The hydrostatic equilibrium addresses the approximate balance between the positive force of the vertical pressure gradient and the negative gravity force and has been widely assumed for atmospheric applications. The hydrostatic imbalance of the mean atmospheric state for the acceleration of vertical motions in the vertical momentum balance is investigated using tower, the global positioning system radiosonde, and Doppler lidar and radar observations throughout the diurnally varying atmospheric boundary layer (ABL) under clearsky conditions. Because of the negligibly small mean vertical velocity, the acceleration of vertical motions is dominated by vertical variations of vertical turbulent velocity variances. The imbalance is found to be mainly due to the vertical turbulent transport of changing air density as a result of thermal expansion/contraction in response to air temperature changes following surface temperature changes. In contrast, any pressure change associated with air temperature changes is small, and the positive vertical pressuregradient force is strongly influenced by its background value. The vertical variation of the turbulent velocity variance from its vertical increase in the lower convective boundary layer (CBL) to its vertical decrease in the upper CBL is observed to be associated with the sign change of the imbalance from positive to negative due to the vertical decrease of the positive vertical pressuregradient force and the relative increase of the negative gravity force as a result of the decreasing upward transport of the lowdensity air. The imbalance is reduced significantly at night but does not steadily approach zero. Understanding the development of hydrostatic imbalance has important implications for understanding largescale atmosphere, especially for cloud development.
It is well known that the hydrostatic imbalance between the positive pressuregradient force due to the vertical decrease of atmospheric pressure and the negative gravity forces in the vertical momentum balance equation has important impacts on the vertical acceleration of atmospheric vertical motions. Vertical motions for mass, momentum, and energy transfers contribute significantly to changing atmospheric dynamics and thermodynamics. This study investigates the oftenassumed hydrostatic equilibrium and investigate how the hydrostatic imbalance is developed using field observations in the atmospheric boundary layer under clearsky conditions. The results reveal that hydrostatic imbalance can develop from the largeeddy turbulent transfer of changing air density in response to the surface diabatic heating/cooling. The overwhelming turbulence in response to largescale thermal forcing and mechanical work of the vast Earth surface contributes to the hydrostatic imbalance on large spatial and temporal scales in numerical weather forecast and climate models.
 NSFPAR ID:
 10481739
 Publisher / Repository:
 American Meteorological Society
 Date Published:
 Journal Name:
 Journal of Applied Meteorology and Climatology
 Volume:
 63
 Issue:
 1
 ISSN:
 15588424
 Format(s):
 Medium: X Size: p. 325
 Size(s):
 p. 325
 Sponsoring Org:
 National Science Foundation
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