Abstract
Data collected by the NCAR Electra aircraft during AMTEX '75 have been analyzed to determine the spectral characteristics of four boundary layer types based on air-sea temperature differences (ΔT) : (a) Strongly convective (ΔT_??_5°C) with mesoscale cellular convection (MCC), (b) Strongly convective (ΔT_??_5°C) without MCC, (c) Weakly convective (2°C_??_ΔT<5°C) and (d) Non-convective (ΔT<2°C) . Comparisons between the spectra of the four types suggest that a boundary layer containing MCC does show some distinguishing characteristics which are not observed to be present in other boundary layer groups. The specific humidity (q) spectra for flight segments containing MCC were characterized by a prominent peak in the wavelength band corresponding to the observed MCC cell size. Also, the high wavenumber end of the q-spectra at the lower level was observed to contain characteristically larger amounts of energy. The most significant new finding of this study was some evidence supporting the presence of a basic convective mode (BCM) with an aspect ratio comparable to those of Benard-Rayleigh convection cells. It is suggested that an organizational process from the BCM to the MCC mode occurs, with energy transferred up-scale from high wavenumbers to low wavenumbers.
The strongly convective boundary layers without MCC were influenced by other synoptic forcing which apparently prohibited the development of MCC. Two of these cases also showed correspondingly larger amounts of energy at the high wavenumber end of the spectrum than in the weakly convective and non-convective cases. Another case near a pattern of MCC cells showed the presence of a strong MCC mode although the MCC cloud patterns were not present in satellite imagery. The Weakly Convective cases were distinguishable in the small amount of energy present throughout the spectrum with little to no evidence of the MCC or BCM modes. The non-convective group also showed little energy present in the spectra at lower levels. Two flight segments at the upper level, however, showed evidence of forced convection due to the presence of a nearby cold front. This was evident in the q-spectra as a prominent peak at low wavenumbers.
