Trade wind inversion. is a highly stable layer (~2 km high) that caps the moist surface layer (often cloudy) from the dry atmosphere above.

Transcription

1 Hilo 9/19/06 2:00 am HST Td T Trade wind inversion is a highly stable layer (~2 km high) that caps the moist surface layer (often cloudy) from the dry atmosphere above. 1

2 Mountain/lee waves in a stable atmosphere A Lenticular Cloud Over Hawai'i 2

3 Precipitation forms in two ways: Warm Cloud Process Cold Cloud Process Warm Cloud Process Common over warm oceans (e.g., near Hawaii). Little or no ice in cloud. 3

4 Drizzle (size < 0.5 mm) Drizzle is generally a warm cloud process. Large drops fall faster Terminal velocity ~ radius (weight ~ r 3 ; area ~ r 2 ) 4

5 How do tiny cloud drops grow large enough for rain? Small droplets descent slowly and evaporate after falling beneath the cloud. Cloud droplet growth by condensation takes a few days to create a raindrop. A raindrop (2 mm) contains 1 million cloud droplets (0.02 mm) Trades It takes only 15 minutes for an air parcel to reach the top of the Koolau range from Kailua and rain, with the SE trades at 15 mi/hr. 12 km (7.5 mi) Lifting by Terrain (Koolau Range) 5

6 Rain formation in warm clouds Collisions between cloud drops can produce large rain drops through coalescence. Collisions occur in part due to different fall rates of large and small drops. Small droplets large surface tension unfavorable for coalescence. Rain formation in warm clouds Capture of a cloud/rain drop in a cloud updraft can give it more time to grow. The drop falls at its terminal speed relative to the air, not the ground. Large drops fall faster. 6

7 Rain drop size and shape Drizzle drops: 100 s of µm; Rain drops: a few mm Small drops are spherical. In large drops, surface tension is no longer strong enough to overcome flattening of falling drop due to pressure effects The Warm Cloud Process STEPS 1. Cooling usually a result of lifting results in saturation of the air. 2. Condensation water vapor condenses onto condensation nuclei after reaching super-saturation. A slow process without relative motion among droplets. 3. Coalescence takes over as the falling drop starts to collide and merge with smaller droplets in its path. 7

8 Cold Cloud Process origin of most rain over mainland U.S. Supercooled cloud drops and ice crystals coexist when 40 < T < 0 C, because of a lack of freezing nuclei to help cloud drops freeze. Ice Crystals & Ice Nuclei Ice melts at 0 o C. Water does not necessarily freeze at 0 o C (supercooled; discovered in 1783 by H. B. de Saussure). Ice crystal formation usually involves ice nuclei that tell the liquid water how to line up as a crystal. Ice nuclei mimic the shape of ice. Are much less common than cloud condensation nuclei Include some clay mineral particles, bacteria, plant leaf detritus and?? Ice crystals themselves are good ice nuclei. 8

9 Saturation vapor pressure is lower over ice Ice binds water molecules more tightly than liquid water Ice-Crystal (Bergeron) Process Ice crystal growth by vapor deposition at expense of water drops For temperatures less than 0 o C, the saturation vapor pressure over ice is LESS than the saturation vapor pressure over super-cooled water. This leads to evaporation of water from supercooled cloud drops and deposition onto ice crystals. 9

10 Ice crystal growth by Accretion Ice crystals fall faster than cloud drops Crystal/drop collisions allow ice crystals to capture cloud drops This process is known as accretion or riming The supercooled drops freeze upon contact with the ice crystal Extreme crystal riming leads to the formation of Graupel, Hail Ice crystal growth by Aggregation Crystals are most likely to stick when a layer of liquid water resides on the crystal surface. Watch for large aggregates/snowflakes when temperatures are close to 0 o C. 10

11 The Cold Cloud Process STEPS 1. Cooling leading to saturation and condensation same as warm cloud case. 2. Freezing As cloud continues to rise, the air temperature cools below freezing to activate freezing nuclei within super-cooled cloud droplets resulting in freezing. 3. Ice crystal growth Deposition saturation vapor pressure around ice crystals is less than around water droplets crystals grow rapidly at expense of cloud droplets. Accretion (capture of water drops ) as they fall through cloud, Ice crystals grow by collecting supercooled droplets that freeze on contact. Aggregation Falling ice crystals collide and stick to other ice crystals snow flakes Precipitation in Cumulonimbus Low liquid water content promotes deposition (diffusion) growth of large crystals. High liquid water content promotes riming and formation of graupel/hail. If the sub-cloud layer is warm, snow or graupel may melt into raindrops before reaching the surface. Even in summer, much of what is rain, when it arrives at the surface of the earth, might have been snow. --B. Franklin (1784) 11

12 Cold and warm clouds seen from space This image over Southern Brazil, taken from the space shuttle by an astronaut in February 1984, shows a mixture of cold and warm clouds. Warm clouds, consisting of individual cumuli clouds are found near the bottom of the image. The image does not show whether they produced rain. The tops of these warm clouds are generally found below 3.1 miles (5 kilometers). Cold clouds, with large anvils rising typically above 6.2 miles (10 km) dominate the top and left portions of the image. The feathery edge of the anvil indicates a cold rain process, called glaciation. These clouds are capable of producing very heavy rain and thunderstorms. Near the center, about a third and half way from the bottom are cumulus congestus clouds, which can produce rain, and can rise to just above the freezing level near 3.1 miles (5 km), and are still considered warm clouds. Notice the way warm clouds are also found underneath cold clouds, as indicated near the center of the picture. --NASA PREFACE of Tropical Meteorology H. Riehl(1954) In July, 1943, the author first saw the tropics when he arrived in Puerto Rico at the Institute of Tropical Meteorology On the first evening some of the staff walked along the beach and admired the beauty of the trade cumuli in the moonlight. Well schooled in the ice-crystal theory of formation of rain, they had no suspicions about these clouds with tops near 8,000 feet where the temperature is higher than +10 C. Suddenly, however, the landscape ahead of them began to dim; then it disappeared; a roar approached as from rain hitting the roof tops. cloud tops with temperatures below freezing were not needed for the production of heavy rain from trade-wind cumulus. Warm Rain 12