storms

Irrigation and Storms in the Inner Mongolian Desert

Images from the DECODE project. Clockwise from top left: Microwave radiometer, wind LIDAR, researcher launching rawinsonde, eddy flux observation system, clouds forming at the boundary line, radar image of convective cells initiating along the boundary, photo of supercell storm growing from the boundary line. Photos courtesy of DECODE team.

A Research Spotlight from 32WAF/20Meso/28NWP

An irrigation oasis in Inner Mongolia, China, is providing unusual, real-world evidence about the effects of sharp vegetation contrasts on local and regional weather. Several presentations at the 32nd Conference on Weather Analysis and Forecasting, the 20th Conference on Mesoscale Processes, and the 28th Conference on Numerical Weather Prediction (32WAF/20Meso/28NWP) discussed findings from the 2022 DEsert-oasis COnvergence line and Deep convection Experiment (DECODE).

In Bayannur City, on the north side of a bend in the Yellow River, sits one of China’s largest irrigated areas: the 2,200-year-old, 769,333 hectare Hetao Irrigation District (HID), which was recognized as a World Heritage Irrigation Structure in 2019. All around this irrigation oasis is arid and semi-arid land, including the Kubuqi Desert to the south. We know that borders between land and water influence weather patterns, but there has been less real-world evidence gathered about the effects of differences in vegetation—and you can’t find a sharper divide than this one.

Image showing treeless mountains and sparsely vegetated foothills next to flat land. In the center of the photo is a dark green area of vegetation which contrasts sharply with the otherwise brown/tan landscape. A road runs to one edge of the green area. At the very right of the image, in the middle distance, is a large, wide building with a bright blue roof.
Above: Aerial view showing part of the Hetao Irrigation District and its sharp contrast with the surrounding desert areas. Video still courtesy of Yijing Liu. Below: Diagram of the juxtaposition between the boundary and its associated convective initiation (CI) and downstream propagation relative to surrounding terrain in Hetao Irrigation District. Image courtesy of Zhiyong Meng.

When cooler air from the Irrigation District meets warm wind from the desert, an atmospheric boundary line can sometimes be seen on radar. During the summer, convection often initiates at this boundary—sometimes leading to impressive storms that can travel long distances. DECODE researchers used a comprehensive set of observations—from radar and satellite to balloon sondes to flyovers—to examine this phenomenon. Their mission was to understand how the boundary forms and under what circumstances it might create unusual weather.

Different views of the boundary. Images courtesy of Zhiyong Meng.

On average, in the three months of summer each year from 2012 to 2016, 60 days produced a boundary, and 44 percent of those boundaries resulted in convective initiation (CI), noted Zhiyong Meng, of Peking University, in her July 20 presentation during Session 16 of 32WAF/20Meso/28NWP. The DECODE field experiment itself lasted 36 days in 2022, from 5 July to 9 August. With two field stations located on the oasis side, and four on the desert side, the teams were able to observe 23 boundaries and 11 occurrences of deep convection initiation, and even one case of a tornado.

Video still with green agricultural fiels and dark storm clouds in the background. A tornado funnel cloud is seen in the right side of the image. The DECODE project logo appears at the top left.
A tornado documented by the DECODE research team. It was generated by a thunderstorm formed at the boundary line. Video still courtesy of Yijing Liu.
Video still shows a bright bolt of lightning in the far left of the image. In the bottom right, a laser wind LIDAR device sits on a rooftop, pointing in the direction of the storm.
Lightning strike during the DECODE experiment. Video still courtesy of Yijing Liu.

Yipeng Huang, of Xiamen Key Laboratory of Straits Meteorology, outlined the most common conditions leading to a boundary/CI in a 21 July presentation. The researchers found that a boundary is most likely on warm summer days, when synoptic forcing is relatively weak, with dominant southerly winds opposing the oasis breeze, and a temperature over the desert that is apparently warmer than over the oasis. They found that along the boundary line between the two masses of air, convection initiation may occur when enough moist air advects north at the west edge of the subtropical high, moves out over the dry desert, and converges with a cool oasis breeze in an environment with large enough instability. Hongjun Liu of Peking University presented the mechanism for this process in a case study on 21 July.

Diagram of boundary formation and convection initiation near Hetao Irrigation District. Image courtesy of Zhiyong Meng.

Meng described “The most beautiful case, on July 29 [2022, when] the boundary produced a CI and the storm became very strong; it actually produced five-millimeter hail in the eastern part of the oasis.” They were also able to observe another storm as it split into two separate supercells. On July 25, a preexisting storm that passed over the area dissipated somewhat, likely due to sinking air over the oasis, then re-initiated strongly once it reached the boundary/convergence line over the desert. On occasion, the boundary would extend over the oasis and strongly increase the precipitation there.

Radar and photograph images of a large thunderstorm forming along the boundary line in the Kubuqi Desert on 29 July, 2022.

In the presentation immediately following Meng’s, Murong Zhang of Xiamen University noted that the team’s real-time forecasts were able to predict the formation of the boundary line in 21 out of 23 cases, although predicting convection initiation was more difficult. They were only able to predict 6 out of 11 CIs, as the numerical model tended to over-predict surface temperature, but under-predict moisture. The observations obtained from DECODE have been used to effectively improve the surface heat flux over the irrigated area, as shown in a presentation by Xuelei Wang of Peking University on the first day of the conference. You can see more from the DECODE team in this video created as part of the project:

With researchers from many institutions* participating, DECODE is an epic undertaking to study a unique natural phenomenon. As field research pioneer Prof. Edward Zipser of Utah University noted after Zhang’s talk, it’s “a program that we want to hear more about.”     

Group photo of the DECODE onsite team at one of the desert stations. Photo courtesy of Yijing Liu.

*DECODE participating organizations include Peking University, Inner Mongolia Meteorological Bureau, Nanjing University of Information Science and Technology, Xiamen Key Laboratory of Straits Meteorology, Xiamen University, Nanjing University, National Satellite Meteorological Center, Foshan Meteorological Bureau, and Jiangxi Storm Hunting Videos Culture Co., Ltd.

Featured image collage: Images from the DECODE project. Clockwise from top left: Microwave radiometer, wind LIDAR, researcher launching rawinsonde, eddy flux observation system, clouds forming at the boundary line, radar image of convective cells initiating along the boundary, photo of supercell storm growing from the boundary line. Photos courtesy of DECODE team.

About 32WAF/20Meso/28NWP

Predicting and understanding storms and other weather events is a complex business with real-world impacts. The American Meteorological Society’s 32nd Conference on Weather Analysis and Forecasting/28th Conference on Numerical Weather Prediction/20th Conference on Mesoscale Processes brought researchers, forecasters, emergency managers, and more together to learn about and discuss the latest scientific developments. The conferences took place in Madison, WI, and online 17–21 July, 2023. Recordings of the sessions are available here.

New Western Storms Scale to Describe Intensity, Potential Impacts of Atmospheric Rivers

Hurricanes are classified by the Saffir-Simpson Scale and tornadoes by the Enhanced Fujita Scale, and now atmospheric rivers—those long, transient corridors of water vapor that fuel flooding rain events each winter in the West, especially California—will also be scaled to enhance awareness and bolster prediction.
The new AR scale ranks their intensity and potential impacts from 1 to 5 using the categories “weak,” “moderate,” “strong,” “extreme,” and “exceptional,” based on the amount of water vapor they carry and their duration. It is intended to describe the strength of ARs as beneficial to hazardous, aiding water management and flood response.
AR-Scale“The scale recognizes that weak ARs are often mostly beneficial because they can enhance water supply and snow pack, while stronger ARs can become mostly hazardous, for example if they strike an area with conditions that enhance vulnerability, such as [where there are] burn scars, or already wet conditions,” says Marty Ralph and co-authors in a paper appearing in the February 2019 issue of BAMS and posted online as an early release today. “Extended durations can enhance impacts,” he says.
Ralph is director of the Center for Western Water and Weather Extremes (CW3E) at Scripps Institution of Oceanography and a leading authority on atmospheric rivers, which were officially defined by the AMS in 2017. The new scale was created in collaboration with NWS meteorologists Jonathan Rutz and Chris Smallcomb, and several other experts. It marks two decades of intensive field research that involved establishing a network of dozens and dozens of automated weather stations to observe ARs in real time and flying research planes through them as they crashed ashore and up and over the mountainous terrain of California, Oregon, and Washington.
Atmospheric rivers are the source of most of the West Coast’s heaviest rains and floods—roughly 80 percent of levee breaches in California’s Central Valley are associated with landfalling ARs. Research shows that a combination of intense water vapor transport for a long duration over a given area causes the biggest impact. But ARs also are primary contributors to the region’s water supply.
The newly created scale is designed to capture this combination, accounting for both the amount of available water and the duration it is available. It focuses on a period of 24-48 hours as its standard measurement. When an AR lasts in an area fewer than 24 hours it is demoted by one category, and if it persists more than 48 hours, it is promoted by a category. Unlike the operational hurricane scale, which has been criticized for inadequately representing the increased impacts of slower-moving, lower-end hurricanes, duration is a fundamental factor in the AR scale. It also aims to convey the benefits of ARs, not just the hazards.
“It can serve as a focal point for discussion between water managers, emergency response personnel and the research community as these key water supply and flood inducing storms continue to evolve in a changing climate,” says co-author Michael Anderson of the California Department of Water Resources.
The scale ranks ARs in five categories:

  • AR Cat 1 (Weak):  Primarily beneficial. For example, a February 23, 2017, AR hit California, lasted 24 hours at the coast, and produced modest rainfall.
  • AR Cat 2 (Moderate): Mostly beneficial, but also somewhat hazardous. An AR on November 19-20, 2016, hit Northern California, lasted 42 hours at the coast, and produced several inches of rain that helped replenish low reservoirs after a drought.
  • AR Cat 3 (Strong): Balance of beneficial and hazardous. An AR on October 14-15, 2016, lasted 36 hours at the coast, produced 5-10 inches of rain that helped refill reservoirs after a drought, but also caused some rivers to rise to just below flood stage.
  • AR Cat 4 (Extreme): Mostly hazardous, but also beneficial. For example, an AR on January, 8-9, 2017, that persisted for 36 hours produced up to 14 inches of rain in the Sierra Nevada and caused at least a dozen rivers to reach flood stage.
  • AR Cat 5 (Exceptional): Primarily hazardous. For example, a December 29, 1996, to January 2, 1997, AR lasted over 100 hours at the Central California coast. The associated heavy precipitation and runoff caused more than $1 billion in damages.

When AR storms are predicted for the West Coast, the scale rankings will be updated and communicated on the CW3E website and its Twitter handle.
“The launch of the AR Scale marks a significant step in the development of the concept and its application,” Ralph commented in an e-mail to the AMS, “and caused me to reflect back a bit on where it came from. All the people and organizations who’ve contributed. The scientific debate around the subject. The creation of a formal definition for the Glossary of Meteorology. The creation of a 100-station mesonet to monitor them in California. The AR Recon effort underway in a partnership between Scripps and NCEP [now NCEI], and in collaboration with the Navy, NCAR, and ECMWF, as well as others.  A number of papers are already in the works using the scale, and we are hopeful that it will prove useful for the public and for officials who must deal with storms in a large area where scales for hurricanes, tornadoes and nor’easters are not very applicable.”