Diagnosis Winds Intro Diagnosis takes the observed winds
Diagnosis Winds
Intro Diagnosis takes the observed winds and investigates the constituents which make up this net wind n Short range forecasting uses this diagnosis and applies some simple techniques to simulate the atmospheric processes n
Diagnosis Take the current and historical observations of the wind n Resolve why certain speeds and directions were reported at various times and locations n These observed conditions should be explained in terms of process and basic fields. n
Contributions of the wind n Calculate n n the geostrophic wind the gradient wind (curvature in the trajectory) isallobaric wind Add the qualitative assessment of any other influences n n Low level stability Surface frictional effects Thermal winds Local effects
Laws of motion n n 1 st law- An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. 2 nd law n n The acceleration of an object is produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. 3 rd law n For every action, there is an equal and opposite reaction
Geostrophic wind Vg Assume straight flow parallel to the isobars for a non-viscous fluid n Third law of motion n Geostrophic wind is defined as the balance between the pressure gradient force and the Coriolis force i. e. the resultant wind n
Geostrophic Wind Vg
Geostrophic Wind
Geostrophic wind Vg n Fairly representative of the winds at the top of the boundary layer as long as the following fields are not large. The curvature of the isobars n The isallobaric field n n Climatology, a value of 50% is representative of the winds observed at the surface
Gradient flow Flow is not always straight - need to add curvature n The resultant wind is a balance between PGF, Coriolis force and centrifugal force n This is the gradient wind V n The gradient wind depends upon the radius of curvature of the trajectory n
Derivation of Gradient wind V See Holton chapter 3 for the derivation
Gradient wind V Assumptions n The contour curvature can be a reliable estimate for the trajectory curvature n Not! n Only true for slowing moving systems n Has major problems with rapidly moving systems n
Vg = geostrophic wind V = gradient wind, n n n Normal cyclonic flow (f. R > 0), |Vg| > |V| Geostrophic is an over estimate of the balanced wind Anticyclonic flow (f. R < 0), |Vg| < |V| The geostrophic wind is an underestimate
Gradient wind n Usually the difference between the geostrophic and gradient wind is less than 20 %
Gradient wind The pressure gradient around a high must decrease linearly toward the high centre n Pressure field is flat with light winds n Pressure gradient near low centres are not constrained to decrease linearly n They can be strong right into the centre n
Gradient wind In theory the gradient wind will be a better approximation of the actual wind and should be used instead of geostrophic wind n To accurately calculate Vg need to know the curvature of the trajectory of the air. n Since this is often unknown, the curvature of isobars is used as an approximation n The gradient wind is only used when there is significant curvature R < 5 degrees of latitude n
Isallobaric wind
Isallobaric wind
Isallobaric wind Vgr = gradient wind n V = instantaneous wind vector sustained by local changes in pressure gradient n Is a vector pointing normal to the isallobars toward isallobaric falls n
Isallobaric Wind Local rate of change of the pressure field n The isallobaric winds can change the wind speed by 10 -15 knots and the directions can be changed n The resultant wind then has a component due to isallobaric gradient n It is perpendicular to the isallobars pointing toward the pressure falls. n
Isallobaric Wind Exercise On the surface chart, analyse the pressure falls and rises. Falls dashed red lines Rises dashed blue lines Zero line purple.
Boundary Layer Effects friction Low level stability n Winds aloft n Surface Frictional Effects n Land Sea Breezes n Mountain and Valley winds n Thunderstorm and Downdrafts n
Low Level Stability n Stability of the air between 3 -5 thousand feet is one of the most important considerations in diagnosing surface winds. n Stability of an airmass can be determined using a suitable tephigram
Low Level stability n Air is stable Little mixing n Rate of downward transport of horizontal momentum is small. n Vertical wind shear is large n
Low level stability n Unstable air n - - Lots of convective mixing so effective exchange of horizontal momentum between layers of the atmosphere. Vertical wind shear is reduced Winds near the surface approach values observed at higher altitudes Turbulent eddies which exchange momentum produce gusty winds
Instability over land Occur during daylight hours n Incoming radiation causes superadiabatic conditions --- convection --- mixing n At night surface based convection will cease due to cooling of layer. n
Stability n The degree of instability and therefore the amount of mixing will depend on: Cloud cover n Albedo of the surface – different degrees of absorption n General state of the atmosphere n ……which will determine the height to which convection will extend.
The 5 stability categories which can be obtained using the low level lapse rate on a tephigram
Instability over water = Tair -Twater
Winds aloft Stability of the boundary layer can be used to estimate which portion of the max winds in this layer will reach the surface. n If unstable, n Diagnose the depth of the unstable layer n Look at the winds at the top of instability n n This will provide an estimate of the max gust which can be expected at the surface
Exercise Find a tephigram which has some instability or use STORM MACHINE & generate one n Analyse the graph to see what speed the winds would be if brought down to the surface n
Web pages Atmospheric soundings n http: //weather. uwyo. edu/upperair/sounding. html n Forecast soundings n http: //weather. admin. niu. edu/machine/fcst sound. html n
Surface Frictional Effects Reduces the speed of the wind n Causes cross-isobaric flow toward a lower pressure n Dependent on the magnitude of the type of the underlying surface n
Land Sea Breezes Local effects n Need to identify localities where there are strong horizontal surface temperature gradients - specifically shorelines. n Mini frontal passage n Change in wind direction and speed n Temperature n
Land Sea Breezes The strength of the breeze depends on: n a) the land-water temperature difference; n b) the strength of the geostrophic wind; n c) the time of day (insolation); n d) the terrain roughness; n e) the curvature of the coastline n
Web pages for sea breezes n http: //www. meted. ucar. edu/mesoprim/sea breez/frameset. htm Sea breeze conclusions in NWP n http: //www. meted. ucar. edu/mesoprim/sea breez/frameset. htm n
Mountain and Valley Winds Important features of the influence of valleys on the wind include: n a) the orientation to the geostrophic wind; n b) angle of incidence of the sun’s rays; n c) the geometrical dimensions of the valley. n
Mountain and Valley Winds n If the geostrophic wind is strong and n Is blowing parallel to the valley n Funnel effect – strengthening of the winds n Is blowing at right angles n Complex flow patterns set up. Wind speed is less than over level land but may get intense turbulence
Mountain and Valley Winds n When the pressure gradient is light Wind flow along the valley. n At night drainage winds blow down the slopes of the valley side and down the sloping floor of the valley. n
Summary n In order to do a produce a good forecast one must diagnose the contributions of: the geostrophic wind n the gradient wind (curvature in the trajectory) n isallobaric wind n Low level stability n Surface frictional effects n Thermal winds n Local effects n
Extra learning n n Gap winds With regard to the origin of gap flows: • Describe the conditions required for geostrophic flow. • Recall that gap winds are typically non-geostrophic. • Describe the origin of the pressure gradients that occur across gaps. • Recall that the thinning of low-level cool air at a gap exit can increase the pressure gradient across a gap. • Recall that adiabatic warming of downslope winds can increase the pressure gradient across a gap.
Extra learning Gap Winds n With regard to forecasting gap winds: • Qualitatively describe how varying the following factors affects wind speed through a gap: * Pressure gradient * Surface roughness * Gap length * Temperature • Describe the horizontal resolution of a mesoscale model required to accurately forecast flow through a gap. Estimated time to complete: 1. 5 -2 h n http: //www. meted. ucar. edu/topics_mountain. php n
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