ATMOSPHERIC STABILITY. Most clouds form as air,, and. What defines air motion and the size/shape of clouds? AIR PARCEL: body of air with &

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1 ATMOSPHERIC STABILITY Most clouds form as air,, and. What defines air motion and the size/shape of clouds? The answer is. AIR PARCEL: body of air with & Thick of it like a It can and No mixing of parcel air with outside air = As the parcel rises: As the parcel sinks: Adiabatic Process & Lapse Rates ADIABATIC PROCESS: is where there is from the parcel. Changes in temperature are due to and only Lapse rate: positive lapse rate: negative lapse rate: Environmental Lapse Rate (ELR): in air temperature with height. Measured with Radiosonde: which measures the vertical profile of :,,,. Launched at and UTC Radiosonde Facts Rises ~ 5 m/s (11 mph) Page 1 of 11

2 Flight can last > 2 hrs Radiosonde can ascend to >115,000 Drift > 125 miles from launch site Bursts at ~ 6 m (20 ) diameter Only ~ 20% of the 75,000 sondes released each year are found and returned. Worldwide: 900 stations U.S.: 92 stations including AK, Pacific and Puerto Rico. Rawinsonde: With attached radar reflector, radio-direction finding equipment permit the tracking of radiosonde. Gives and at various altitudes. Dropsonde (or Dropwindsonde) Dropped from. Used to collect weather data esp. over areas. Used by Parachute slows flight of dropsonde. Info radioed back to. Back to lapse rates these apply to the parcel now. Dry Adiabatic Lapse Rate (DALR): applies to the parcel 9.8 C / 1000 m or 5.5 F / 1000 ft If the parcel of air is, the rate of cooling ( ) or warming ( ) is A parcel of air will rise and at the DALR until the is reached. At this point, the air is and further results in ( ) Remember, Extra heat from causes air to cool at a rate (cooling is ) This lower rate is constant (depends on how much is in the air). This rate is called Wet Adiabatic Lapse Rate (WALR): average value is ~6 C / 1000 m or 3.3 F / 1000 ft Page 2 of 11

3 so, if the parcel of air is, we will use the. Again, this value varies over and by. DETERMINING STABILITY Stability: determines the likelihood of clouds, precipitation and thunderstorms We compare the adiabatic lapse rates of a (wet and dry) with the environment's lapse rate ( ) 4 forms of stability: absolute, absolute,, and Compare Environmental Lapse Rate to DALR and WALR = Absolutely stable = Absolutely unstable = Conditionally unstable DETERMINING STABILITY Compare the temperature of the to the surrounding air. ABSOLUTE STABILITY Parcel is than its environment, it is more and will to its original level. comparing rates: Clouds tend to have and ; Stable air usually occurs in the part of the day. Note that movement of the parcel upward means it is than the surrounding air, so it back to its original altitude. Also, in movement of the parcel downward, it becomes than the surrounding air, and to its original altitude. Stable air vertical motion ABSOLUTE INSTABILITY Parcel is than its environment, it is less and will comparing rates: Page 3 of 11

4 Clouds that form in this situation show ; Unstable air usually occurs with warmer air temperatures Note that movement of the parcel upward means it is than the surrounding air, so it Also, in movement of the parcel downward, it becomes that the surrounding air, and continues Unstable air vertical motion CONDITIONALLY INSTABILITY Condition for instability is: comparing rates: A rising air parcel cools to the point where it reaches. Elevation of cloud base: From this point upward the air cools at the. NEUTRAL STABILITY This condition occurs when the or The air parcel will neither or. Causes of a Stable Atmosphere A., change in temperature between surface and air aloft is. B. Occurs when air aloft or. 1. warm air with little surface change 2. lower layer a. nighttime Page 4 of 11

5 b. cold air advection at c. air moving over 3. entire layer ( ) Causes of an Unstable Atmosphere A., temperature drops rapidly B. Air aloft or surface 1. lower layer a. warming due to b. warm air advection at c. air moving over 2. entire layer Stuve Diagram Page 5 of 11

6 CLOUD DEVELOPMENT There are four mechanisms that are responsible for the development of clouds PRECIPITATION Lift air to form clouds (air cooled to ) Further lifting of air produces around Condensation nuclei (aerosols ~ 1µm):,, or from burning of fossil fuels. are common condensation nuclei. Cloudy weather mean precipitation Terminal velocity: constant downward speed attained by a particle due to a balance between gravity acting downward and air/fluid resistance acting upward. Explains why cloud sized droplets/crystals do not fall as precipitation until they are larger. - for a particle to fall as precip., its terminal velocity > rising motion in cloud - cloud droplets and ice crystals are so small => low terminal velocities In order for precipitation to occur the must grow larger. PRECIPITATION PROCESSES There are two processes that stand out in the growth of cloud droplets into drops big enough to fall as precipitation. 1) 2) Page 6 of 11

7 COLLISION-COALESCENCE PROCESS Temperatures are freezing. Cloud drops colliding with other cloud drops producing drops. have different terminal velocities faster falling droplets will,, and with slower falling droplets in their path. Collision efficiency: fraction of all droplets in the path of a falling droplet that comes in contact with the larger droplet. - varies with the of droplet. for large droplets (>80 um in diameter) ICE-CRYSTAL (BERGERON) PROCESS Both and must co-exist in clouds at temperatures freezing. Water droplets existing at temperatures freezing are called. water vapor molecules surround each vs. around each in the of a cold cloud. This causes the water vapor to move from the toward the A vapor pressure that is is. Therefore, vapor deposits on the. Ice crystals at the expense of the This process is called or. PRECIPITATION TYPES RAIN Diameter greater than or equal to 0.5 mm. Any larger and they break up into smaller drops. Associated with and DRIZZLE Diameter less than 0.5 mm Usually associated with clouds Page 7 of 11

8 VIRGA Rain that o Air under cloud is and rain/snow begins to o Amount of vaporization increases with: in cloud base height RH of ambient air SNOW A solid form of precipitation composed of in a complex hexagonal form BLIZZARD Characterized by and (> 30 knots) A severe blizzard: T 10 F, winds > 45 mph, and visibility near zero FALLSTREAKS Ice crystals that before reaching the ground Similar to FLURRIES Light snow showers SNOW SQUALL Brief, but heavy snow fall SNOW GRAINS Flat or elongated opaque white ice particles Form in similar way to drizzle, but prior to reaching the ground SNOW PELLETS Used to be called Soft conical or spherical opaque grains of ice Form when supercooled cloud droplets collide with an ice crystal and freeze SLEET Snowflake falls and encounters a layer of air The snowflake The raindrop may fall back through a surface layer of air and the liquid into These ice pellets usually when they strike the surface Page 8 of 11

9 FREEZING RAIN The cold surface layer beneath a cloud may be to freeze raindrops as they fall The raindrops are when they reach the surface Droplets come into contact with a frozen surface, HAIL Hail is produced in a Large frozen raindrops grow by accumulating liquid droplets ( ) Strong updrafts carry the frozen drops high above the freezing level. A coating of ice forms and they grow larger and start to fall. The droplet may get caught in the violent updraft to repeat the cycle. Eventually, the ice will be large enough to escape the updraft and will come down to Earth. WEATHER RADAR: LOCATING PRECIPITATION Reflectivity Mode WSR 88-D emits short pulses of microwave energy with wavelengths of At these wavelengths, radar pulses are readily by rain, snow, or hail, but not significantly by Falling precipitation reflects some of the radar signal back to a receiving unit, where it is processed and displayed on a computer screen as a Some radar echoes may be caused by on ground (ground clutter) typically this is electronically subtracted from display Doppler Effect The shift in of sound or electromagnetic waves that accompanies the motion of the wave source(s) or wave receiver. Velocity Mode (Doppler) Determines air motions within a weather system. There is a and EM waves coming from a moving source. o Doppler radar can detect this shift, and displays it as Doppler radars monitor movement of precipitation particles directly or from the radar unit. Page 9 of 11

10 Doppler radar allows advance notification of and indicate motion directly the radar, while and indicate motion directly from the radar. Phased Array Weather Radar Phased array radar uses multiple beams that are sent out simultanteously. Scans much faster (30 sec. vs. 5-7 min) Can focus on a severe weather feature More detailed examination of storm evolution MEASURING PRECIPITATION Rainfall and snowfall are routinely measured in terms of accumulation depth over a specified time interval, usually and every Measurements are made with gauges or remotely by weather radar or satellite sensors Standard non-recording rain gauge o Cylinder with a cone-shaped funnel opening o Can resolve rainfall into increments as small as o A measures depth of water in cylinder o Rainfall is measured manually at a fixed time and then the gauge is emptied Tipping-bucket rain gauge o Consists of a free-swinging container with compartments, each of which can collect of rainfall o Each compartment alternately fills with water, tips, and spills its contents, tripping an electronic switch that marks a chart or sends a signal to a computer Weighing-bucket rain gauge o Calibrates the weight of accumulating rainwater in terms of water depth o Marks chart on a clock-driven drum or sends electronic signal to a computer o May melt frozen precipitation with antifreeze or a heater Snowfall Measurement We want snow depth that falls over a certain time period, the of the snowfall, and the at observation time New snowfall accumulates on a and is measured with a ruler Page 10 of 11

11 The melt water equivalent can be determined by weighing-bucket gauge measurements or melting snow in a non-recording gauge As a very general rule, Actual ratio depends on crystalline form of snowflakes and the temperature of the air through which the snow falls The depth of snow on the ground is determined by an at several locations Remote Sensing Weather Radar Reflectivity is proportional to raindrop and rainfall rate is proportional to the of the raindrops Reflectivity data is converted to rainfall rates by estimating raindrop size distribution Remote Sensing Satellite Tropical Rainfall Measuring Mission (TRMM) satellite uses radar, a microwave imager, and a visible/ir scanner to estimate rainfall Page 11 of 11