Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 4:46 PM GMT on January 16, 2013
I am an avid cyclist, but I would think twice about cycling through the heavy smog that shrouded Beijing, China, the past few days (check out the AP photograph below). Originally, smog referred to the reduction in visibility caused by the combination of smoke and fog, but, in more modern times, smog has come to describe a cauldron of pollution and fog that sometimes pose very serious health hazards in large urban and industrial areas.
Motorcycling through the smog in Beijing on January 12. Courtesy of the Associated Press.
Smog typically forms in weather patterns dominated by high pressure. The recent siege of smog over Beijing and other parts of eastern China was no different, as shown by the 12Z GFS model analysis of MSL isobars (note the gaggle of highs centered over eastern China). The lack of a surface pressure gradient over this region indicates that winds were generally light. In turn, weak surface winds curb the dispersion of any pollutants in the lower troposphere. Indeed, the meteogram at Beijing (ZBAA) from 08Z on the 12th to 09Z on the 13th (see image below) confirms the presence of light winds.
The meteogram for Beijing, China, from 08Z on January 12 to 09Z on January 13, 2013. To the right of present weather and restrictions to visibility, I circled (in red) freezing fog and haze. Courtesy of the University of Wyoming.
To understand the connection I'm making between the dispersion of pollutants and wind speeds, I'll tell you a story about my commute to Emory University in Atlanta when I was in graduate school there in the mid 1970s. Each day, I rode my bike from my house to my office in the math department, usually leaving my house around 7 A.M. and leaving my office around 4 P.M. My commute included a stretch of dirt road, and, on clear mornings with light winds (high pressure), I dreaded when cars passed me because choking dust would just hang in the air the rest of ride. In the late afternoon, however, when there was a breeze (usually a result of momentum mixed downward from faster winds aloft), dust dispersed rather quickly after a car passed me on the dirt road. I hope my story helps.
By the way, the present weather symbol at 12Z on January 12 on Beijing's meteogram (circled in red) corresponds to freezing fog that deposits rime ice (supercooled water drops in the fog freeze on contact with cold surfaces). Note that 12Z temperature was 19 degrees Fahrenheit, so, yes, fog droplets were supercooled (revisit my blog about the issue of freezing). In this case, the sky is discernible (pdf file of WMO icons).
A typical vertical profile of temperature associated with smog episodes includes a subsidence inversion on the eastern flank of a high-pressure system, where the air is sinking from the upper troposphere to roughly 850 mb. To understand how subsidence inversions form, I recommend you carefully go through this instructive flash animation, courtesy of Penn State's certificate program. At any rate, subsidence inversions act like a lid, preventing air below the inversion from mixing upward.
In this case, there really wasn't a classic subsidence inversion (see 12Z skew-T below). In my view, the upper-air pattern at 12Z on January 12 was not conducive to subsidence through a deep layer of the troposphere (note the short-wave trough over eastern China on this 500-mb GFS model analysis at 12Z on January 12). Rather, there was a deep stable layer from the ground up to 850 mb (an inversion to roughly 900 mb and a roughly isothermal layer above the inversion), which was similar to the early morning conditions through which I rode my bicycle to Emory University in the mid 1970s. Clearly (no pun intended), pollutants, dust, fog, haze...you name it...that was "added" to the lower troposphere over Beijing just hung in the air. And, without stronger winds to disperse the smog, pollutants just built up over time, posing a serious health threat.
The 12Z GFS model skew-T showing the temperature (red) and dew-point (green) soundings over Beijing, China, on January 12, 2013. There was a temperature inversion in the lowest layer of the atmosphere and a roughly isothermal layer above it, making the lower troposphere highly stable and thereby trapping low-level pollution. Courtesy of NOAA.
Jeff Masters pointed out this MODIS visible satellite of China at 03Z on January 14, 2013. The relatively thin haze layer (compared to thicker low clouds over eastern China that appear bright white) appears as a grayish layer (Beijing is located near the northern tip of the mass of low clouds over eastern China).
I received an e-mail from a former (and great) student who graduated from the certificate program at Penn State, Winn Soldani. Winn is a businessman who travels a great deal. Here's his e-mail (it's really incredible):
Haven't read your full blog, but saw the topic. I was in Beijing last week...Arrived late on the 10th and departed on the 12th. The pollution was like nothing I've ever imagined. It was so bad that visibility INDOORS (e.g. in the airport arrivals hall) was affected. Horrible.
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