Solving the Mystery of Nighttime Temperature Drops in Valleys

  The National Cross-Country Ski Center and the National Biathlon Center located in the Guyangshu Stadium Group in Chongli District, Zhangjiakou City once undertook the two major events of cross-country skiing and biathlon in the 2022 Beijing Winter Olympics. According to the original schedule, the competition time of some minor events of the two major events is arranged in the evening, so the accurate forecast of low temperature at night will directly affect whether the competition can be held on time. Take cross-country skiing as an example. This so-called “Snow Mountain Marathon” requires long-distance gliding on rolling hills and mountains. It has high requirements for athletes’ skills, speed and endurance, as well as severe weather conditions. . Because of the long duration of the entire competition, if the playing field is cold, athletes will be exposed to cold weather for a long time, which will increase their risk of hypothermia. Therefore, the International Olympic Committee stipulates that when the temperature of the competition venue is lower than -20°C, cross-country skiing competitions will be suspended.
  In fact, the ancient poplar competition area finally adjusted the competition time of 8 events due to the low temperature. For example: “Nordic biathlon men’s individual cross-country skiing 10 kilometers”, from 19:00-19:50 on February 15th to 18:30-19:05 on the same day; , from 19:00-20:20 on February 16 to 17:15-18:35 on the same day; and so on.
  For valley areas, in addition to snowfall and high winds that cause low temperatures, cold pools also cause low temperatures. Among the eight events mentioned above that were adjusted due to low temperature, the cold pool phenomenon that occurred from February 14th to 16th was very obvious, causing the minimum temperature in the ancient poplar competition area to drop to about -26°C. Not in advance.
Why is it so cold in the valley at night

  Under normal circumstances, the temperature decreases with the increase of altitude, and the vertical lapse rate of the temperature is about 0.6 degrees Celsius per 100 meters, so people often say: “It’s not cold at high places.” But under the influence of the cold pool effect, located in In the low-altitude valleys, the phenomenon of “the low places are extremely cold” will appear at night. What has actually happened?
  Before explaining this phenomenon, we need to understand the reasons for the formation of “sea breeze” and “land breeze”. The land-sea breeze is a mesoscale local circulation that occurs near the coast and is ubiquitous worldwide, often occurring on sunny days with weak background winds. During the day, the temperature rises, the land heats up quickly, the land temperature is higher than the ocean temperature, the air near the surface of the land is heated and expands, and the water vapor in the same layer of the ocean is replenished, forming a “sea breeze”; at night, the temperature drops, the land cools down quickly, and the land temperature When the temperature is lower than that of the ocean, the air near the surface of the land is cooled and sinks, and the air near the surface spreads to the ocean, forming a “land breeze”.
  Likewise, in and around mountains, thermal differences can cause “valley winds.” On sunny days with weak background winds and in mountainous areas with complex terrain, if the drop in surface temperature after sunset is greater than the drop in air temperature, the temperature near the surface will be lower, and the cold air near the surface will form a density flow under the action of gravity and flow along the Converging down the slopes, they eventually form cold pools in the valley. At this time, although the altitude of the valley is lower, the temperature drops very fast, sometimes it can drop 4-5°C in an hour, so there is an abnormal phenomenon that the temperature of the valley at night is lower than that of the mountain top. In addition, under the background of weak cold air, the strength of the cold pool determines the degree of low temperature – the deeper the cold pool and the longer it lasts, the lower the temperature will be. Multiple meteorological observations conducted by the Beijing Winter Olympics meteorological forecasting team have shown that there is a large deviation between the model forecast and the actual observation in the nighttime low temperature forecast of the Guyangshu competition area, precisely because of the formation of cold pools on the complex terrain.

Brightness and temperature distribution on the surface of Barringer Crater, Arizona, USA at 21:00 on October 19, 2013

  Cold pools occur all over the world, and the research on cold pools in my country is still in its preliminary stage. Foreign meteorologists have made some observations and studies on the cold pool. For example, in October 2013, relevant American researchers conducted relevant observations and experiments in Barringer Crater, Arizona. Two Doppler wind lidars coordinate observation and continuous scanning to measure the change of wind field in the two-dimensional vertical profile of the crater. In addition, the experimenters deployed three thermal infrared cameras, two operating side-by-side on the northern rim of the crater and one operating on the southern rim. The purpose of this observation is to study the nocturnal evolution of the atmospheric structure in the basin under the background of static and steady circulation, including the formation and development of cold pools. Observations and studies have shown that, in most cases, cold pools tend to occur during the period from sunset to eve, under clear conditions, few clouds, and weak boundary layer wind speeds. Of course, there are many factors that affect the formation, duration and temperature difference of cold pools, such as the wind speed and depth of day and night winds, local topographic features and soil moisture, sunlight exposure, partial shadow and surface energy budget, etc.
  Because of the complexity of the cold pool, mountainous weather forecasts are quite different from plain weather forecasts, and drastic changes in the weather can easily affect athletes’ physical functions, so weather forecasts for the high and low points of the venue are crucial. In order to better provide meteorological support, 81 sets of ground meteorological observation stations have been deployed in the Zhangjiakou competition area, which can provide various meteorological data with high temporal and spatial resolution. The weather forecast team also carried out a series of cold pool experiments, established a cold pool circulation forecast model, and produced weather service products as detailed as the track (including hourly forecasts within 24 hours for each track and 3-by-3 forecasts for 48 to 72 hours. hourly forecast), etc.
  After years of practical accumulation and theoretical research before the competition, the meteorological forecast team’s forecast of the local microclimate of the competition venue has become more accurate. The International Olympic Committee spoke highly of the work of the Zhangjiakou Weather Forecasting Team, believing that the weather forecast was reliable and the event broadcast was wonderful.