Lapse Rates

Lapse rates are progression at which air temperature changes with increasing / decreasing height in the atmosphere.  The rate is considered positive when temperature decreases with elevation, zero when temperature is constant with elevation, and negative when temperature is increasing with elevation.  While there are two different class of lapse rates, normal and adiabatic. The difference between normal and adiabatic lapse rates determine the vertical stability, or instability, of the atmosphere. That is, an air parcel’s tendency to embrace or prohibit vertical motion.

Environmental Lapse Rate—non-rising air that is affected by radiation, convection, and/or condensation. It averages about 6.5°C/km.

Dry Adiabatic Lapse Rate—rate of cooling with increasing altitude. It is constant at about 10°C/km.

Moist Adiabatic Lapse Rate—air, saturated with water vapor, is not constant but is determined by the combined effects of expansion cooling and latent heating (LH) because saturated air cools slower than dry due to the heating produced by condensing water vapor. Is always less than the dry adiabatic lapse rate.

Due to the fact that density differences are affected by the differences between the adiabatic lapse rates and the environmental lapse rate, one may notice that absolute instability occurs when the environmental lapse rate (Г_E) exceeds the dry adiabatic lapse rate (Г_D) [i.e. Г_E > Г_D]. Whereas, absolute stability occurs when the environmental lapse rate (Г_E) is less than the wet adiabatic lapse rate (Г_W) [i.e. Г_E < Г_W]. However, when the environmental lapse rate (Г_E) falls between the wet adiabatic lapse rate (Г_W) and the dry adiabatic lapse rate (Г_D) [i.e. Г_W < Г_E < Г_D] the atmosphere is considered conditionally unstable, as you can see from the picture below.

Absolute & Conditional Instability

Instability is a race to get cold between the parcel and the environment, and we want to environment to win. We could help the environment win by making the environment cool more slowly and / or make the parcel cool at a slower rate. The parcel method, for example, talks about the parcel being a hypothetical box that does not allow any transfer of heat in or out but, allows only adiabatic temperature changes.

The stability of the parcel is dependent on the parcel’s motion after a forced displacement. As the parcel undergoes adiabatic change, its temperature is compared to the surrounding environment to relate differences in density. If the parcel returns to its original position it is considered stable, whereas if the parcel continues moving away from its original position it is considered unstable. Moreover, if a parcel is displaced but remains at its new position it is considered neutral.

Due to the fact that density differences are affected by the differences between the adiabatic lapse rates and the environmental lapse rate, one may notice that absolute instability occurs when the environmental lapse rate (Г_E) exceeds the dry adiabatic lapse rate (Г_D) [i.e. Г_E > Г_D]. Whereas, absolute stability occurs when the environmental lapse rate (Г_E) is less than the wet adiabatic lapse rate (Г_W) [i.e. Г_E < Г_W]. However when the environmental lapse rate (Г_E) falls between the wet adiabatic lapse rate (Г_W) and the dry adiabatic lapse rate (Г_D) [i.e. Г_W < Г_E < Г_D] the atmosphere is considered conditionally unstable, as you can see from the picture below

On the other hand, especially with regard to the potential for severe storm development, another type of stability becomes important: potential instability. While, static stability (discussed above) considers what happens to a small parcel (box) of air when lifted or lowered while the surrounding air is kept in place, potential instability contemplates what happens when an entire layers of air are displaced upward [i.e. a mass of warm air displaced upward by the movement of a cold front].

Atmospheric Stability and Instability

            Instability is a race to get cold between the parcel and the environment, and we want to environment to win. We could help the environment win by making the environment cool more slowly and / or make the parcel cool at a slower rate. The parcel method, for example, talks about the parcel being a hypothetical box that does not allow any transfer of heat in or out but, allows only adiabatic temperature changes.

The stability of the parcel is dependent on the parcel’s motion after a forced displacement. As the parcel undergoes adiabatic change, its temperature is compared to the surrounding environment to relate differences in density. If the parcel returns to its original position it is considered stable, whereas if the parcel continues moving away from its original position it is considered unstable. Moreover, if a parcel is displaced but remains at its new position it is considered neutral.

            Due to the fact that density differences are affected by the differences between the adiabatic lapse rates and the environmental lapse rate, one may notice that absolute instability occurs when the environmental lapse rate (Г_E) exceeds the dry adiabatic lapse rate (Г_D) [i.e. Г_E > Г_D]. Whereas, absolute stability occurs when the environmental lapse rate (Г_E) is less than the wet adiabatic lapse rate (Г_W) [i.e. Г_E < Г_W]. However when the environmental lapse rate (Г_E) falls between the wet adiabatic lapse rate (Г_W) and the dry adiabatic lapse rate (Г_D) [i.e. Г_W < Г_E < Г_D] the atmosphere is considered conditionally unstable, as you can see from the picture below.

            On the other hand, especially with regard to the potential for severe storm development, another type of stability becomes important: potential instability. While, static stability (discussed above) considers what happens to a small parcel (box) of air when lifted or lowered while the surrounding air is kept in place, potential instability contemplates what happens when an entire layers of air are displaced upward [i.e. a mass of warm air displaced upward by the movement of a cold front].

Forecasting: Short Waves in the Upper Atmosphere and Their Effect on Surface Conditions

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt 

Forecasting: Potential Instability

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Atmospheric Stability & Instability

Stability & Instability

·         Instability - Possessing the ability to move away from the original position; allows convection and enhances vertical motions.

·         Stability - Possessing the ability to return to its original position; suppresses convection.

·         Inversion - Temperatures increasing with increased altitude, or height. A negative lapse rate.

·         Lapse rate (Γ) - The change of temperature with a change in height. Rate of decrease in temperature. Getting colder with height = Positive lapse rate. dT / dZ = temperature changing with height.

 

·         Adiabatic – not heat

·         Isotherm – A line of constant temperature.

·         Isothermometer – A line of constant dew point temperature.

·         Barometer - measures pressure

 

Air Parcel’s

·         “Blob” of air

·         Expands and contracts freely

·         Rises and sinks through the atmosphere

·         If it is warmer it is less dense then the air which makes it rise (compression)

·         If it is colder it is more dense then the air which makes it sink (expansion)

·         Temperature will change from pressure – does not exchange from inside and outside the box

·         Temperature is NOT a measure of heat

  

Types of Heat Transfer

1.    Conduction

a.    The transfer of sensible heat from a warm object to a cool object through contact

2.    Convection

a.    fluid/anything that is flowing

3.    Radiation

How Pressure changes with height?

a.    dp / dZ < 0

b.    Pressure decreases with height, which is a negative change, that is less than

1^st Law of Thermodynamics

̶        Heat In = Work + delta-T

a.    Adiabatic process:        

Warm air is less dense then cold air ONLY at the same pressure

Thunderstorm: Variables & Ingredients

Ø  Variables needed for Severe thunderstorms:

1.    Moisture

2.    Instability

3.    Lift

4.    Wind shear

Ø  Bob, from Texas

̶        Launches weather balloons (radiosonde)

̶        Radioing back temperature, dewpoint, etc…

̶        Thermodynamic diagrams

Ø  Wind shear

1.    Speed Shear

o   Winds increasing speed with height

2.    Directional Shear

o   Winds changing direction with height

Ø  Development

̶        Cumulus Humilius

̶        Cumulus Congestus

̶        Towering Cumulus – not precipitating

̶        Cumulonimbus (Cb) – precipitating

Ø  Texture

̶        More “cauliflower” the stronger the updraft

̶        “rock hard towers” implies that most of the cloud is in the liquid phase

̶        Updraft liquid weakens or reaches high in the troposphere = liquid freezes = giving cloud a “glaciated” texture (considered fairly weak)

Ø  Anvil

̶        Crisp

̶        Fuzzy

Ø  Vertical Shear

̶        Increases longevity and organization

̶        Strong shear = storm-scale rotation by tilting horizontal vorticity into vertical vorticity

̶        Too much shear = the storm cannot organize (“orphan anvils”) = CAPE is too weak and shear is too strong

Ø  Flanking Line

̶        Flanking line leading into the main updraft

̶        Main cell SW is tilted due to the environment shear

Ø  Boundaries (pg. 307)

̶        Describes fronts

̶        The leading edge of thunderstorm outflow

̶        Leading edge of the sea breeze

̶        Any other lines marking the junction of 2 airmasses

Ø  Creating Boundaries

̶        Differential heating of air either over surfaces with different properties, such as water, and lands, forests and fields, urban and rural landscapes, or over surfaces heated differently  (land over cloudy versus clear skies)

Ø  Occlusion = Cold air rapping around a cove

Ø  WER = Not a lot of precipitation/ at all

Ø  Anvil à Sinus Cloud à Made from ice crystals

Ø  More evaporation = High LCL’s = Relative humidity is lower towards the ground

Ø  LCL = Helps indicate the relative humidity of the sub-cloud layer

Ø  Wet Bulb Zero = Sleet = Frozen Rain

Ø  Verga = Rain that evaporate before hitting the ground

Ø  BRN (Bulk Richardson’s Number): CAPE is too weak and the shear is too strong

̶        Sweet spot: 10-45 BRN

                 BRN = CAPE / Shear

Ø  What 3 influences does dry air have on severe weather? 

̶        More evaporation = Stronger downdraft

̶        Dryer air in Mid-level of atmosphere tends to promote large hail growth

̶        Connectivity unstable (will learn in unit 2)

Ø  Single Cell Thunderstorms

̶        Single cell storms are dominated by buoyancy processes

̶        Sometimes called “air mass” t-storms, these storms are poorly organized and pose relatively little threat to the public (lightning and hail)

̶        Typical of afternoon thunderstorms

̶        Updrafts form in relatively random locations

̶        The dominant forcing feature is instability since they form in a low-shear

̶        Goes through the cycle within 30-60mins

̶        Severe weather threats minimal

̶        Pulse Severe Storm

Ø  Severe single cell thunderstorm

̶        Forms in a low shear environment

̶        Taller updraft/More instability

̶        More intense reflectivity/More intense core

̶        Longer lasting

̶        Precipitation takes longer to descend to the ground/Stronger updraft

̶        Vertical Integrated Liquid (VIL) is larger

̶        “Popcorn Severe”

Ø  What are the differences with ordinary thunderstorm and a “pulse” severe thunderstorm?

1.    Taller updraft/More instability

2.    More intense reflectivity/More intense core

3.    Longer lasting

4.    Precipitation takes longer to descend to the ground/Stronger updraft

̶        Both form in a low shear environment

Ø  Land Spouts

̶        Single cell thunderstorm can create them

̶        Horizontal shear causing vertical vorticity that stretches and causes a tornado

̶        Tend to have a double vortex (thin core and translucent on the outside)

̶        Usually weak, not always