Good morning everyone, this is Brandon brady and I am continuing my series on Baroclinic and Barotropic Weather Environments today. I want to thank everyone who post comments and leave positive feedback about the blog that I insert here on zoomradar. You all are awesome. I want to thank my friend and meteorological colleague Jeff for his support and leadership as well. Ok with that being said let’s continue on the baroclinic and barotropic series.
1. Last week we learned that a barotropic environment was a tropical like climate, the type of weather that everyone sees on a postcard with white sands and palm trees on a tropical island. For the most part that is spot on right, because except for deep convection(thunderstorms), weather systems in the tropics tend to be decoupled in the vertical. (Decoupled means separated or lacks instability) Because of no large scale divergence patterns occur over a deep layer there is a complete disconnect. Therefore, no large scale divergence occurs, or very little vorticity. In fact, except for the ITCZ or the inter-tropical convergent zone at the equator, there is weak vorticity that occurs at the equator. Even though there is weak advections, weak or small vorticities and homogenous uniform temperatures in a typical barotropic weather environment, weather systems occur here, such as, high and low pressure systems. I will discuss these later.
2. last week we learned about how baroclinic systems are much more dynamic than barotropic systems. Baroclinic systems are driven by strong temperature gradients, and strong temperature advections that includes CAA or cold air advection, WAA – warm Air advection. Baroclinicity is also defined by different fronts cold fronts, warm fronts and occluded fronts. Probably the greatest example of baroclinicity is the Mid-latitude Cyclone which relies on thermal advections to mix gradients and cause instabilty. Baroclinic systems are Driven by Baroclinic Instability. Baroclinic Instabilty is defined by the steep difference in temperature between the North and South Poles and the Equatorial regions of earth. If you are a weather enthusiast like me, than a baroclinic weather environment is your cup of tea. Tornadoes, severe thunderstorms, winter storms, essentially mid latitude cyclones is the essence of baroclinic systems.
The second part of the blog will include how meteorologists use the Barotropic and Baroclinic Weather Model Systems in Forecasting.
1. Equivalent Barotropic Model – The equivalent barotropic Model is an older forecasting tool used by meteorologists to relate the vertical structure of wind field to the Vorticity tendency at the mean level of 500mb. Now remember folks vorticity means spin(counterclockwise) and the 500mb level is at 18000 feet. Both of these terms are important to forecasters because it tells you if a low pressure system is present and at 500mb it shows you where these systems are going. Now the equivalent barotropic model only shows simple relationships between the geostrophic wind(balance b/w PGF and Coriolis) and Height lines/Pressure Gradients. Essentially the Equivalent Barotropic Model is a simplification of the “Real” Atmosphere. And remember like in the definition of barotropic winds are unidirectional and wind speeds increase with height. There is little shear here because winds are only in ONE directon. Thus isotherms and isobars are always parallel to wind vectors. No thermal advections and thus NO CYCLOGENESIS. If you are a Meteorology student or ma professional meteorologist you can read a theta chart and see if the wind vectors are small(arrows) and they don’t cross the isotherms/isobars its a calm environment, no advections occurring. Nevertheless, this MODEL is still very good for depicting the development and movement of mid – to upper level waves. In the Real Atmosphere with Uni-Directional shear the Equivalent Barotropic Level is located at the 500mbs.
1b. Disadvantages of the Equivalent Barotropic Model
A. There are many approximations and many calculations – This can lead to inaccuracies and many mistakes.
B. Analysis level – This Forecast model is fixed at 500mb. In real world applications, forecasters look at every level from 100mb to 200mb to make accurate weather forecast predictions in the mid-latitudes. In fact they vary in millibar levels from equator to pole.
C. Vertical profile – The vertical profile is too simple. The is no turning of wind, no temperature advections, no mesoscale or smaller development only SYNOPTIC Scale processes. Synoptic meaning larger scale.
I wanted to show everyone a simplistic diagram and depiction of a barotropic model. I know this is a model of the sea surface, however I can translate it to the atmosphere where height lines and in waves (black lines) and the wind vectors in blue lines. The barotropic model has winds in an unidirectional manner not crossing the isotherms/isobars and the baroclinic model has strong advection thermally. source oc.nps.edu. I do want to add one more graphic because it goes along what I am talking about. I hope this graphic makes sense. If there are any questions please post them in the comment section.
2. Baroclinic Model –
The Baroclinic Model is the main driver of cyclone development. In the development of Low pressure at the Mid-Latitudes low pressure at the center closes themselves quicker and easier at the surfae than at the mid levels(500mb). Still a trough develops or a deepening trough occurs. Nevertheless, intensification of upper level troughs and Ridges do occur in the Model.
A. Surface – 500mb Development of Cyclones – For Baroclinicity to occur horizontal thermal advection needs to occur. Furthermore Low level Advection near the surface causes lift and vorticity generation. basically you need Instability and counterclockwise rotation to occur if a deep baroclinic low pressure is to occur. In the baroclinic model 500mb Vorticity changes are due to advection. What is a really interesting fact about the cyclone development and baroclinic model is that the development of troughs at 500mb is MOST INTENSE on the COLD SIDE of Cold fronts. Thermal Advection is largest here. Negative 500mb height tendencies and positive surface pressure tendencies occur WEST and SOUTHWEST of the surface cyclone respectively. You can tell this when you see the “dry slot” in major winter storms that affect the Northeast in the winter.
This is a great depiction of baroclinic zones with TEMPERATURE ADVECTIONS. The isobars and isotherms show which way the therms are advecting. The reds indicate warm air advection, and the blue indicate the cold air advection. The black lines indicate where they are going. In the eastern U.S. the Warm air is headed northeast, and the Western U.S. the Cold air is headed outh and East.
I want to share with all of you one of the reasons why I study meteorology. I truly love Northeast snowstorms, and I am going to provide you with some in depth knowledge of baroclinicty and explosive cyclogenesis. These ingredients go into making these storms thrive and explode.
B. Explosive Cyclogenesis Baroclinicity at the Extreme – The Northeast snowstorm or Nor’Easter – Explosively developing cyclones can display pressure falls of 10-20 mbs over 12 hours. Central pressure can fall to 960-970 mbs that are accompanied by Hurricane force winds and snowfall accumulations of 25-35 inches. Due to their destructive capabilities the most rapidly developing cyclones occur over a moisture source in a maritime environment(Atlantic ocean) have been separately categorized. These baroclinic monster storms can be defined as a large drop in Central pressure 12mb in 12 hours minimum, they occur almost always over maritime areas, usually confined to the cold season where temperature differential is at the extreme – Cold Land and Warm Gulf Stream. These storms tend to be located 250 miles downstream of the 500mb trough and on the COLD SIDE of the westerly prevailing winds. These storms form along a STRONG AIR TEMPERATURE or SEA SURFACE GRADIENT – land and water temperature differential. THIS is the essence of baroclinic weather environments. These storms are always under forecasted in regard to intensity, however numerical weather model prediction is getting better.
The Axis of Maximum frequency – of these monster storms occur slightly east of the Gulf Stream off the Mid to north Atlantic Coast, along the Kuroshio Current in the North Pacific off the east Coast of Japan towards southwest Alaska, and even in Washington and Oregon at times.
There is another component to these explosive cyclogenesis Low Pressures(Nor’Easters)
C. Coastal Front – The coastal front is a weak trough that lies approximately parallel to the coastline. This creates confluence of easterly flow from the Ocean and north ot Northwesterly land flow. Confluence means a coming together of air at the surface, it has no where to go but up and RISE. Ths results in Large Horizontal Temperature Gradients – the essence of Baroclincity. This coastal font has a larger temperature gradient believe it or not than the gulfstream SSTs. Thats incredible. Due to the coastal front location these incredible Low Pressures track follow the coastline or the Gulfstream and usually arrive around the “benchmark” location of the 40/70 LAT/LON line an are associated with EXTREME Thermal gradients.
This is a classic example of a Mid-Atlantic and Nor’easter snowstorm. Coastal Front and rapidly rising air in New England depicted by higher and colder cloud tops. Awesome.
The source of my blog is Mississippi State notes from Synoptic meteorology class.
Next week is the last part of my Barotropic series I will provide more examples to better paint a picture for everyone. Enjoy the weather its the only weather you got
CIAO for now. Brandon p Brady