Why Cloud Droplets Don t Fall

Transcription

1 Introduction to Climatology GEOGRAPHY 300 Tom Giambelluca University of Hawai i at Mānoa Precipitation Processes Why Cloud Droplets Don t Fall Cloud droplets are too small to fall to the ground: Low terminal velocity. For a round drop: Gravitational force is proportional to the cube of the drop radius Drag force is proportional to the square of the radius and the square of the velocity As a drop accelerates, drag force increases until it eventually equals gravitational force, and the velocity becomes constant at the terminal velocity Both gravitational force and drag force increase with drop size, but gravitational force increases more Therefore, large drops have higher terminal velocities than small drops Evaporation Droplets and Drops Droplets to Raindrops Growth of droplets to raindrop size can occur by one of two processes: In Warm Clouds: Collision-coalescence In Cool and Cold Clouds: Bergeron process 1

2 Collision-Coalescence All cloud droplets falling at their terminal velocities Terminal velocity is a function of drop size If all drops the same size, no collisions A relatively few large drops can initiate the process Bergeron Process Saturation vapor pressure over ice is lower than that over liquid water. Droplets can remain liquid at temperatures below freezing, if freezing nuclei are not in sufficient abundance. These the air will contain a mixture of liquid drops and ice. Because of the difference in saturation humidity, the air will become supersaturated with respect to the ice. This will lead to rapid sublimation and rapid growth of the ice particles. The depletion of water vapor will cause the air to become unsaturated with respect to the liquid droplets, leading to evaporation of water from the droplets. This process effectively transfers water from a large number of small droplets to a small number of large ice particles. Rainfall Measurement Radar Standard raingage Recording raingages weighing type float type tipping bucket optical tipping bucket raingage 2

3 Mapping Rainfall The Rainfall Atlas of Hawai i 1986 RAINFALL Update the monthly rainfall data base Correct station geographical coordinates for station locations Use state of the art techniques to fill gaps Develop independent estimates of spatial rainfall patterns Radar rainfall Mesoscale meteorological model estimates Vegetation-based estimates PRISM Merge (fuse) all estimates using raingage station means as the standard Incorporate assessments of uncertainty into the process of producing the fused rainfall maps Spatially variable uncertainty in each of the predictor data sets Uncertainty in the station 30-yr means Gap filling error Small sample error Location error PREDICTOR DATA SETS Radar Rainfall Radar rainfall estimates MM5 model rainfall estimates PRISM rainfall estimates Vegetation-based rainfall estimates 3

4 MM5 Rainfall July-August 2005 example PRISM Rainfall (Nguyen, 2006 ) Chris Daly, PRISM, Oregon State University Vegetation-Based Rainfall Making the New RF Atlas Maps 4

5 Measuring Drop Size Laser disdrometer: Instrument to measure drop size, drop velocity, and rainfall intensity Kazuki Nanko (Tsukuba University, Japan), has developed a laser disdrometer and uses it to measure the characteristics of throughfall (rainfall under a vegetation canopy). Splash detachment can cause severe soil erosion on bare soils. Drop size and velocity determine the kinetic energy of drops, and hence the amount of damage done to the soil. The size of throughfall drops is generally much larger than raindrops in the open. As a result, splash erosion can be very high under forest canopies if there is no protective ground cover. Laser Disdrometer Design of Kazuki's Disdrometer: Disdrometer Measurements Miconia Study Dr. Nanko is collaborating with us on a study of the impacts of the invasive tree species Miconia. Miconia has very large leaves that almost completely shade the ground. As a result little or no ground cover survives under its canopy. The exposed soil is subject to direct impacts of throughfall drops. Throughfall drop size seems to be larger for trees with large leaves. Therefore, we were interested in seeing how big throughfall drops are under Miconia. 5

6 Drop sizes for rain and throughfall under Miconia Note that significantly more large throughfall drops are observed under Miconia and strawberry guava, another invasive tree in Hawai`i, than under native ōhi a. 6