Meteorological Influences on Arizona Precipitation Mark Sinclair Meteorology Dept. Embry-Riddle Aeronautical University

Talk outline How precipitation occurs Winter storms The summer monsoon El Niño/La Niña Global warming Precipitation trends

What causes precipitation? We need – An abundance of low-level moisture – A lifting mechanism Caused by a combination of atmospheric dynamics and local terrain effects In winter (October-May), most precipitation is from winter storms Summer precipitation comes from the southwest Monsoon (July-Sept)

35% of PRC precipitation occurs during the summer monsoon The remaining 65% comes from winter- type storms Annual average for Prescott is 19 inches Two precipitation regimes

Winter storms In winter, precipitation is from cyclonic storms Lifting occurs east of migratory upper level troughs (next slide) Is helped by warm, moist low-level airflow Winter precipitation supplies much of the water needed to recharge the water storage systems of Arizona and replenish soil moisture – Less runoff and evaporation in winter cf. summer – Slow snow-melt especially good for recharge

The recipe for winter precipitation in Arizona Lifting and precipitation occurs in shaded region Upper-level flow Warm, moist low-level flow Warm sea temperatures off coast An active trough over S. California

Satellite picture for a northern Arizona rain event, 12 Nov 2003

Orange represents monsoon precipitation Monthly frequency of mid-latitude cyclones

Prescott winter (NDJFMAM) precipitation since 1950 Strong cyclonic storms near AZ since 1953 Individual years Running mean Below normal every year since 1994 A weak correlation between winter cyclones & Prescott precipitation Average

Trends? Note that, although cyclonic storms have been increasing slightly, Prescott winter precipitation has decreased in recent rears A cyclonic storm does not guarantee a major precipitation event Wind direction relative to terrain is important Sea temperatures off California also important Arizona has warmed in the last 100 years, resulting in less precipitation falling as snow

Prescott winter (NDJFMAM) precipitation (in) Trend is in per decade 100-year trend A clear decrease in winter precipitation at Prescott

Best fit Crown King Best fit for Prescott Crown King trend is –5.0 inches per decade 56 inches in 1915 avg 12 inches in 1993 Prescott trend is – 2.0 inches per decade 33 inches in inches in 1993 Snowfall decreases in mountainous regions due to a warming trend (next slide) Prescott and Crown King snowfall

Average temp trend is 0.45 F per decade 50 F in F in 2002 Minimum temp trend is 0.65 F per decade 34.5 F in F in 2000

Orographic enhancement In mountainous regions, precipitation is enhanced on the upwind side of mountains, with a rain shadow on the leeward side This is why Crown King gets 29 inches and Prescott Airport gets only 13 inches of precip Precipitation amounts increase with – Increasing wind speed – Increasing relative humidity of upwind flow – Increasing dew point of upwind flow

How mountains affect precipitation – Precipitation max on upwind side – Rain shadow on downwind side Mountain Airflow Rain shadow

How precipitation is enhanced on the upwind side of mountains – the “seeder-feeder” effect

The summer Monsoon During July, August and half of September, precipitation comes from thunderstorms associated with the summer monsoon Convection occurs in moist southerly flow Because of its high intensity/short duration, most summer rainfall runs off quickly and/or evaporates – Less replenishment of soil moisture and recharging of aquifers than winter precip

The recipe for monsoon precipitation in Arizona Middle-level flow Low-level surge of warm, moist air H Mid-level Gulf moisture enters AZ from the east Westerlies well to the north High pressure belt moves north of AZ, bringing mid- level easterly flow to AZ H Convection breaks out over AZ

No trend in monsoon precipitation, however it has become more variable Last four years below normal July + Aug + Sept

Which factors impact longer- term precipitation trends? El Niño/La Niña Pacific Decadal oscillation Climate change

El Niño SST anomalies

La Niña SST anomalies

DRY

WET

Wet El Nino winters Dry La Nina winters Dry El Nino winters Wet La Nina winters Only impacts winter precip, not summer

Note that between 1950 and 1976, there were strong La Niña’s and weak El Niño’s Blue = La Niña Red = El Niño Between 1976 and 1998, there were strong El Niño’s and weak La Niña’s Many scientists believe that we are shifting back to a prolonged period of strong La Niña’s and weak El Niño’s, like 1950 to This slower trend is related to the Pacific Decadal Oscillation ?

? Prescott winter precipitation since 1950 ?

Climate change Our climate is, and has always been, changing Most year-to-year variability is natural Global warming is one recent component Hypothesized to be caused by increases in CO 2 that result from fossil fuel burning and deforestation (“greenhouse effect”) Winners and losers – Costs include rising sea levels, changes in climate patterns – Benefits include longer growing season, more CO 2 for crop growth, increased drought and pest resilience

Drought and Global Warming A stronger hydrological cycle – Estimated 7-15% increase in global precipitation – Precip increases at high latitudes, smaller decreases at low and middle latitudes (Arizona)_ – Estimated 5-10% increase in evapotranspiration – Potential for more severe, longer-lasting droughts in continental regions like Arizona Prediction uncertainty remains great – Models can not yet resolve details of factors like mountains & vegetation changes that influence regional precipitation changes

Prescott annual precipitation (in) Some graphs of Arizona precip …

Crown King annual precipitation (in) Crown King winter precipitation (in)

Flagstaff region annual precipitation (in) Flagstaff region winter precipitation (in)

Jerome annual precipitation (in) Jerome winter precipitation (in)

Williams annual precipitation (in) Williams winter precipitation (in)

Seligman annual precipitation (in) Seligman winter precipitation (in)

Conclusions Two precipitation regimes – winter precip from cyclonic storms & summer monsoon convection Prescott area winter precip seems to be decreasing slightly, but much local variability Snowfall decreasing because of a warming trend More precip during El Nino, less during La Nina, with more La Nina’s on the way