Image Credit: BBC
Over the past few years, you’ve probably heard about climate change. And global warming. But what’s the difference? Is there even one? Why has the name seemingly changed? The use of both of these terms can be confusing, so let’s get them sorted out.
Global warming is the term that was originally most widely used to refer to the observed rapid changes in Earth’s climate system. It refers to the fact that overall, the planet is getting warmer. While there are some areas of the planet experiencing a cooling trend in their climate, the Earth as a whole is warming, even when these cooling pockets are taken into account. However, this warming is only a piece of the story of what is currently happening to our planet. Global warming is, in fact, a symptom of climate change.
The term “climate change” refers to the effects that global warming is having on the planet. The climate system involves more than just the temperature of the atmosphere. It also encompasses moisture in the atmosphere, and where and when it rains. Additionally, increasing global temperatures has a number of physical effects on the planet, such as melting land ice, sea level rise, and intensifying hurricanes, just to name a few. These, in turn, affect Earth’s climate. Thus, scientists use “climate change” to refer to all of the changes that are resulting and will result from increased global temperatures. Climate change is an umbrella term, under which global warming falls along with the other effects associated with an overall warming planet.
The media has shifted from global warming to climate change in their stories simply because “climate change” is a broader term that encompasses all changes and is more accurate if something like sea level rise or changing precipitation is being discussed. Scientists, however, have been using both terms for a long time. Each of these terms serves a different purpose for scientists in talking about what is happening to our planet. “Global warming” hones in on temperature changes, while “climate change” helps discuss other changes occurring in the Earth’s systems. While the two terms have been used interchangeably in public discourse and are closely related, it is important to remember the distinction between them.
©2019 Meteorologist Margaret Orr
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Image Source: https://www.bbc.com/news/science-environment-47144058
Spring is the season when severe weather kicks up again. In some areas, like the Central United States, it also means that tornado season will be starting up soon. And while this fact is common knowledge, the science behind why this season experiences such a noticeable uptick in severe weather events isn’t discussed as frequently. In this article, we will be exploring the key ingredients required for severe thunderstorms like supercells to form and how they come together at much greater rates during the spring months than at other times of the year in the mid-latitudes.
Ominous skies over eastern Colorado in May 2018.
The key ingredients for supercell thunderstorms are: instability, a source of lift, moisture, and wind shear. Instability refers to the general alterations of atmospheric stability. For instance, if you were to draw out a parcel of air, said parcel would rise, fall, or remain in its current location depending on how stable (or unstable) its surrounding environment was.
Source: Columbia University
The atmosphere can become more unstable via surface heating; whenever sunlight warms the ground, certain parcels at and near the surface will warm at a faster rate than their surrounding ones. It is these lighter, warmer parcels that will rise and will continue to do so until they enter a cooler environment. They themselves will then begin to cool and become heavier until they eventually sink back down to the surface where they can be heated up again and repeat this convective process.
Source: Columbia University
In order for storms to initiate, there must be a source of lift that allows for parcels of air to be vertically driven into the atmosphere. As such, the solar radiative heating that was recently described is one of many sources of lift that are available to prospective storms. If there is both sufficient heating and enough moisture over a localized area, then we can further expect for these parcels to vertically carry that water content along with them, leading to the vertical mixing of both heat and moisture across several layers of the atmosphere. These parcels will eventually condense and produce isolated showers and thunderstorms over those locations. Once they die, their outflow can help to produced localized areas of lift that can allow for a few more of these sorts of thunderstorms (oftentimes called garden-variety thunderstorms) to develop. These sorts of environments can be routinely found in tropical and subtropical areas such as Florida or the Caribbean, especially during the summer months. This makes spring unique for areas in the mid-latitudes in that it is around that season that plants and other forms of vegetation begin to grow and mature. In states such as Illinois and Indiana, which host large amounts of agricultural production, the added vegetation provides moist soil and added surface moisture content.
Source: Thomson Higher Education
That being said, these types of thunderstorms are almost never severe; once the environment runs out of surface moisture to mix into the atmosphere by the mid-to-late afternoon hours, then any storms that did develop will soon die out. In fact, these sorts of thunderstorms tend to have life expectancies of just under an hour. In order for storms to maintain themselves, they require another ingredient that has not yet been discussed.
We have already discussed one primary lifting mechanism: solar radiation. This process, however, only accounts for isolated thunderstorms to develop, given that such thunderstorms are limited to pockets where the air has become sufficiently unstable enough for parcels to rise and condense. If we take a look at North America, the return of spring re-introduces warmer air masses to the rest of the continent, while still experiencing cold spells whenever arctic air masses sink down into the lowe-48. The fronts that divide the two air masses themselves become large-scale lifting mechanisms that can also trigger thunderstorms. Moreover, geography also can play a significant role in providing sources of lift, as they are natural barriers that an air mass must overcome by rising on the wind side of the mountain before descending again on the leeward side. If the ascending air mass has sufficient moisture content and there is enough radiational heating, then the end-result can be oragraphically-induced thunderstorms.
Source: Thompson Higher Education
Starting around the start of spring, low-level moisture tends to also be advected (transported) from the Gulf of Mexico and into the Great Plains of North America. This introduction of moisture into the environment allows for any prospective storms to have a greater source of moisture than they otherwise would, aiding in the production of very sharp contrasts between moist and dry air masses. It is these sorts of boundaries between the two air masses that lead to the development of a dry line, which commonly develops in the Great Plains when this sort of moisture advection occurs. This area of the world also harbors a unique topographical setting which further allows for the production of dry lines; the elevation of the Great Plains gradually increases from east to west given they cover an area that’s stretches from the Mississippi River Basin all the way to the front-range of the Rocky Mountains. As such, when Gulf Moisture moves across the Great Plains, it slows down and fills in the areas which host lower elevations. As it does so, this air mass then comes in contact with much drier air from the American Southwest, which descends into the Great Plains from areas of higher elevation. Given that this drier, denser air mass, is descending into a moister, lighter air mass, the end result is a source of lift that can enable for much larger areas of ascent that can cover large swaths of the Plains on a much greater scale than what is observed from isolated, tropical and subtropical thunderstorms.
Source: Kendall/Hunt Publishing
We have already discussed three of the key ingredients that are required for severe thunderstorms: a source of lift, instability, and moisture. And as one would imagine, some of the most severe thunderstorms on the planet are a result of strong sources of lift, such dry lines. Nevertheless, the final ingredient required for supercells to initiate given that without it such a thunderstorm would not be able to maintain itself: wind shear.
Source: Earth Sky
Wind shear is essentially the change in wind speed and/or direction with respect to height. In the case of isolated thunderstorms that occur in the tropics, this ingredient is nearly non-existent given that the air masses of such environments tend to be stable with the exception of the localized areas of enhanced heating where the storms were able to initiate from. As such, once the thunderhead reaches its highest level in the atmosphere, all of the parcels that have cooled and condensed will sink back down to the surface. This downdraft will then cut off any supply of warm, moist air that initiated the thunderstorm, and produce an outflow boundary that may help to produce a new one in its wake. Under conditions in which there is greater wind shear, however, such a thunderstorm would have a downdraft that sinks further away from the core of the storm. As such, these thunderstorms require sources of lift that can produce sufficient enough wind shear given that this difference in the location between a thunderstorm’s updraft and downdraft will then in turn allow for a thunderstorm to maintain itself for a much longer period of time, strengthen, and become a much more severe thunderstorm that can potentially produce hail, damaging winds, and of course tornadoes.
Late-afternoon supercell over Nebraska
Given this requirement, supercells are most likely to be produced over areas with strong, well-defined air mass boundaries, such as surface fronts or dry lines. As such, the mid-latitudes, which include areas like the Great Plains of North America, are prime locations for such thunderstorms to develop in the spring. This season is when these boundaries begin to develop at greater rates across the continent while the reintroductions of vegetation, along with increased radiative heating, ensure that the environment is much more prime for supercells and the severe weather hazards that come along with them.
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©2019 Meteorologist Gerardo Diaz Jr.
April Showers, Bringing May Flowers? …. Global Precipitation Climatology (Photo Credits: Sharon Sullivan, Global Precipitation Climatology Project)
April showers bring May flowers - or so we’ve been told - this phrase can be traced back to the mid-1500s from a collection of writings known as “A Hundred Good Points of Husbandry” - “Sweet April Showers do spring May Flowers”. But, with portions of the Southwestern U.S. still in moderate to severe drought categories, what does that mean for May flowers?
Precipitation is defined by the AMS Glossary as “liquid or solid phase aqueous particles that originate in the atmosphere and fall to the earth’s surface”. Precipitation is important for understanding climate evolution and hydrological applications ranging from agriculture to flooding. Extensive research has been undertaken in recent decades to develop a global precipitation climatology. The main source of precipitation climatology is from the Global Precipitation Climatology Project (GPCP), a joint effort that attempts to merge data from 6000 rain gauge stations, geostationary and polar-orbiting satellites, radar, and sounding observations in order to estimate total monthly rainfall.
Rain gauge observations have been used to estimate precipitation rate, however, the global distribution of gauges is population dependent and assumes there is no potential losses from the melting of frozen precipitation. Radar can be used to estimate rain rate through comparison of hydrometeor size to the radar reflectivity factor, but beam blocking, attenuation, and great distances from the radar can leave gaps in precipitation climatology. While satellites may tend to underestimate precipitation compared to gauges and radar, they can infer precipitation amounts over oceans, complex terrain, and sparsely populated areas by determining cloud types/depths through the visible and infrared channels.
Annual mean precipitation distributions in mm day-1. The global mean is 2.61 mm day-1, or 37.480 in year-1 (Global Precipitation Climatology Project)
On global, regional, and local scales, precipitation is controlled by the availability of water vapor, temperature, aerosols, cloud type, dynamics (latitude), and orography (local). The Northern Hemisphere tropics precipitate on average 50% more than the Southern, due to a greater proportion of land mass. Precipitation tends to follow a diurnal cycle, but this pattern is regionally dependent. During spring, the best of both precipitation dynamics - winter and summer - converge. The jet stream remains strong, holding onto the cold winter chill, as sunlight warms the lower atmosphere. The Rocky Mountain Range in the central United States typically experiences high-frequency precipitation events in the mid-afternoon summers and early fall (suggesting that high surface temperatures drive convective processes), while early morning rainfall may occur in Thailand and the Bay of Bengal during monsoon season. For New Mexico in particular, the summer monsoon months receive almost half the average precipitation for the year. The poles typically have lower precipitation amounts (<1 mm day-1) due to the lower water vapor content, as colder climates aren’t able to hold as much water vapor in the atmosphere above. Another important phenomena associated with global precipitation variability on interannual time scales is El Niño-Southern Oscillation (ENSO), where changes in sea surface temperature can modify the position of storm tracks in the Northern Hemisphere.
Thus, an accurate precipitation climatology is essential to improve model & satellite verification, make short-term forecasts more effective, and even help to predict shifts in global precipitation patterns in the future. Just as a reminder than even the most unpleasant of things (in this case, the heavy rains of April) can spring forth even the most enjoyable of things - the abundance of May flowers.
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©2019 Meteorologist Sharon Sullivan