Friday, January 9, 2015

Madden-Julian Oscillation

The Madden-Julian Oscillation (MJO) is a periodic fluctuation of convective activity in the tropics that impact the activity of the monsoon and the formation of tropical cyclones in various basins throughout the world.

Near the equator, weather is more chaotic and less structured than at midlatitudes. At midlatitudes, large cyclones and, sometimes, attached frontal boundaries, govern weather patterns. However, the formation of the vortices of these cyclones requires an effect called the Coriolis effect. The Coriolis effect, in essence, induces rotation in midlatitude regions because the rotation of the Earth carries points at different latitudes at different speeds. But at the equator and the area immediately around it, this effect is not significant enough to cause rotation under usual circumstances. Thus weather patterns in the tropics are rather caused by undulations of the Intertropical Convergence Zone (ITCZ), which seemed to produce shower and thunderstorm activity randomly.

However, some evidence contrary to the idea that the fluctuations were random has been known since antiquity. India's weather, for example, includes clearly demarcated dry and wet seasons in association with the monsoon, despite the fact that southern India's weather is predominantly affected by the activity of the ITCZ. In the 1970's, when more meteorological data became available, scientists Roland Madden and Paul Julian, after whom the oscillation is named, noted that there was a recurring cycle between increased precipitation and suppressed precipitation in the tropics, particularly notable in the Indian Ocean and the Pacific Ocean. More data emerged over the coming years, revealing that the anomalies in tropical precipitation tended to follow the equator, moving east around the globe over time.

The areas of increased and decreased thunderstorm activity associated with the MJO span several thousand miles; in areas of increased activity, the MJO is said to be in positive phase, and in areas of suppressed activity, in negative phase. Each "phase" propagates eastward around the globe at 4-8 meters per second, and therefore takes 30-60 days to travel around the Earth. This time interval, though not an exact value, is the period of the MJO, or in other words, the time necessary for the precipitation anomalies to return to roughly their initial conditions. At any given time, there are about 1-2 areas of increased convection and 1-2 areas of decreased convection that span the tropics.

The above water vapor imagery from the NOAA shows a dry air mass over much of the central Atlantic and Caribbean. The increased tendency for dry air masses to appear in an area of the tropics is thought to correlate with the negative phase of the MJO.

The MJO, as well as governing the onset of the monsoon in India and other locations, can also have an effect on tropical development, even in the Atlantic and Pacific basins. Dry air is often devastating to tropical cyclones, invading their circulations and weakening or even dissipating them. Though some parts of the typical Atlantic and Pacific hurricane seasons are more active then others, in any particular season, the tropical cyclone activity may modulate with the MJO, increasing during positive phase and decreasing in negative phase. In support of this theory, it is a well documented fact that the Atlantic basin tends to be quiet when the Eastern Pacific basin is active and vice versa. This is because these neighboring basins tend to be experiencing opposite phases of the MJO at any given time.

The study and modeling of the MJO is very important to meteorology becuase this oscillation brings order to the chaos of tropical weather. Greater understanding of the MJO will aid long-term and large-scale forecasts, and improve our ability to antipicate tropical cyclonogenesis.


Thursday, January 1, 2015


Derechos are a meteorological phenomenon consisting of long lines of heavy thunderstorms persisting for several hours and traveling a large distance. Unlike typical pop-up thunderstorms which are usually localized, derechos bring all the threats of severe thunderstorms—heavy rainfall, hail, high winds, and possibly tornadoes—to large swaths of land. Thus derechos can be particularly damaging to life and property.

The word "derecho" is Spanish for "straight", and this name reflects the nature of the wind damage associated with the storms. For reasons discussed below, derechos cause "straight-line wind damage". For a thunderstorm complex to be a derecho, the complex must cause severe wind gusts (where "severe" refers to winds in excess of 50 kt or 58 mph) along a swath of land extending at least 250 miles in length.

The conditions that spawn derechos often involve the collision of moist tropical air and cool air from the polar regions, often along a cold frontal boundary. The warm, unstable nature of the tropical air causes it to rise near the border of the two air masses as the cold air slides beneath it. This process forms cumulonimbus clouds, very large clouds that can stretch from the lower atmosphere to over ten miles high. Such clouds are characteristic of thunderstorms, so storms tend to form along the frontal boundary, giving rise to a long ridge of clouds called a shelf cloud:

A shelf cloud, representing the leading edge of a line of thunderstorms

Another defining feature of derechos is the curvature the line of thunderstorms that develops as the system progresses. The curved radar signature left by these storms are called bow echoes, due to their likeness to archer's bows. This phenomenon is illustrated below by a composition of time-step radar images from a derecho on June 29, 2012.

Seven consecutive radar images are superimposed, showing the development of the bow echo. In addition, the image displays wind gusts observed at different locations during the progress of the derecho. Since several of these in fact were hurricane-strength gusts (exceeded 73 mph), and the storm traversed over 450 miles during its lifetime, this system does indeed meet the qualifications of a derecho.

As noted above, the hallmark of a derecho is straight-line wind damage. The sources for such winds is the downburst, a phenomenon in which cool air descends quickly to ground level and spreads in all directions, sometimes causing extremely high winds. Downbursts occur when a large air mass is rapidly cooled by the evaporation of water, the sublimation of ice directly into vapor, or the melting of ice crystals. Since all of these processes are endothermic, or require energy from the surrounding environment, they cause the air in the vicinity to cool. The mass of air, having been cooled, is now heavier than the surrounding air and accelerates toward ground level. When it reaches the ground, it is forced outward in all directions, causing winds spreading uniformly from the impact point, hence the straight-line winds.

The life cycle of a downburst

Within a downburst, which may span distances of several tens of miles, there are sometimes smaller-scale features known as microbursts, which are a few miles in length and contain especially intense winds. On a yet smaller scale are the burst swaths that sometimes occur in microbursts, areas of just thousands of square feet in which the wind speeds may rival those of a tornado. The straight-line winds of a derecho are also responsible for the bow echoes that appear on radar. Winds emanating from near the center of the squall line fan out and cause its edges to bend into the bow shape.

Meteorologists and climatologists, using data from the past 30 years, have analyzed how often and where derechos occur, predominantly within the United States.

The above map shows the frequency of derechos in different areas of the United States. Virtually no derechos have occurred in the west, and the most active area for derechos is the southeastern plains, which periodically have multiple derechos in a single year. Note also that these storms are most common in the area stretching from Minnesota to western Ohio during the warm season, and in an area from eastern Texas through the southern Gulf states during the colder months. It is likely that derechos occur regularly outside the United States, but very few thunderstorm events have been formally classified as such.