Structure and Heat Energy Budget of Mixed Layer Capped by Inversion during the Period of Polar Outbreak over Kuroshio Region

Abstract

The observational study on the thermal and kinematical structure of the cold air-mass and its transformation over Kuroshio region is made for the period of the strong cold air outbreak (23-27 February 1974) based on the data from AMTEX '74.
The relatively shallow (1-2km depth) layer of cold air-mass is capped by the well developed inversion layer. The inversion layer distinctly separates the untransformed steady westerly layers aloft from the transformed northerly wind layer below. The vertically uniform profile of θe and V in the subinversion layer, except the lowermost layer, indicates strong vertical mixing there. This “mixed layer” is also characterized by the convective instability due to the large value of θe in the lowermost layer adjacent to the sea surface. The analyzed period is subdivided into “developing stage of the mixed layer” (23-24 February) and “quasistationary stage of the mixed layer” (25-27 February). The “developing stage” is char- acterized by the increase of the thickness of the mixed layer and the upward motion. The “quasi-stationary stage” is characterized by the quasi-stationary state of the mixed layer and the downward motion. The inversion base is, on the whole during the quasi-stationary stage, lower over the northwestern and higher over eastern part of AMTEX area, except some local rising of the inversion base.
The budget calculation indicates that the horizontal advection of heat and moisture (cold and dry advection) and the apparent heat and moisture sources are very large in the subinversion layer and they abruptly decrease at the inversion base. The vertical distribution of the convective transport of the heat energy is also obtained. Detailed analysis on the thermodynamical balance in the mixed layer is made for the quasi-stationary stage.
The time variation of the static stability in the mixed layer and inversion layer during the developing and quasi-stationary stages is also analyzed on the basis of the heat energy budget calculation.