is distal, containing a large fraction of finer grain

sizes (silt and clay) and lacking sufficient pebble- and

cobble-sized grains to form desert pavement. Susceptibility in these areas is also greatly influenced by the

condition of the vegetation canopy, except where there

is no vegetation. Delta and dry lake bed areas are also

sites of greatly increased susceptibility if the soil surface

is disturbed. In bolson and delta areas the sporadically

spaced desert shrubs inhibit deflation under their canopy, resulting in the soil being typically banked up

around each plant, such that each occupies its own little

hillock. This effect was observed to be most pronounced

in the delta areas (East Mesa and Yuma Desert) where

hillocks of 2-m relief are not uncommon. It is reported

(G. Davis 1992, personal communication) that susceptibility of dry lake beds west of Blythe has been increased following high-rainfall events, which bring into

the lake beds a renewed supply of fine-grained, unconsolidated sediment; once dried out, these vegetation-free

areas are particularly susceptible to dust entrainment.

West of Yuma the Algodones Dunes extend northwestward along the eastern edge of the Imperial Valley.

While these dunes are certainly a local dust source, they

constitute a lag deposit of mostly sand-sized sediment,

thought to be less prolific a source of blowing dust than

the East Mesa area lying immediately to the west. The

East Mesa area is underlain by sediments containing a

large percentage of silt- and clay-sized material. Areas

of the Colorado River delta, including the Imperial Valley, which are now subject to intensive year-round irrigated agriculture, were probably very significant

sources of blowing dust before they were irrigated and

placed into production. In this regard the original U.S.

Army Signal Corps meteorological log books from Fort

Yuma, now in the possession of the Arizona Historical

Society (courtesy of C. Brooks, curator), contain a daily

record of dust storms for some years in the late 1800s.

From 4 October, 1875 through 4 October 1876, a total

of 55 dust storms occurred at Fort Yuma, twice the

average annual occurrence in 1948–78 and more than

the total number of events recorded at Yuma during any

year between 1948 and 1982.

8. Variation of mean hourly wind speeds and

threshold mean hourly wind speeds with wind

direction

Mean hourly wind speeds vary with wind direction.

This variation no doubt contributes to the noise level

associated with derived threshold MHWSs, and it is

logical to relate these variations in MHWS versus direction to the dust-yielding potential of upwind source

areas. At Blythe, the most prolific source areas with

significant fetch lie to the south of the airport along the

Paloverde Mesa, and to the west in Chuckwalla Valley

(Ford Lake and Palin Lake) (Fig. 1). Dust events having

the lowest MHWSs at Blythe are those associated with

south and west winds; dust events having the highest

MHWSs at Blythe are those associated with north-northwest to northeast winds (Fig. 13). At Yuma, the obvious

potential source areas for dust having a long fetch lie

west to northwest of Yuma (East Mesa) and south-south-
SEPTEMBER 1997 HOLCOMBE ET AL. 1171

FIG. 13. MHWS for each dust event occurring at Blythe, plotted

as a function of wind direction. Wind directions 1–16 correspond to

the points of the compass, proceeding clockwise with 1 being north

and 16 being north-northwest. Filled triangles show means of hourly

wind speeds for each wind direction. Horizontal line shows mean

wind speed for all dust events.

FIG. 15. Mean annual percentage of time that winds in excess of

566 cm s21 are blowing from the directions shown, at Yuma Marine

Corps Air Station.

FIG. 16. Mean annual percentage of time that winds blowing from

the directions shown are causing reduced visibility (visibility less

than 11 300 m) due to blowing dust, at Yuma Marine Corps Air

Station.

FIG. 14. MHWS for each dust event occurring at Yuma, plotted as

a function of wind direction. Wind directions 1–16 correspond to the

points of the compass, proceeding clockwise with 1 being north and

16 being north-northwest. Filled triangles show means of hourly wind

speeds for each wind direction. Horizontal line shows mean wind

speed for all dust events.

east (SSE) to south-southwest of Yuma (Yuma Desert

and Gran Desierto) (Fig. 1). Dust events having the

lowest MHWS at Yuma are those associated with westsouthwest to west-northwest winds and northeast (NE)

to east winds; dust events having the highest MHWSs

at Yuma are associated with north-northwest to northnortheast winds and with southeast to south-southeast

winds; the high MHWS associated with east-southeast

winds is probably not significant because of the small

number of east-southeast events (Fig. 14). The same

variation observed in MHWS versus wind direction is

also apparent in the variation in threshold MHWS versus

wind direction at Yuma (Summary of Meteorological

Observations—Surface, 1991; courtesy of Chief Warrant Officer Davis, weather officer, Yuma Marine Corps

Air Station). For percentage of time when MHWSs in

excess of 566 cm s21 are blowing, percent frequencies

are highest for the north and south-southeast directions

(Fig. 15); for percentage of time when dust is blowing,

percent frequencies are highest for the west and westnorthwest directions (Fig. 16). Percent frequency of time

that dust is blowing exceeds percent frequency of time

that MHWSs in excess of 566 cm s21 are blowing when
1172 JOURNAL OF APPLIED METEOROLOGY VOLUME 36

FIG. 17. For Blythe, comparison of R2 and zero intercept between

the raw dataset (as in Fig. 8) and the dataset in which all dust events

are adjusted for differences in mean wind speed associated with each

wind direction.

FIG. 18. For Yuma, comparison of R2 and zero intercept between

the raw dataset (as in Fig. 9) and the dataset in which all dust events

are adjusted for differences in mean wind speed associated with each

wind direction.

winds are from the west, west-northwest, northeast, eastnortheast, and east; obviously, the threshold MHWS for

blowing dust is less than 566 cm s21 for these directions

(Figs. 15 and 16). Percent frequency of time that dust

is blowing is less than percent frequency of time that

MHWSs in excess of 566 cm s21 are blowing when

winds are from the northwest, north-northwest, north,

north-northeast, southeast, south-southeast, and southsouthwest; the threshold MHWS for blowing dust is

therefore greater than 566 cm s21 for these directions

(Figs. 15 and 16). The Yuma pattern suggests highersusceptibility source areas for dust west of Yuma and

east to northeast of Yuma, and lower-susceptibility

source areas for dust north to northwest and south to

southeast of Yuma. This pattern correlates with the occurrence of high-susceptibility sources west and east of

Yuma and the absence of a high-susceptibility source

north of Yuma, but it does not correlate with the occurrence of a high-susceptibility source south to southeast of Yuma (Fig. 1). There is no obvious explanation

for this seeming contradiction. Possibly the local, convective, short-duration summer storms moving up from

the Gulf of California, which are associated with southeasterly winds, are not as efficient at transporting dust

for given wind speeds as the winter frontal systems,

which account for most of the westerly and northerly

winds.

We have seen that MHWSs vary with wind direction

(Figs. 13 and 14) and that these variations derive from

susceptibilities to blowing dust in the upwind source

areas. It follows that if one adjusts or normalizes

MHWSs associated with each wind direction to a common mean, scatter resulting from differences in source

areas would be reduced, and the resulting ‘‘normalized’’

dataset would contain a clearer signal of the effect of

the vegetation canopy on susceptibility to blowing dust.

Accordingly, a third regression analysis was done as

described previously, using the entire dataset for each

station, with each event adjusted or normalized by an

amount equal to the difference between the MHWS for

each direction and the MHWS for all events from each

station. The difference between the mean for each wind

direction and the overall mean is shown in Figs. 13 and

14. As predicted, correlation is improved between

threshold MHWS as a function of prior precipitation

summed over the various 1–12-month precipitation intervals. Results are shown in Figs. 17 and 18. In all but

the 7-month window at Yuma, correlation remained

about the same or increased; for all windows, correlation

increased at Blythe. For the 5-month window at Blythe,

R2 increased from 0.766 to 0.850 (R 5 0.922). Zero

intercept for windows having a high correlation increased to a little over 200 cm s21 at Yuma and about

250 cm s21 at Blythe.

9. Threshold friction velocities estimated from

threshold mean hourly wind speeds

Potential for blowing dust at a specific location at a

specific time is proportional to wind energy available

for transport of that dust. Available wind energy may

be expressed in terms of friction velocity, a term that

describes the wind energy profile and can be derived

from measured wind speed, instrument height, and an

index of surface roughness. Theoretical aspects of friction velocity and definitions are provided in Stockton

and Gillette (1990) and MacKinnon et al. (1990). Conditions for dust entrainment in the atmosphere are met

when friction velocity exceeds threshold friction velocity at a specific site at a specific time. For the purposes

of this study friction velocity can be regarded as proportional to measured MHWSs. Threshold friction velocity can be regarded as proportional to a threshold
SEPTEMBER 1997 HOLCOMBE ET AL. 1173

FIG. 19. (a) Ratio of mean extreme hourly wind speed to MHWS

u, plotted as a function of MHWS, for all 1948–78 dust episodes

occurring at Blythe, California, for which mean extreme hourly

wind speed was recorded. (b) Ratio of mean extreme hourly wind

speed to MHWS u, plotted as a function of MHWS, for all 1948–

78 dust episodes occurring at Yuma, Arizona, for which mean

extreme hourly wind speed was recorded.

MHWS expressed in the same units as MHWSs measured at the weather stations.

Threshold MHWSs at times of zero prior precipitation (zero intercepts) lie consistently in the range

of 150–180 cm s21 (Fig. 10) for the 4–7-month windows of prior precipitation associated with the lower

envelope enclosing all dust events, and in the range

of 400 cm s21 for the 3–6-month windows of prior

precipitation associated with the lower envelope enclosing the main body of dust events. This value is

taken to be representative of the threshold MHWS

required to blow dust from an unvegetated, desert

surface during times of drought and optimum conditions for dust entrainment in the source area. Conditions associated with the lower of the zero intercepts

probably include natural or anthropogenic soil disturbance. At Yuma, using the known anemometer

height z of 1005 cm, a surface roughness factor z0 of

1 cm, and a von Ka ́rma ́n’s constant k of 0.4, a threshold MHWSs Ut of 180 cm s21 and 400 cm s21 equate

to threshold friction velocities (u*t of 11 cm s21 and

23 cm s21, respectively, using the equation:

U*t K

u 5 . t * ln(z/z )0

Wind tunnel tests have been conducted to determine

threshold friction velocities for various types of desert

surficial sediments (Gillette et al. 1980). Results show

that threshold friction velocities in the range of 10–15

cm s21 are substantially below the expected ranges of

threshold friction velocities as derived from wind tunnel

experiments; threshold friction velocities of 30–50 cm

s21 are required to erode loose, dry sand with minimal

crusting, such as would occur on the surface of sand

dunes or a surface covered with a layer of aeolian sand.

Inasmuch as we would expect threshold velocities to be

exceeded for an event of sufficient scale to drop visibility below 11 300 m in a whole area, it suggests to us

that in the Yuma area the actual wind speeds controlling

dust entrainment are from higher-speed gusts well in

excess of mean hourly wind speeds recorded at the

weather station.

For about half the dust events used in this study,

‘‘extreme wind speeds’’ were observed and recorded

hourly on the MF-10A log sheets. A comparison between the MHWS and the mean extreme wind speed

for each dust episode for which extreme wind speeds

were recorded is shown in Fig. 19. From threshold

MHWSs of 180 cm s21 and 400 cm s21

, threshold mean

extreme wind speeds of 450 cm s21 and 800 cm s21,

respectively, are obtained, using the ratio between mean

extreme wind speed and MHWS of 2.5 and 2.0, respectively, from Fig. 19. Threshold mean extreme wind

speeds of 450 cm s21 and 800 cm s21 yield threshold

friction velocities (u*t

) in the range of 25 cm s21 and

46 cm s21, respectively. These threshold friction velocities compare favorably with the lowest ranges of those

obtained from wind tunnel threshold friction velocities

required to erode loose dry sand [30–50 cm s21; Gillette

et al. (1980)]. We postulate that for those dust events

for which MHWSs lie at or near threshold MHWS the

extreme wind speeds for these same dust events, which

are recorded on hourly weather observations, are of the

right magnitude to compute valid threshold friction velocities.

10. Conclusions

Mean hourly wind speeds (MHWSs) recorded during

blowing dust events (as defined by visibility falling below 11 300 m) at Yuma, Arizona, and Blythe, California

during the period 1948–78 lie principally between 300

and 1300 cm s21 at Yuma and between 400 and 1600

cm s21 at Blythe, with the mean of MHWSs being 850

cm s21 and 929 cm s21 at each station, respectively.

Significant numbers of blowing dust episodes have

occurred during every month of the year at both stations,

but the highest number of events during the period of
1174 JOURNAL OF APPLIED METEOROLOGY VOLUME 36

study have occurred during February–April, July–August, and November. Mean frequency of dust events was

27 per year at Yuma and 13 per year at Blythe.

Threshold MHWSs for blowing dust increase with

increases in prior precipitation in proportion to the inhibiting effect of the vegetation canopy, which blooms

following periods of increased precipitation. Because of

the time required for the vegetation canopy to fully develop, and the persistence of the vegetation canopy once

developed, correlation between threshold wind speed

and prior precipitation is highest for 4–6-month windows of prior precipitation at both stations.

Many of the dust events associated with unusually

low MHWSs recorded at Blythe and Yuma during the

1948–78 time frame are probably also associated with

human-induced disturbances of the desert floor. These

events can be correlated in time with periods of heavy

construction on interstate highways and with preparation of additional land surfaces for irrigation.

We have used the 5-month (Blythe) and 6-month

(Yuma) windows of prior precipitation to observe or

compute threshold MHWSs. Threshold MHWSs are

very similar at both stations during times of drought

(zero intercept at 150–180 cm s21 for all events including low wind speed events; zero intercept at 400

cm s21 for the main body of events). For comparable

levels of prior precipitation, however, higher-threshold

MHWSs are derived at Blythe as compared to Yuma,

probably reflecting a better-developed stand of vegetation during most times because of the higher longterm mean of precipitation at Blythe.

Since threshold MHWSs for blowing dust lie well

below the recorded wind speeds during most dust events

at both stations, we conclude that windstorms of sufficient strength to blow dust are more frequent at Yuma

than at Blythe, but that windstorms occurring at Blythe

have, on the average, higher wind speeds. We also conclude that the overwhelming majority of strong-wind

events occurring at both stations will be accompanied

by blowing dust.

Because threshold MHWSs lie significantly below

MHWSs recorded in most dust events at Yuma, we conclude that mean annual precipitation could increase substantially at this station without significantly reducing

the occurrence of blowing dust episodes. At Blythe, on

the other hand, substantial increases in future mean annual precipitation would raise the threshold MHWS to

the point that conditions for blowing dust would be

substantially reduced.

Many of the infrequently occurring periods of elevated precipitation correlate in time with El Nin ̃o–

Southern Oscillation events, which typically repeat every 2–8 yr. This correlation is especially pronounced at

Blythe where seven of nine periods of anomalously high

precipitation correlate with El Nin ̃o–Southern Oscillation events.

Susceptibility to blowing dust varies areally in the

Sonoran Desert region. Mountains, irrigated agricultural areas, wetlands, rivers and lakes, and urban areas

are characterized by low susceptibility; basin areas

underlain by bolson deposits and pediments have susceptibility ranging from low to moderate; and dry lake

beds and nonirrigated portions of the Colorado River

delta are highly susceptible to blowing dust. Disturbance of the desert soil surface greatly increases susceptibility to blowing dust, particularly in areas of

desert pavement underlain by fine-grained sediments.

Average MHWSs, and threshold MHWSs for blowing

dust, vary with wind direction at Blythe and Yuma. This

variation introduces a noise level into the effects of

vegetation and prior precipitation on threshold wind

speeds for blowing dust. It is obviously related to variations in the susceptibility of upwind source areas in

most instances, but in one or more instances it may

possibly be related to storm type. At Yuma threshold

MHWSs are less than 566 cm s21 for westerly and easterly winds and greater than 566 cm s21 for northerly

and southeasterly winds. Removal of this ‘‘noise’’ due

to variation in susceptibility of source areas results in

a higher correlation between threshold MHWS and prior

precipitation.

Threshold friction velocities computed from MHWSs

recorded during dust events are significantly lower than

those obtained from wind tunnel experiments over loose

desert soils. Threshold friction velocities computed from

mean extreme wind speeds, on the other hand, are comparable to those obtained from wind tunnel experiments.

We conclude that the mean extreme wind speeds recorded during blowing dust episodes yield friction

speeds in line with those required for entrainment of

dust in the atmosphere in significant amounts.

Acknowledgments. Log sheets from Changery’s

(1983) study were obtained from Anthony J. Brazel,

Laboratory of Climatology, Arizona State University,

who generously provided copies of copies of these

documents, the originals of which have been lost.

Valuable knowledge of local soil conditions was provided by Bobbi Stevenson, USDA Soil Conservation

Service for Yuma, and by Gerald Davis, Palo Verde

Irrigation District for Blythe. A historical summary

of weather at Yuma was provided by Chief Warrant

Officer Davis, weather officer at the Yuma Marine

Corps Air Station. An examination of early weather

logbooks kept by the Meteorological Sergeant at Fort

Yuma was kindly arranged by Carol Brooks, Arizona

Historical Society.

REFERENCES

Brazel, A. J., 1989: Dust and climate in the American southwest.

Paleoclimatology and Paleometeorology: Modern and Past

Patterns of Global Atmospheric Transport, M. Leinen and M.

Sarnthein, Eds., NASA ASI Series C, Vol. 282, Kluwer Academic Publishers, 65–96.

Changery, M. J., 1983: A dust climatology of the western United

States. U.S. Nuclear Regulatory Commission Rep. NUREG/CR-
SEPTEMBER 1997 HOLCOMBE ET AL. 1175

3211, 25 pp. [Available from National Climatic Data Center, 151

Patton Ave., Room 120, Asheville, NC 28801-5001.]

Chepil, W. S., 1956: Influence of moisture on erodibility of soil by

wind. Proc. Soil Sci. Soc., 20, 288–292.

Cole, J. E., G. T. Shen, R. G. Fairbanks, and M. Moore, 1992:

Coral monitors of El Nin ̃o/Southern Oscillation dynamics

across the equatorial Pacific. El Nin ̃o: Historical and Paleoclimatic Aspects of the Southern Oscillation, H. Diaz and V.

Markgraf, Eds., Cambridge University Press, 349–375.

Crosswhite, F. S., and C. D. Crosswhite, 1982: The Sonoran Desert.

Reference Handbook on the Deserts of North America, E. L.

Bender, Ed., Greenwood Press, 163–295.

Gillette, D. A., and R. Passi, 1988: Modeling dust emission caused

by wind erosion. J. Geophys. Res., 93, 14 233–14 242.

, J. Adams, A. Endo, D. Smith, and R. Kihl, 1980: Threshold

velocities for input of soil particles into the air by desert soils.

J. Geophys. Res., 85, 5621–5630.

MacKinnon, D. J., D. F. Elder, P. J. Helm, M. F. Tuesink, and C.

A. Nist, 1990: A method of evaluating effects of antecedent

precipitation on duststorms and its application to Yuma, Arizona, 1981–1988. Climate Change, 17, 331–360.

Quinn, W. H., V. T. Neal, and S. E. Antunez de Mayo, 1987: El Nin ̃o

occurrences over the past four and a half centuries. J. Geophys.

Res., 92, 14 449–14 461.

Stockton, P. H., and D. A. Gillette, 1990: Field measurement of the

sheltering effects of vegetation on erodible land surfaces. Land

Degradation and Rehabilitation, Vol. 2, John Wiley and Sons,

77–85.

Yuma Daily Sun, 1978: 2 August.