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May-June 2014

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Weather Queries

Q: Middle and high clouds travel with weather systems, but do the lower clouds near the surface do the same, or develop and dissipate as the system moves through any one location? Also, here in Southern California, I hear forecasts such as “low clouds and fog spreading inland during the night and mornings.” Do these low stratus and stratocumulus clouds actually move inland, or do they develop as the sea breeze front moves inland?

Robert Price
Sun City, California

These questions are alike, in that they both refer to the persistence of clouds.

The context for the first question is the normal progression of waves in the upper tropospheric westerlies, which are accompanied by high and low pressure systems at the surface. The westerlies are strongest, usually featuring an embedded jet stream, during the winter, when the contrast between warm air at low latitudes and cold air at high latitudes is greatest. The wavy pattern of the jet stream consists of ridges and troughs. Ridges are regions where contours, following the flow, curve in the clockwise direction (anticyclonically) in the Northern Hemisphere. Large ridges usually feature poleward excursions of the flow. Troughs are regions where contours curve in the counterclockwise direction (cyclonically) in the Northern Hemisphere. Large troughs usually feature equatorward excursions of the flow. A ridge axis is a line that intersects flow contours at the point of maximum anticyclonic curvature. A trough axis is a line that intersects flow contours at the point of maximum cyclonic curvature.

With the above definitions, we can discuss cloud formation and movement. Upward motion usually precedes a trough axis. Downward motion usually precedes a ridge axis. Clouds form when rising air cools sufficiently to cause condensation of water vapor. If the cloud becomes deep enough, liquid droplets or ice crystals will grow and eventually fall from the cloud as precipitation. Clouds dissipate when sinking air warms sufficiently to cause evaporation of droplets or sublimation (phase change from solid to vapor) of ice crystals. Because upward and downward motions associated with winter storms often persist for hours and occupy areas hundreds of kilometers across, the middle and high cloud shields often maintain their shapes for extended periods, though individual cloud elements within the larger cloud mass morph continually.

The following figures illustrate the foregoing concepts using satellite images and weather maps for the mid-troposphere. Figure 1 is an infrared image of the United States and southern Canada at 2031 UTC (Coordinated Universal Time, also called Greenwich Mean Time, the local time in Greenwich, England) on February 2, 2014. 2031 UTC is afternoon in the United States. An infrared image depicts the temperature of the target—either the ground if the sky is clear or the tops of clouds. Temperatures are color-coded according to the color bar at the top. The darker the shade of gray, the higher the temperature. Yellows depict temperatures in the –30°C to –40°C range, and reds in the –40°C to –60°C range. Not everything colored yellow is cloud. From northern Alberta to northern Manitoba, Canada, the surface temperature is below –30°C (–22°F), as shown by the large, rather featureless expanse of yellow in that area. When viewing an animated sequence of infrared images, it is easy to distinguish between clouds and very cold ground because the clouds move.


Figure 1. Composite infrared image from two United States GOES satellites for 2031 UTC, February 2, 2014. The color bar at the top is the key to the target temperatures displayed in the image.

Figure 2 helps to explain, at least partially, the distribution of middle and high clouds in Figure 1. The dark contours show the altitude of the 500-mb pressure surface above sea level at 0000 UTC on February 3. These contours closely follow the wind flow. Four trough lines are marked in bold purple: one about to enter the Pacific Northwest, another off the California coast, a third stretching from eastern Manitoba to eastern Minnesota, and a fourth in the Central Plains from Nebraska to Texas. High clouds, arising from upward motion, precede the trough in each case except in the Pacific Northwest. Extensive cloudiness precedes the latter trough, but the cloud tops are not quite cold enough to be colored (but see Figure 3, which shows that these clouds have thickened just six hours later). The long swath of high clouds from southern Illinois to New England arises from the presence of a strong frontal zone, overlain by a jet stream. Atmospheric flow is essentially straight in this region, and the extensive cloudiness is caused by upward motion along the front rather than the approach of a trough in the westerlies.


Figure 2. Height of the 500-mb pressure surface above sea level at 0000 UTC on February 3, 2014. The solid black curves are height contours spaced 60 meters apart. These contours follow the wind flow fairly closely. The dashed red contours are isotherms, spaced 2°C apart. All contours are based on measurements taken by radiosonde balloons, which measure temperature, dewpoint, pressure, and wind. Data are plotted at each balloon release point, with temperature (°C) at the upper left, dewpoint (°C) at lower left, height in tens of meters (“549” means 5,940 meters) at the upper right. Wind direction is depicted by the staff extending upwind from the station circle; wind speed (knots) is indicated by the barbs on the staff, with a solid flag indicating 50 knots, a full barb 10 knots, and a half barb 5 knots. The bold dashed purple lines indicate trough axes.

Subsequent figures illustrate the temporal continuity of the large cloud shields associated with the troughs. Figure 3 is an infrared image for 0231 UTC on February 3—six hours later than the image in Figure 1. Cloud tops in advance of the trough in the Pacific Northwest have become higher and colder, the cyclonic circulation in high clouds is still evident in the trough nearing the California Coast, and high clouds are increasing in southern California and Nevada. The cloud shield associated with the trough in southern Canada has crossed the border into Ontario and maintains its basic shape. The cloud shield in advance of the trough in the Southern Great Plains has expanded and consolidated just east of the Mississippi River.


Figure 3. Composite infrared image from two United States GOES satellites for 0231 UTC, February 3, 2014. The color bar at the top is the key to the target temperatures displayed in the image.

In Figure 4, for 0831 UTC, high cloudiness associated with the trough approaching the Desert Southwest has become disorganized, but the curvature in the mid-level clouds suggests that the trough line runs from near Point Conception (the prominent outward bulge in the California coastline west of Santa Barbara) and extends southwestward. High cloud shields to the east of the other three trough lines remain well defined and cohesive. The extensive patch of yellow in the western Canadian provinces shows the southward progression of surface temperatures less than –30°C as the night wears on.


Figure 4. Composite infrared image from two United States GOES satellites for 0831 UTC, February 3, 2014. The color bar at the top is the key to the target temperatures displayed in the image.

Figure 5 is the 500-mb chart for 1200 UTC on February 3. A comparison of Figures 2 and 5 shows that all four troughs have advanced significantly. They are, after all, embedded in strong, wintertime flow.


Figure 5. Height of the 500-mb pressure surface above sea level at 1200 UTC on February 3, 2014.

Figure 6, an infrared image for 1431 UTC on February 3, just two and one-half hours behind Figure 5, shows a clear relationship between the positions of the trough lines and middle and high clouds to their east, except perhaps for the trough in the Southwest. A streak of high clouds from extreme northern Baja to southwest Utah is related to the trough, but the other streaks to the east are associated with the strong, almost straight-line jet stream (135 mph) in the area at about 35,000 feet (not shown).


Figure 5. Height of the 500-mb pressure surface above sea level at 1200 UTC on February 3, 2014.

To summarize the foregoing, rising motion in the middle and upper troposphere usually precedes mid-latitude troughs embedded in the prevailing westerly flow of winter. The rising motion prompts the development of large and deep cloud shields, which travel at the speed of the trough and maintain their identity, sometimes for a day or two. Cloud features within a large cloud shield are constantly merging, separating, and morphing. The most important point is that the clouds stay with the large-scale vertical motion.

Now to address the question, “Do lower clouds move with upper air troughs, or do they form in advance of trough passage and dissipate afterwards?” Troughs embedded in westerly flow usually move with the flow. The four troughs just examined are in this category. They are called short-wave troughs, as distinguished from long-wave troughs, which move much more slowly (the flow moves through them); they can cover half the United States, and can dominate the weather pattern for weeks on end. Discussion here is confined to the influence of short-wave troughs on low-level cloudiness.

Short-wave troughs usually travel with the speed of the mid-tropospheric winds—not as fast as jet stream winds in the high troposphere, but faster than low-tropospheric winds. It follows that clouds in the lowest kilometer or two move more slowly than the short-wave trough, and so should form as the trough approaches and dissipate as the trough recedes. Figure 7 illustrates this idea. It is a vertical cross-section through the troposphere, aligned with the jet stream. It shows idealized air trajectories relative to the moving trough. At low levels, air rises toward the trough axis and sinks behind it. Jet stream air in the high troposphere approaches the trough from behind, reaches its lowest point near the trough axis, and rises ahead of it. This juxtaposition of rising and sinking air on opposite sides of the trough axis usually results in abrupt clearing of middle and high clouds after trough passage. In Figure 7, it is worth noting that the trough axis need not be vertical. For example, a trough axis tilted backward (toward the left) with altitude would indicate a deepening (intensifying) trough.


Figure 7. A vertical cross-section aligned with the jet stream, showing idealized air flow in the upper and lower troposphere relative to a moving short-wave trough. Note large-scale lifting ahead of the trough and sinking behind it.

 

The fall velocity of individual ice crystals or tiny droplets that comprise middle and high clouds is hardly perceptible, so that, once these clouds form, they can persist for many hours and drift with the wind, with little or no updraft to sustain them. Low clouds typically undergo more rapid changes once they form because temperature and humidity near the ground are strongly influenced by the diurnal heating cycle, land-sea boundaries, and hills or mountains.

Here is a typical scenario as an upper trough and associated surface low-pressure system approach and pass by:

  • 1. High clouds first, slowly thickening and lowering as a warm front approaches.

  • 2. Steady precipitation, moistening the air close to the ground and hastening the formation of low clouds in gently rising air. These stratus clouds form in the precipitation; they are not imported from elsewhere.

  • 3. Warm front passage, followed by at least partial clearing. Cumulus clouds may form and grow in the warm sector. Their shape and depth change rapidly. If they grow tall into thunderstorms, their anvil tops may be swept rapidly forward by high winds in the upper troposphere. Most such clouds are ephemeral.

  • 4. Cold front passage with showers, followed by stratocumulus clouds. Colder air following the front, passing over warmer and perhaps moist ground, creates a well-mixed layer in the lowest kilometer or so, where convective (up-and-down) motions occur freely. Under these conditions, stratocumulus clouds, their vertical growth limited by a frontal inversion, form over large areas and often persist for one or two days. They may partially disappear at night but reform the next day as solar heating rejuvenates the mixed layer. Sinking air in the lower troposphere behind the trough eventually causes their demise.

In summary, nearly all clouds depend upon rising air for their formation. Large expanses of high clouds form in the gentle, but steady, upward motion ahead of short-wave troughs. Though these cloud shields usually maintain their identity for a day or two, individual clouds within them are morphing constantly. Low clouds tend to form in air moistened by precipitation as a short-wave trough approaches and prior to warm front passage. Extensive stratocumulus clouds form in the well-mixed layer following cold-front passage. These may persist for a couple of days until sinking air well behind the trough causes them to evaporate.

The second question is, “Do coastal stratus clouds in southern California actually move inland, or do they develop as the sea breeze front moves inland?” Such clouds are confined to what is called the marine layer—a shallow layer of air, moistened and chilled by the relatively cold ocean surface, capped by a sharp inversion (increase of temperature with altitude), and commonly found along the California Coast, especially during the warm season. Figure 8 shows afternoon stratus clouds off the California coast from San Francisco at the upper left, to north of San Diego, which lies just off the lower right-hand corner. In this afternoon image on April 22, 2013, from the GOES 15 satellite, stratus clouds have encroached on the shoreline at center, whereas they remain a few miles offshore from Santa Barbara to San Diego.


Figure 8. A large expanse of stratus clouds, capping the marine layer, off the coast of southern California. This is an image at visible wavelengths acquired by the GOES 15 satellite at 2230 UTC on April 22. 2013.

 

Coastal stratus is a semi-permanent feature off the southern California coast during the warm season. It tends to move inland in the evening and disappear during the morning, but it does not necessarily accompany the wind shift that marks the sea-breeze front.

Daytime heating of the land leads to lower pressure over land than over water. This draws marine air inland, but as this air passes over warmer ground, it heats up from below, and rising bubbles or thermals develop within it. As these reach the top of the marine air layer, they mix with the warmer, drier, desert-like air above, causing evaporation of the stratus clouds and limiting their inland penetration. This may persist for several hours. As the sun sinks toward the western horizon, the radiative energy balance reverses over land. More energy is lost from the ground through infrared radiation than is absorbed at visible wavelengths from the sun. The ground begins to cool, and thermals no longer exist. This allows the coastal stratus to move inland unhindered. In addition, the rapid decrease in moisture at the top of the marine layer promotes cooling by means of infrared radiation from the cloud tops, thereby strengthening the inversion and protecting marine air from mixing with the warm, dry air above. Marine stratus may persist beyond daybreak, until the sun again begins to heat the ground and erode the stratus clouds.

In summary, large patches of stratus clouds persist for days offshore during the warm season, but they seldom move bodily onshore or offshore. Mixing of marine and desert air and radiative effects strongly influence the appearance and disappearance of these clouds over land.

A fascinating look at the daytime evolution of stratus off the southern California coast is available at http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2010/09/100920_g15_srso_socal_vis_anim.gif.

I thank John M. Brown of NOAA's Earth System Research Laboratory (ESRL) for help in answering these questions. John C. Osborn, also of ESRL, assisted with Figures 1 through 6.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 530 Walnut Street, Suite 850, Philadelphia, PA 19106.

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