Lecture8_102011 - Recap of FCST 4 Recap of FCST 4...

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Unformatted text preview: Recap of FCST 4 Recap of FCST 4 Knoxville, TN L 1003.5 mb Forecast verification for KTYS Forecast verification for KTYS GFS MOS NAM MOS High 65F, Low 46F, POP ~100% High 62F, Low 46F, POP ~100% Difficult to determine which model has the stronger front … main differences are in the dewpoint (GFS high 30s, NAM mid­low 30s), the wind speed, and perhaps the wind shift. VERIFICATION 62­45­23­1.96”! SOUNDINGS SOUNDINGS MSC 243 Lecture #8 20 October 2011 Key West: 15” of rain Key West: 15” of rain NWS Miami: “Strongest tornado to hit Broward or Miami­Dade ounties since the March 27, 2003 Brownsville / Liberty City tornado” Counties since the March 27, 2003 Brownsville / Liberty City tornado” http://www.srh.noaa.gov/media/mfl/SunrisePlantationTornado.pdf Something is wrong here. What is a sounding ? What is a Weather balloon (“radiosonde”) rises and measures conditions of the atmosphere at different altitudes Why do we need soundings ? Why do we need soundings ? We don’t know a lot about the atmosphere above the ground but we need to know the state of the atmosphere at many levels to make a forecast! Soundings provide meteorologists and the models with information about what’s going on. Weather balloons (rawinsondes) launched Isobars, Isotherms, Dry Adiabats Isobars, Isotherms, Dry Adiabats Skew­T chart is a graphical display of an upper air sounding for a particular ground station. Isobars ­ straight horizontal solid lines at pressure levels → height information. Isotherms ­ straight diagonal solid lines sloping from lower left to upper right. These lines are labeled in °C The concept of Skew­T means that the temperature is not plotted vertically but angles off to the right at a 45° angle. Dry adiabats ­ slightly curved solid lines of constant potential temperature (in K), sloping from the lower right to upper left. Isobars, Isotherms, Dry Adiabats, Inversions Isobars, Isotherms, Dry Adiabats, Inversions Isobars are lines of constant pressure. Isotherms are lines of constant temperature. IS O TH ER M ISOBAR DR YAD IA BA T Dry adiabats are lines of constant potential temperature. Temperature Temperature Temperature Temperature at a specified pressure. At pressure find T reading Read value of isotherm Label in °C Potential Temperature Temperature a parcel of air would have it brought adiabatically to the reference pressure At pressure find T reading Follow dry­adiabat down to 1000mb Read value of isotherm Dew Point Temperature Temperature at which a parcel of air will become saturated if it is cooled. At pressure find Td reading Read value of isotherm Label in °C Moist Adiabats, Mixing Ratio Moist Adiabats, Mixing Ratio Moist adiabats ­ slightly curved dashed lines sloping from the lower right to upper left. These lines represent paths that saturated air follows and represents the rate of temperature change in a parcel of saturated air rising pseudo­adiabatically. Pseudo­adiabatically means that all the condensed water vapor is assumed to fall out immediately as the parcel rises. Mixing­ratio lines ­ slightly curved dashed lines sloping from the lower left to upper right. These lines are labeled in grams per kilogram; grams of water vapor per 1000 grams of dry air. Moist Adiabats, Mixing Ratio Moist Adiabats, Mixing Ratio MI XIN MOIST-ADIABAT G- RA T IO Moist adiabats represent the lines of constant temperature change in a parcel of saturated air rising pseudoadiabatically (Sometimes known as saturation adiabats) Mixing-ratio lines are lines of constant water content. Moisture Moisture Mixing Ratio “w” The ratio of the mass of water vapor (in grams) to the mass of dry air (in kilograms). At pressure find Td Read value of mixing ratio line Label in g/kg Saturation Mixing Ratio “ws” The water vapor content of the air if it were saturated. At pressure find T Read value of mixing ratio line Label in g/kg Temperature and Moisture Temperature and Moisture here: at 700 mb Data from a Skew-T available at each level are: - Dew Point - Mixing Ratio - 313K Potential Temperature - ­1 C 5 g/kg 11 g/kg Temperature Saturation Mixing Ratio Clouds are more likely to occur when T and Td are similar Relative Humidity Relative Humidity Relative Humidity The ratio (in %) of the amount of water vapor in a given volume to the amount that the volume would hold if saturated. At pressure find Td (w) Note value of mixing ratio line At pressure find T (ws) Note value of saturation mixing ratio line RH = w/ws x 100 Inversions Inversions Portion of a sounding where the atmospheric temperature or increasing with height. Types of inversions: Radiation Inversion A thermally produced surface based inversion formed by the rapid cooling of air in contact with the surface as compared to the upper layers. Favorable conditions include: Long nights or periods of low solar radiation. Clear skies. Dry air, as moist air absorbs infrared energy. Little winds. Inversions Inversions Subsidence Inversion A mechanically produced inversion formed by adiabatic warming of a layer of sinking air. Favorable conditions include: Strong anticyclones or stable air masses that force air to sink. Frontal Inversion The transition layer between a cold air mass and the warmer air mass above it. Favorable conditions include: Warm air overriding cold air. Where is the inversion? Where is the inversion? Rising parcels that encounter an inversion are cooler than their environment, and stop rising. Inversion Parcel Motion Parcel Motion As a parcel moves up and down, so long as it is not saturated, its temperature will change at the dry adiabatic lapse rate. As a parcel moves up and down, so long as it is not saturated, it will conserve its ‘mixing ratio’, the ratio of water molecules to air molecules. Parcel Motion Parcel Motion If a parcel becomes saturated, continued cooling will result in condensation into water vapor. The latent heat released by condensation offsets the cooling from expansion, and the parcel will rise at the moist adiabatic lapse rate. This moist adiabatic lapse rate is generally around 5­6 C / km, not as high as the dry adiabatic lapse rate. Cloud Formation Cloud Formation First, consider an air parcel at the surface. When it is warmed at the surface, it is buoyant: it rises and its temperature falls. If no heat is added or released, the parcel rises “dry adiabatically”. Adiabatic cooling of rising air is the dominant cause of cloud formation… [On a Skew­T diagram, move up along the dry adiabat from the surface] Cloud formation Cloud formation The rising unsaturated air parcel reaches the lifting condensation level (LCL), where it begins to condense. At the LCL, the air parcel becomes saturated as its temperature reaches the dew point (of the parcel). This is where the cloud base exists. LCL = the height at which a parcel of air would become saturated if lifted dry adiabatically (point at which saturation would Lifting Condensation Level (LCL) Lifting Condensation Level (LCL) LCL (~ 860 mb) • • • • • • At surface pressure find T Draw a line up parallel to dry adiabat At surface pressure find Td Draw a line up parallel to mixing ratio line The intersection of the 2 lines is the LCL Read pressure and label in mb Above the LCL: Cloud Above the LCL: Cloud Formation Now that the air parcel is saturated at the LCL, it might be able to rise even higher under the right conditions Condensation is a warming process: latent heat release may assist further rising. But, this is partially offset by the cooling due to expansion. As the parcel rises above the LCL, it cools at a slower rate: the moist adiabatic lapse rate. Above the LCL: Cloud Above the LCL: Cloud Formation Under favorable instability conditions, the air may rise to a level where it becomes warmer than its environment. This is the Level of Free Convection (LFC) The level beyond (above) which the air parcel becomes buoyant. An air parcel above this level can rise upward even in the absence of any forced lifting ON A SKEW­T: Start at Lifting Condensation Level (LCL) and follow up the moist adiabat until you reach the temperature line from the sounding Key Concepts: Buoyancy Key Concepts: Buoyancy Buoyancy: upward force that acts on a parcel of air due to density difference Higher potential temperature and water vapor content increase buoyancy. Precipitation acts to decrease buoyancy. Main thing to look at: is the temperature of the air parcel warmer or cooler than the environment (i.e. the sounding temperature)? Atmospheric Convection Atmospheric Convection As an air parcel moves up from the LFC, its temperature is higher than that of the environment, and it is positively buoyant. Q: What is the air parcel going to do? A: It will rise further, with no extra help! The air parcel will continue to cool at the moist adiabatic lapse rate. Atmospheric Convection Atmospheric Convection The air parcel remains just saturated. Hence, as it continues to rise, it condenses more water and forms more clouds and precipitation. Therefore, convection is associated with clouds and rainfall. The larger the difference in temperature between the parcel and environment, the faster the updraft, the faster the condensation, and the more severe storms! Atmospheric Convection Atmospheric Convection This condensation continues until the air parcel finally reaches the Equilibrium Level (EL). EL = The height at which the temperature of the buoyant parcel again becomes equal to the environmental temperature. The EL is where the cloud anvil forms. Sometimes, air parcels with upward momentum may push up through the EL, but then they are heavier than their surroundings and sink back. ON A SKEW­T: From the point where the LFC was found, follow a moist adiabat up until crossing the temperature line again. That level is the equilibrium level, at which a parcel of air no longer accelerates upward. Levels: LCL < LFC < EL Levels: LCL < LFC < EL EL LFC LCL ...
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This note was uploaded on 01/08/2012 for the course MSC 243 taught by Professor Majumdar,s during the Fall '08 term at University of Miami.

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