3. (20 pt) In March 2014, an unusually strong surface low-pressure system moved

Question

3. (20 pt) In March 2014, an unusually strong surface low-pressure system moved across the state of Pennsylvania. At 21Z on March 12, 2014, the central pressure of the low had dropped to 984 mb, and pressures in the wake of the low rose quickly. with a maximum 3-hr pressure rise of about 9 mb. The sea-level pressure (in mb) and sea-level pressure tendency (in mb/3 hr) charts at 21Z on March 12, 2014, are found in the Lab 8 folder on ANGEL. Using these charts, we want to compare the magnitude of the geostrophic wind to the magnitude of the isallobaric wind (which is a part of the ageostrophic wind). In this case, in western Pennsylvania, the isallobaric wind is about as large as it ever gets (note the tight packing of isallobars there), so that's where we'll focus our attention.

Explanation / Answer

The geostrophic wind is determined by the gradient of the isobars. On a pressure surface the gradient of the isohypses reflects the tilt of the pressure surface. If this tilt changes with pressure then also the geostrophic wind will change with pressure in magnitude and/or direction. The real wind differs from the geostrophic wind by an ageostrophic wind that is dominated by a term proportional to the height tendency (or pressure tendency).  This portion of the ageostrophicwind is known at the ISALLOBARIC WIND. Due to this pressure gradient term the isallobaric wind is larger than the geostropic wind in pensylvania.

a) pressure gradient is dp/dz= -pg = -1.2*10^-3*9.8= -11.76*10^-3.

given that, distance= 250 Km, density= 1.2 Kg/m^3, and f= 10^-4 /sec

geostropic wind Vg= (1/pf)*dp/dz= (1/((1.2*10^-3)*(10^-4))*(11.76*10^-3) = 9.8*10^-4 m/sec.

b) geostropic winds were differ from calculated winds, due to geostropic winds exist in locations where there are no frictional forces and the isobars are striaght. However, such locations are quite rare. Isobars are almost always curved and are very rarely evenly spaced. This changes the geostrophic winds so that they are no longer geostrophic but are instead in gradient wind balance. They still blow parallel to the isobars, but are no longer balanced by only the pressure gradient and Coriolis forces, and do not have the same velocity as geostrophic winds.

pressure gradient= 9mb/3hr/250Km = 9*10^3/3/3600/250*10^7= 43.2*10^-4 dyne/cm^2-sec

isallobaric winds Vi= (1/(rho*f^2))/dp= (1/((1.2*10^-3)*10^-8))*(43.2*10^-4)= 36*10^7 = dyne cm/gm.

This means that in a high pressure system or ridge, the gradient wind blows parallel to the isobars faster than geostrophic (supergeostrophic) speed.

Related Questions

Navigate

• Browse (All)
• Browse 3
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.