Discover and read the best of Twitter Threads about #METR171a
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Conservation of potential vorticity can explain typical ridge/trough airflow patterns associated with large mountain ranges #SJSU #METR171a #SynopticMeteorology
And contrary to what you may have learned in grade school, this continental-scale trough ridge pattern, induced by the Rockies, is probably more responsible for Western Europe's mild winters than anything having to do with the Gulf Stream #SJSU rmets.onlinelibrary.wiley.com/doi/abs/10.125…
Ridges above mountains result from compression of air columns, inducing negative relative vorticity and troughs on the lee side result from expansion of air columns inducing positive relative vorticity. #SJSU #METR171a #SynopticMeteorology
Potential vorticity is the vorticity an air column would have if it were brought adiabatically to some standard latitude and stretched or shrunk vertically to some standard depth #SJSU #METR171a #SynopticMeteorology
Isentropic potential vorticity is conserved under adiabatic frictionless flows #SJSU #METR171a #SynopticMeteorology
Stretching a column of air vertically can ‘spin up’ vorticity via conservation of angular momentum just like an ice skater rotating faster when they bring their arms closer to their body #SJSU #METR171a #SynopticMeteorology
Air parcels tend to flow along surfaces of constant potential temperature (isentropes). #SJSU #METR171a #SynopticMeteorology
Warm air advection is associated with air parcels ascending upsloping isentropes. This is associated with adiabatic cooling, condensation and precipitation. #SJSU #METR171a #SynopticMeteorology
Cold air advection is associated with air parcels descending downsloping isentropes. This is associated with adiabatic warming and fair weather. #SJSU #METR171a #SynopticMeteorology
The quasi-geostrophic height tendency equation is a prognostic equation that tells us how synoptic-scale systems are likely to evolve in time: strengthening/weakening of highs and lows #SJSU #METR171a #SynopticMeteorology
Pressure height falls and thus the development of lows (or the weakening of highs) are associated with positive vorticity advection by the geostrophic wind and/or increasing warm air advection with height or decreasing cold air advection with height by the geostrophic wind
Pressure height rises and thus the weakening of lows (or development of highs) are associated with negative vorticity advection by the geostrophic wind and increasing cold air advection with height or decreasing warm air advection with height by the geostrophic wind
The quasi-geostrophic omega equation tells us that vertical motion in the atmosphere is related to non-geostrophic circulations that arise in order to restore thermal wind balance #SJSU #METR171a #SynopticMeteorology
Differential positive vorticity advection by the geostrophic wind and warm air advection by the geostrophic wind are associated with upward vertical motion and precipitation if sufficient moisture is present #SJSU #METR171a #SynopticMeteorology
Differential negative vorticity advection by the geostrophic wind and cold air advection by the geostrophic wind are associated with downward vertical motion and fair weather #SJSU #METR171a #SynopticMeteorology
Jet stream is maximized at mid-latitudes because that is where the meridional temperature gradient is the highest and it is maximized in the upper troposphere because that is where the temperature-gradient-caused pressure gradient is the highest #SJSU #METR171a
Jet stream is wavy because of conservation of absolute vorticity. When it meanders north, planetary vorticity increases, inducing a negative curvature vorticity to compensate. When it meanders south, planetary vorticity decreases, inducing pos curvature vorticity to compensate.
Rossby waves travel to the west relative to the background flow and longer waves travel faster to the west. Since the background flow is to the east, longer waves appear to move more slowly to the east and the longest waves can move to the west.
Dominant terms in vertical momentum equation are vertical pressure gradient force and gravitational force. #SJSU #METR171a #SynopticMeteorology
Vertical motion is of primary importance in weather forecasting because the source of heat and moisture for storms is the surface. Horizontal divergence of the wind field aloft necessarily implies vertical motion through conversation of mass. #SJSU #METR171a #SynopticMeteorology
Potential temperature tells us the temperature a parcel of air would have if it were brought down to a reference pressure level. Potential temperature is useful for diagnosing the stability of the atmosphere. #SJSU #METR171a #SynopticMeteorology
The momentum equations that we work with are just f=ma applied to the atmosphere. They say that acceleration of a parcel of air is the sum of the forces applied to that parcel (per unit mass): the pressure gradients force, the Coriolis force, and friction. #SJSU #METR171a
Scale analysis allows us to focus on which terms are most important for describing the main features of the weather phenomena we are interested in. #SJSU #METR171a #SynopticMeteorology
Scale analysis reveals that synoptic-scale weather systems are close to being in geostrophic balance where their motion is characterized by a balance between the pressure gradient force and the Coriolis force. #SJSU #METR171a #SynopticMeteorology
Math review (derivatives of vector and scalar fields)
1) The gradient is the directional derivative (del) applied to a scalar field. The gradient vectors represent both the local direction of greatest increase of the scalar field and the local magnitude of that increase
1) The gradient is the directional derivative (del) applied to a scalar field. The gradient vectors represent both the local direction of greatest increase of the scalar field and the local magnitude of that increase
The divergence is the directional derivative (del) applied to a vector field via the dot product. The divergence values measure the local spreading of a vector field. #SJSU #METR171a #SynopticMeteorology
The vorticity is the directional derivative (del) applied to a vector field via the X product. Vorticity values are a local measure of the propensity of adjacent vectors to produce rotation. The vorticity represents both the spatial orientation & the magnitude of that rotation.
Math review (vectors)
1) Vector fields embed information about the magnitude and direction of a quantity at each grid point in space (represented by their origin) – they are not line segments in that space.
#SJSU #METR171a #SynopticMeteorology
1) Vector fields embed information about the magnitude and direction of a quantity at each grid point in space (represented by their origin) – they are not line segments in that space.
#SJSU #METR171a #SynopticMeteorology
2) The dot product is the measure of similarity between vectors – it accumulates interactions in the same direction
3) The cross product is a measure of the difference between vectors – it accumulates interactions in different directions
I'll be tweeting the main points that we discussed in each of the meetings of the Synoptic Meteorology class I am teaching at SJSU. Here are today's, where the broad topics were the human role in forecasting and statistical postprocessing.
#SJSU #METR171a #SynopticMeteorology
#SJSU #METR171a #SynopticMeteorology
1) The value-added from human forecasters has diminished over time as dynamical models have improved/become higher in resolution and statistical postprocessing has become more sophisticated.
2) A meteorologist (or A.I. system) should be able to recognize, through experience (learning) on past data, that a given dynamical model will tend to be off in a given direction under a particular circumstance (e.g., GFS winter high temperatures during a nor'easter tend to be _)
