Henry Jones

May Lamb

10/19/22

EV 333 Dr. Gratz

Report on the December 22, 2021 - Jan 1, 2022 California Atmospheric River Event

Introduction

In December of 2021, two atmospheric river (AR) events within two weeks of one

another led to more than ten feet of snowfall throughout much of the Sierra Nevada mountains

across central and northern California (Putnam et al. 2021). Snowpack levels rapidly increased,

but lower elevations faced the brunt of extreme rain which led to landslides and flash floods

(Percy, N., Antonios, C. 2021). These storms brought unprecedented amounts of precipitation on

the Pacific coast, particularly in the mountains, and reached inland as far as Wyoming and

Colorado (Castelleno et al. 2022). The heaviest snowfall was seen in the second event, dating

December 22, 2021 to January 1st, 2022, near Lake Tahoe in California, where over ten feet of

snow fell in this short span according to the Castelleno et al., Center for Western Waters and

Weather Extremes (CW3E) detailed event summary. Snowpack surged statewide to 154% of

average annual snowpack and caused major travel disruptions and power outages, in addition to

flooding and landslides experienced at lower altitudes. The UC Berkeley Central Sierra Snow

Lab set a new total snowfall record for December snowfall of 214 inches, and most areas around

the Continental Divide reported above-normal snowpack conditions after January 1st (Castelleno

et al. 2022). While record precipitation events such as these bring many sociological dangers and

natural hazards such as flooding and mudslides, ARs are vital to the hydrological system in the

western US.

Along the western coast of North America, and in particular California, some 40-85% of

annual precipitation is a result of landfalling ARs and the orographic precipitation they bring

after uplift along the coastal and inland mountain ranges (Berwyn 2019). Upon reaching coastal

ranges, the AR-borne water vapor rises over the terrain, cools, and condenses to then fall as rain

or snow. Thus while interesting in their own right as meteorological phenomena, ARs are highly

correlated with drought, dry season wildfires, and are largely the lifeline of the entire hydrology

of these regions.

As defined by Ralph et al. (2018), ARs are long, thin filaments of significant water vapor

content which are several hundred kilometers wide and often one to three thousand kilometers in

length. A dedicated science of ARs has emerged in the last two decades as satellite imagery has

dramatically improved AR diagnostics and forecasting, and the immense hydrological

significance of ARs along the western coast of the US and other mountainous, arid locations has

been realized (Ralph et al. 2020). ARs often transport more water than the Amazon River in

concentrated water vapor, leading to immense amounts of precipitation (Berwyn 2019). Today,

AR researchers continue to investigate the physical mechanism responsible for AR genesis,

proliferation, and termination.

While originally thought of as a “channel flow” of intensely humid air originating in the

tropics, scientists are proving the actual dynamics of vapor transport to be more complex

(Sodemann et al. 2020). According to these researchers, the majority of AR-transported water

vapor arises from global-scale, longitudinal water vapor convergence along the mid-latitudes and

the subtropics, challenging the simplistic pipeline conception. Due to the rising of air masses

with poleward direction, water is continually condensed and replenished along the lifecycle of a

poleward atmospheric river. Despite the unknowns in the complexity of AR dynamics, Figure 1

(Ralph et al. 2020) illustrates the modern understanding of AR formation: cold fronts (typically

from polar regions) meet warm, humid air which is then uplifted in a cyclic manner to create

condensation. Given the advances in AR understanding, efforts have been made to standardize

ARs for forecasting purposes.

In light of the US west coasts’ tenuous relationship with ARs, scientists have developed a

comprehensive scale very similar to hurricane categorization to communicate the spectrum of

hazards possible. Key metrics for AR event intensity and duration include vertically integrated

water vapor transport (IVT) and vertically integrated water vapor (IWV) or total precipitable

water (TPW). IVT is mainly used in the AR scale as it depends less on surface elevation, is more

directly related to precipitation outcomes, and conveys vapor flux similar to land rivers. ARs are

identified with IVT values of 250 to 1250 kg m^–1 s^–1, whose five sublevels correlate with

weak(1), moderate(2), strong(3), extreme(4), and exceptional(5) level ARs. Category 3-5 ARs

represent the most dangerous storms with moderate to great risk of hazardous conditions, while

the lowest categories bring mostly beneficial precipitation (Ralph et al., 2019). This particular

event made landfall as a moderate to strong AR.

On December 22nd of last year, this AR made landfall over California, strengthening as it

spread across the southwestern US, and majorly contributed to a massive snow event. AR 2 and

3 conditions were observed over coastal Southern and Central California and the inland reach of

this AR created similar conditions in parts of Southern Arizona and Colorado (Castelleno et al.

2022). After the AR dissipated, a series of upper-level shortwaves brought further rain and snow

to California and the extended western states. Shortwaves are pressure waves embedded within

the planetary long waves caused by the uneven heating of the Earth’s surface by the Sun. These

waves are less than 3,700 miles long (long waves are generally 4,000-5,000 miles long) and their

high energy and velocity is responsible for their control of synoptic scale weather systems. In

fact, major precipitation bands are typically centered near short waves passing overhead

(Weather 2021). Thus while the AR was dissipating from December 25th to 31st, several

shortwave troughs aided by a stationary ridge over the Central Pacific Ocean traveled over

California (Castelleno et al. 2022) and dropped heavy precipitation and snow along the coastal

mountain ranges, the Cascades, the Sierra Nevada, and the Intermountain West.

These extreme snow levels in combination with high winds caused hazardous whiteout

driving conditions and major closures on highways, toppled trees, and consequent power outages

for thousands of people. The storms of this event fortunately did not cause widespread flooding,

but according to the Castelleno et al. (2022), multiple rivers rose above the action stage and the

snow runoff actually helped restore below-normal reservoir levels to near normal and combatted

ongoing drought and water shortages. Despite this, the danger of AR-related storms is

demonstrated in California’s history, including the Great Flood of 1862, which partially

destroyed Sacramento and the causeway failure of the Oroville Dam in 2017 (Alvi 2017, Dowd

2022). Furthermore, nearly half of all flood damage, costing around $23 billion, in the Western

United States between 1978 and 2017 was caused by only 10 of the most intense atmospheric

rivers during that time (Berwyn 2019). Global warming is already intensifying the rain and

snowfall from atmospheric rivers, and as the climate continues to warm these extreme ARs will

become wetter, bigger, and overall more intense and devastating for communities according to

Berwyn (2019). With regards to the late December 2021 AR and shortwave precipitation event,

this report is an investigation of the meteorological factors and dynamics of this record breaking

precipitation event, as well as the sociological impacts regarding climate change and climate

justice.

Methods

In conducting our analysis, we chose to include five figures (2-6) to best explain the

spatio-temporal evolution and dynamics of the extreme precipitation event including an AR and

strong upper-level short wave activity. With the abundance of publicly available water vapor

satellite imagery and the event analysis conducted at the Center for Western Water Extremes

(Castelleno et al. 2022), we synthesized these detailed data in conjunction with surface analyses

( NWS surface 2022) and HYSPLIT aerosol trajectories ( Hysplit 2022) to provide a continuous,

spatio-temporal analysis of the event. Figures 2 and 5 were acquired through the CW3E detailed

event summary (Castelleno et al. 2022). The animated figures 3 and 4(b) are animations of

MIMIC-TPW satellite data (Space 2021) which we created from a short Python Jupyter

notebook given by the Season Chaos blog (Besong 2022). To generate Figure 6, we computed

HYSPLIT back trajectories from initial locations 4000m (about 13000 ft) above mean sea level

in an area just west of Lake Tahoe (Figure 6(d)). The surface elevation is around 6000 ft on

average in the starting region, so an elevation of about 4000m above sea level was chosen to

investigate the connection between the surface air below and the upper level above at 500hPA in

Figure 5 which is about 18,200 ft above sea level ( Air Pressure 2015) according to the US

standard atmosphere. The starting region was chosen as it includes Truckee, California, from

which there are many meteorological observations, and Donner Summit on Interstate-80, which

recorded the highest snowfall totals over the course of the event (Rogers 2022).

Results

Sourced from the CW3E’s December 22- Jan 1 event summary (Castelleno et al. 2022),

Figure 2 and its animation linked in the caption show IVT and IWV vector plots off the coast of

California. The presence and magnitude of the landfalling AR which marked the start of this

record-breaking precipitation event is easily visualized with the shading of IVT and IWV

contours. Both water vapor content (b, IWV) and flux (a, IVT) enable easy visualization of the

atmospheric river which initiated this precipitation event. The low pressure system west of the

Oregon coast can be seen in both parts of the figure and also in the surface analysis in Figure 4.

The animation demonstrates this low pressure system contributed to the northeastern, inland

penetration of the AR. Figure 3 and its animation illustrate the evolution of the total precipitable

water (TPW) content over the eastern Pacific as the AR makes landfall along Oregon and

northern California and ultimately moves south until reaching the Baja peninsula. While TPW is

useful in understanding AR dynamics, Figure 3 does not provide insight into the heavy

precipitation resulting from upper-level shortwaves. Figures 5 and 6 illustrate this phenomenon

after the AR dissipated over central and northern California by December 24, 2021. The

comparison of the surface analysis and TPW at 03Z on December 23, 2021 in Figure 4 confirms

the importance of cold polar fronts in AR evolution which was given in Figure 1. As mentioned

previously, the surface low just west of the western Washington coast in Figure 4(b) confirms

this AR to be cyclonic in its evolution, as is typical of ARs impacting California (Sodemann,

2020). Figure 5 demonstrates the first three of five shortwaves which passed over California

immediately after the AR and brought heavy precipitation. As is expected, 500-hPA level

contour divergence occurs in the regions where absolute vorticity is highest which shows

potential for uplift and therefore precipitation. Finally, the calculated HYSPLIT trajectories in

Figure 6 confirm the presence of the stationary ridge over the northeastern pacific and the

multiple shortwave troughs over California in the days after the ARs dissipation.

Discussion

As seen in the Figure 3 animation, the event was initiated when a stream of moisture

from the eastern Pacific off the coast of British Columbia, CA moved south and narrowed and

intensified in the process. By December 19 (Fig. 3a), the water vapor intensified to be a weak,

likely unclassified AR making landfall on the northern coast of Oregon. As the TPW images

progress over 24 hour periods in Figure 3, the weak AR moved southward and intensified.

The strong, counter-clockwise wind vectors in the IWV plot in Figure 3, and the

tightening contours of increasing geopotential height in the surface analysis in Figure 4a point to

a very large high pressure system south of the Gulf of Alaska and a localized, yet intense low

pressure system off the coast of Northern California, where the AR made landfall on December

20. In the days prior, it's most likely the offshore low pressure system and cold front moved

further and further south toward California corresponding with the southward movement of the

TPW stream in Figure 3 (and animation). The positioning of the high and low pressure systems

thus acted as a perfect conveyer belt for cold polar air to wrap clockwise around the Gulf of

Alaska southward, until moving counterclockwise around the low pressure system (Fig. 4a).

After moving south of the low pressure system, the cold air was drawn north and eastward,

moving the cold front needed for AR formation (Fig. 1) to allow for the ARs landfall on

California as seen in the Figure 2 and 3 animations. Simultaneously, the low pressure system

acted as a warm conveyor belt, as moisture from the tropical latitudes is accelerated and brought

alongside the cold front barrier in a very common extratropical AR phenomenon (Sodemann

2020, Fig. 2.8). As mentioned in Sodemann (2020), cyclonic ARs such as these comprise the

most intense precipitation events in California, and their impact on historic flood seasons,

snowpack, and reservoir levels are well documented. As the Figure 2 and 3 animations show, the

AR moved southward and was largely absent from California by December 24, at which point

upper-level shortwaves began to dominate California’s weather.

As mentioned in the CW3E event summary and seen in Figures 5 and 6, while the

landfalling AR marked the beginning of this event, the upper-level shortwave activity in the

following days contributed similarly to the record precipitation. The short wave analysis by the

CW3E in Figure 5 shows significant upper-level troughing and the resulting increased absolute

vorticity over California on December 25-28. Shortwaves are known to be the primary

instigators of precipitation events on a global scale as they are essentially upper level fronts

which generate positive vorticity (Haby) and therefore the potential for uplift and/or instability.

It’s also likely that the AR-imported moisture in the previous few days provided a more humid

backdrop for the shortwaves, which further increased precipitation. After the fifth shortwave, the

troughing dissipated and moved southward marking the end of the precipitation event

(Castelleno et al. 2022). To further bolster our analysis of the short wave precipitation, we

computed Hysplit back-trajectories as seen in Figure 6 which clearly show the stationary ridge

over the northern Pacific and the subsequent troughing and uplift (noted in the bottom subplots

of subfigures a,b, and c). This stationary ridge likely played a large role in the numerous

shortwave events. By remaining still, the faster, more energetic shortwaves seem to have coursed

through the long waves in the area and repeatedly uplift air above the Sierra’s instead of

migrating along with a moving long wave. By January 1, 2022, California finally got a break

from precipitation but the event left a multitude of hazards remaining in its wake.

In addition to the meteorological and hydrological significance for California, this AR

and short-wave precipitation event had a significant sociological impact. The three day closure of

Interstate 80 and Highway 50 during the holidays is estimated to have cost $300 million in

goods, while at least 50,000 homes lost power in the Sierra Nevadas, with an estimated 21,000

who remained without power for eight days or more (Castelleno et al. 2022). This AR event in

late December was echoed by another causing widespread downpours later in January, triggering

mudslides that further damaged roads and homes. The deluge was severe enough to wash out a

150 foot section of Highway 1 in Big Sur, which is highly vulnerable near the sea (Berwyn

2022). With temperatures well below freezing, many feet of snow, and major interstates closed in

the areas hit hardest, conditions were dangerous for everyone, but particularly so for the most

vulnerable.

For the elderly, disabled, homeless, and poor, snow storms like this pose particularly

dangerous threats as there may not be places to go, people to help, or resources to find help. And

while this is true of every winter storm, extreme events such as these only exacerbate the risks

and lengthen discomfort. In addition, while the risk of large snowfall is immense, the threat of

urban flooding and hydrological insecurity is likely even greater.

California is especially vulnerable to atmospheric rivers with its Mediterranean climate,

experiencing wet winters and long, dry summers, leading to long term water scarcities and higher

likelihood of extensive drought periods. As noted previously, ARs are extremely influential in

California’s precipitation totals year to year, bringing 92% of the West Coast’s heaviest rainfall

days and accounting for 85% of California’s multi year precipitation variance (McDermott

2022). Climate change is clearly predicted to strengthen and increase the frequency and intensity

of ARs in years to come, which will have an increasingly huge impact on the land, ecosystems,

and communities in California. As warming oceans release more moisture to the air through

elevated evaporation, and warmer temperatures in the air enable the air itself to hold more

moisture, atmospheric rivers are poised to carry even greater amounts of water vapor and make

landfall as higher category events. A study in 2018 by Duane Waliser from the NASA Jet

Propulsion Lab shows a long-term trend of ARs bringing high levels of moisture to the shores

over the last 70 years. Even with variation in usual climate cycles like El Niño, Waliser’s

research shows ARs are expected to be about 25% wider and 20% longer than they are now

(Berwyn 2019). While new research continues to bring to light the impending dangers of ARs for

California, the state is also contending with rapid population and economic growth, significantly

increasing water demands as drought prevails, forcing State and Federal agencies to innovatively

balance water rights and supplies to meet these demands.

In anticipation of these conflicts and scarcities, California is working to mitigate flood

damage risk by improving floodplain management, while also recognizing the critical

importance of ARs in forming the base of the State’s entire water resources. In order to

accurately predict the extent of these future storms, it is critical that meteorologists and related

scientists learn more about ARs and their potential impacts and are doing just that to determine

the best methods for preparation and mitigation in the future. Current models predict that

megastorms could realistically flood thousands of square miles in a statewide disaster and cost

over $700 billion (Porter et al. 2010). At the forefront of mitigation efforts is therefore flood

protection, including improving runoff flows and their spatial distribution, as well as expanding

and better utilizing reservoirs and water storage spaces (Persad 2020). These strategies are slow

to plan and enact, and are very costly, but are extremely important and increasingly time

sensitive, as demonstrated by the intensity of the AR event in California last December that we

have discussed at length, and that broke snowpack records and endangered vulnerable

populations and affected entire communities.

M. Lamb and H. Jones collected data and compiled studies together. Lamb researched

atmospheric rivers and defined them, summarized the event in the introduction, and wrote the

discussion concerning climate justice, the impacts of ARs on people, and their relationship with

climate change. Lamb formatted sources into references below.

Jones performed HYSPLIT trajectory analysis in addition to generating figures 3 and 4(b),

collecting surface analysis images, and compiling figures. Jones contributed to the introduction

and wrote the methods, results, and meteorological components of the discussion.

Figures

Figure 1 . Vertical structure of a typical AR viewed over the Pacific ocean from the seminal work

of Ralph et al. 2004 . In (a), the large-scale frontal composition of an AR is seen as the moist

interface between a cold polar front and a warmer air mass of tropical or subtropical/mid-latitude

origin. The AR is heading towards the west coast of the US and can be seen to be related to the

cyclone near the Gulf of Alaska which appears to have spun the cold front in the AR into the

southern latitudes in a counter-clockwise manner. Ralph et al. 2004 present the modern

understanding of AR vertical structure in (b), showing the cold front and the warm, moist air

interface generating uplift and cloud condensation. The orange region and contour in (b) show

vertically integrated water vapor flux (IVP) which show that the most intense region of vapor

transport occurs at altitudes of about 1 km.

(a) (b)

Figure 2. Vertically integrated water vapor transport (IVT) and vertically integrated water vapor

(IWV) at 00Z December 20, 2021 provided in the CW3E event summary (Castelleno et al.

2022). IVT accounts for wind speed and direction while IWV is simply a measure of water vapor

content. IVT magnitudes of 250 kg m^-1 s^-1 is generally used as a minimum threshold for AR

diagnostics, and clearly the thin filament of IVT far exceeds this threshold in many places along

the AR. Animation can be viewed here .

(a) (b)

Figure 3 . Total precipitable water for the northern Pacific Ocean at 00Z on December 19-22,

2021(Besong 2022). We can clearly see the narrow band of water vapor flowing to the northeast

into northern California and Oregon and the general evolution of water vapor content over the

north eastern Pacific Ocean. Animation can be viewed here .

(a) (b)

Figure 4. Surface Analysis (a) and Total Precipitable Water satellite image from TPW-II (b,

Besong 2022) for 03.00Z December 23, 2021. The cold front seen in (a) clearly marks the

boundary of a significant change in humidity as the southern side of the boundary has much

greater total precipitable water (the scale for (b) is seen in Figure 4). While we expect colder

temperatures to cause precipitation and thus a decrease in TPW for a constant absolute humidity,

this cold front has the added significance of being within an AR where water vapor is being

transported along the boundary. At this point in the event, the AR itself is weakening while still

bringing significant precipitation.

Figure 5. Plots of 500mb levels along with wind barbs and absolute vorticity at 4pm PST on

December 25, 26, and 28 generated and used by the CW3E event summary (Castelleno et al.

2022). The plots are annotated to show the first three shortwave events which quickly followed

the AR and brought much of the precipitation for the event.

(a) (b)

Figure 6. Corresponding HYSPLIT back-trajectories for the first three shortwave events with a

starting elevation of 4000m above sea level at 00Z for December 26(a), 27(b), and 28(c). The

subplots below the trajectories indicate parcel altitude in meters above sea level at the

corresponding time.

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