SYNOPTIC CONSIDERATIONS OF 

 THE LOW-LEVEL JET 



The sharpness of the wind maximum tends to be 

 enhanced when the geostrophic wind decreases with 

 height (Blackadar 1957). Following this observation, it 

 is useful to examine an expression describing the change 

 of the geostrophic wind with height. Consider the follow- 

 ing analysis: 



_^ = -pg (hydrostatic eq.) (2) 

 dz 



p = pRT(eq. of state) (3) 

 fVg =-_i As. (geostrophic wind) (4) 

 P an 



where n is normal to and increases to the left of the 

 velocity vector. Now eliminate p using eqs. (2) and (3), 

 then: 



P az RT 

 and then differentiate with respect to n 



an \p~ dz/ RP an (5) 

 Now write (4) as 



fV 



J_ i£ = - __a. 

 p an RT 



and differentiate with respect to z 



RP az RT az RP an 



or 



^ = \dT_ - _a_ ar 



az T dz fT an 



The first term on the right of (7) is less than one-tenth 

 of the second term. Neglecting this term: 



--^ ? (8) 



a z f T an 



which is a form of the thermal wind equation. According to 

 equation 8, for the geostrophic wind to decrease with 

 height above the jet maximum, the temperature to the left 

 of the geostrophic wind must increase along n. Since low 

 pressure is also to the left of the geostrophic wind this 

 requires the presence of a warm low-pressure system. 



Observations have shown this to be the case. Means 

 (1952) found that, for the central United States, 

 supergeostrophic winds are found in low-level jets where 

 isotherms are approximately parallel to the streamlines 

 with warmer air toward lower pressure. Hoecker (1965) 



noted the meteorological conditions during three 

 southerly low-level jet systems included warm low- 

 pressure area to the west (left) of the jet. Also he noted 

 that the occurrence of a surface nocturnal inversion 

 allows greater increased low-level vertical shear, which 

 favors a higher jet speed for a given initial pressure 

 gradient. 



It is important to note that the thermal wind component 

 has a direction parallel to the isotherms of the mean 

 temperature of the layer considered with high temperature 

 to the right. This is just opposite of the situation for a 

 decrease of the geostrophic wind with height. Therefore, 

 in this case, the thermal wind opposes the geostrophic 

 wind. This was proposed in a theoretical hodograph 

 developed by Blackadar and found in observations by 

 Hoecker (1965). In two cases Hoecker found that the 

 thermal wind vector almost directly opposed the sea level 

 geostrophic wind. Also, he states, "The opposition of the 

 sea level geostrophic and thermal wind vectors is indicative 

 of a warm low-pressure system in the region and since a 

 warm low pressure system is shallow, the geostrophic wind 

 (as well as the real wind) should decrease with height." 

 Hoecker suggests (based on his observation in the 

 Oklahoma-Texas area) that "if an adverse thermal wind 

 exists at about 1800 GST along with a southerly low-level 

 flow, and if the adverse thermal wind can be forecast to 

 persist during the following hours of darkness, the 

 boundary-layer jet system (speed maximum at about 300 m 

 above the ground),. . .can be expected to occur that night." 



The jet wind does not necessarily have to occur at night. 

 According to Blackadar (1957), the jetlike profile may 

 occur even in the daytime and conversely, the jet effect 

 may not occur at all during the night if the geostrophic 

 wind increases too rapidly upward. Rider (1966) observed 

 low-level jet winds at the White Sands Missile Range. He 

 observed that, although the low-level jet was 

 predominantly a nocturnal phenomenon, with the nose of 

 the jet near the height of the nocturnal temperature 

 inversion, significant low-level wind maxima are some- 

 times found in the daytime. Also, there were a few cases 

 where a jet formed even though lapse conditions prevailed, 

 and there were cases where a temperature inversion 

 developed during the night but a significant low-level 

 jet was not evident. 



REPORTED JET-WIND 

 OBSERVATIONS 



Measurements made at Silver Hill, Md., by Gifford 

 (1952) provided clear evidence that windspeeds at the 

 level of the jet maximum (2,000 ft [610 m]) are considerably 

 supergeostrophic. Similar conclusions may be drawn from 

 the analysis of low-level wind maxima at 0300 local time 

 at San Antonio, Tex. (Blackadar 1957). Some other early 

 measurements made at O'Neil, Nebr., during a 6-week 

 period in the fall of 1953, illustrated several well-developed 

 low-altitude wind jets (Barad 1961). Barad asks, "How did 

 so strong and distinct a pattern go so long undetected?" 

 The answer is simply that standard observation techniques 

 were inadequate to find it. Observations on a 1,400-ft 

 (427-m) tower near Dallas, Tex., clearly showed the 



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