Snow Extent in Northern Hemisphere now Highest Since 1967

Written by Renato R. Colucci on December 5, 2022. Posted in Current News

Snow extent in the Northern Hemisphere at the end of November represents an important parameter for the early winter forecast. This year snow extent is running much higher than average and according to existing global estimates, it is now beyond the highest ever observed since 1967.

Winter forecast, especially in its early phase and in Europe, might be strongly influenced by such a large snow extent, although many other factors need attention.

Northern Hemisphere snow extent is currently indeed very high, now at about 41 million square kilometers, according to the NOAA/Rutgers Global Snow Lab. The most recent snow cover information is given in the image below.

The Weekly Northern Hemisphere snow cover extent for the winter season 2022-2023 in purple is plotted together with the mean (grey dashed line), maximum (blue), and minimum (orange) snow cover extent for each week. Mean weekly snow cover extent and extremes were calculated using the 56-year period from October 1966 to July 2022.

Looking at the below Rutgers Daily Snow Extent map, it is clearly noticeable how Russia is completely covered in snow now. Snow is also seen overwhelming all of Canada, and Alaska, as well as a good portion of the Lower 48. This is an important parameter for the early winter forecast.

You could probably forecast a colder-than-normal winter based on Autumnal Northern Hemisphere snow cover extent alone.

It’s the largest snow extent in decades.

Having so much snow on the ground means any arctic outbreak is going to be a little bit colder. But why should be like this?

The image below shows the daily snow cover departure from the long-term average. In blue and red are respectively shown the positive (more snow extent than the long-term average) and negative (less snow extent than the long-term average) anomalies. In Eurasia vast areas with larger snow extent is present especially over the eastern European countries boarding Russia and Central China.

In North America snow extent for the end of November is more south than normal, especially in the North West and North East sectors of the U.S: Small and scattered negative anomalies are less significant in the picture, mostly affecting central Mongolia, North East China, and the central U.S.

According to the Finnish Meteorological Institute, also the total snow mass for the Northern Hemisphere is tracking comfortably above the 1982-2012 average. This result is based on the current Northern Hemisphere snow-water equivalent relative to the long-term mean and variability.

Snow extent grew pretty fast in the last 30 days, as it should generally be in this period of course, but 2022 is setting a clearly very favorable season for snow on the ground.

See the source article to view a video showing this – Ed

By the way, as a curiosity, the temperature is always colder on the ground in winter and at night in calm conditions, that is without or with the very weak wind. That’s because the Earth warms up and cools off much faster than the atmosphere does.

More, colder air is denser, therefore the air near the ground is colder at night than the higher air. That’s why for instance your thermometer says +1 or +2 degrees Celsius, but you might see frost on the ground.

Increasing snow extent prior to official start of winter season

Extensive snow extent early in the season is an indicator of persistent cold as we head into winter proper, mainly due to albedo and emissivity feedback.

Albedo is defined as the fraction of light that is reflected by a body or surface. On average, the Earth reflects about 30 percent of the sun’s radiation, but when fresh snow covers the ground that value nearly triples and it is close to 1.0. In fact, snow has the greatest albedo on Earth, from less than 0.60 for wet and melting snow to greater than 0.85 for fresh snow.

The albedo of fresh snow could be as higher as 0.95, which means it reflects back to space 95 percent of the sun’s radiation, keeping the ground and air much cooler.

It represents an important parameter in climatology since recent albedo decreases in the Arctic have increased heat absorption at the surface leading to the so-called Arctic Amplification.

Practically, Arctic lands and sea are warming faster than other environments on Earth due to the positive feedback induced by the darker surface of the land and sea in summer, where there is no snow or ice on the ground or sea ice melted.

Because of the changes in the solar elevation and the global radiation with time and geographical location, the albedo effect on the surface radiation balance, thus the surface temperature, also changes spatially and temporally.

But besides albedo, snow has also high emissivity that cools the ground. Over the thermal infrared part of the radiative spectrum, snow acts almost as a “blackbody.” The emissivity of snow varies from 0.96 to 0.99, with an average of about 0.98.

Without going too much into the details, we can say that such value is generally higher than any other land surface emissivities. This means, when the ground is covered in snow the higher emissivity of a snow surface causes an increase in the outgoing longwave radiation, thus cooling the snow surface.

The overall impact of snow cover on the ground thermal regime depends on several factors: the timing, duration, accumulation, and melting processes of seasonal snow cover.

In general, cold, dry, and clear-sky conditions generate higher emissivity of snow surface resulting in surface cooling and very often the development of a low-level temperature inversion. This is the way how the well-known Siberian High forms at this time of the year.

The Siberian High is a massive pool of cold dry air that accumulates in the northeastern part of Eurasia from September until April. It is usually centered on Lake Baikal.

It reaches its greatest size and strength in the winter when the air temperature near the center of the high-pressure area is often lower than −40 degrees Celsius. The Siberian High is the most important atmospheric center of action in Eurasia during the winter months.

The Siberian High could strongly affect the weather in western Europe when specific patterns develop and typical retrogressive flows occur producing unusually frigid conditions over the “old continent”. In the image below we can see an example from early February 2012 when a historical cold outbreak affected Europe and the Mediterranean.

The associated air mass moves in the low troposphere being formed by very cold, dry, and dense air. The Siberian High is defined as a surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs.

Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs.

On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface (roughly 5500 meters) about midway up the troposphere. Because they weaken with height, these high-pressure systems are cold.

The intensity of the Siberian High, experienced a marked decrease in the last 50 years, showing a strong correlation with the temperature rise during the same period. In general, the Siberian High and the winter temperature in Europe have an almost perfect mirroring relationship during 1960–2010, suggesting a direct impact of the former on the latter.

Note the significant upticks in the recent measurements.

Nevertheless, numerous studies have found recently an unexpected phenomenon of extensive fall and winter cooling over central Eurasia not fully reflecting the nature of climate variability over time.

A team of scientists has shown how autumn cooling in Eurasia is likely influenced by the Pacific Decadal Oscillation (PDO) and Siberian high (SH). Since 2004, the strengthening of the PDO and SH explains approximately 54 and 18 percent of the autumn cooling in Eurasia, respectively.

The relationship between autumn snow cover and the ensuing winter temperatures is generally much stronger for Eurasia than for North America. For Eurasia the average snow cover extent during the autumn explained as much as 52 percent of the variance in the winter (December–February) temperatures compared to only 12 percent for North America, a study found.

Nevertheless, when the average winter snow cover is correlated with the average winter temperature the relationship is better for North America than for Eurasia. As much as 46 percent of the variance in the winter temperature is explained by the winter snow cover in North America compared to only 12 percent in Eurasia.

This is because extensive snow cover over North America decreases surface air temperature directly via local diabatic cooling.

More, coincident with the strengthened Siberian High and the accelerated East Asian jet stream in January, and December lower surface air temperature in North America decreases surface air temperature over midlatitude Asia.

Hence, December North American snow cover could potentially be exploited for sub-seasonal predictability over Asia.

See more here severe-weather.eu

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