Subseasonal Vertical Correlations with MBL Cloud Top Heights over the Southeastern Pacific and Cloud Top Height/Vertical Velocity Relationships in a Baroclinic Atmosphere Using Satellite and Re-Analysis Data

Abstract:

MBL cloud top height (Z_top) is a reflection of the balance between several large-scale terms, including subsidence, which in isolation tends to suppress Z_top, and entrainment, which deepens Z_top, and is inversely proportional to stability. With the use of gridded daily MODIS L3 cloud data and state-of-the-art reanalysis data from ERA-Interim, we extend beyond the traditional constraints of defining stability/dynamic variables with one or two fixed levels by instead computing vertical correlations relative to Z_top. Along a southeast (SE) Pacific cross section centered at 20˚S from the coast to 140˚W, ϴ negatively correlates with Z_top not only just above cloud top, but throughout the lower free troposphere. Below cloud top, ϴ correlations with Z_topare fairly weak.

More confounding relationships are found between profiles of pressure vertical velocity (ω) and both Z_top and MBL cloud fraction (CF); especially over the stratocumulus (Sc) and transitional regions, stronger ω (either near Z_top or aloft) is associated with deeper MBL clouds, anomalously low ϴ near and above Z_top, and slightly reduced CF. Over the trade cumulus region, enhanced subsidence is associated with an anomalously cold MBL, increased stability, and suppressed Z_top.

For further elucidation, we visit the concept of a baroclinic atmosphere, in which the trough/ridge axes and ϴ anomalies tilt with height, albeit in opposite ways. Along our cross section, in the case of an anomalous surface high to the east of an upper-level high, an eastward tilt with height of ϴ is documented, especially over the transitional and Sc cloud regimes. Anomalous ridging aloft is strongly correlated with suppressed Z_top.

Over the transitional and Sc regimes (east of ~105˚W), maximum ω anomalies are located directly between the upper-level anomalous ridge and trough axes. The coldest ϴ anomalies near and above Z_top are located just east of the maximum subsidence, corresponding to reduced stability, deeper Z_top, and a slight reduction of CF. Thus, the baroclinic structure between 100˚W and 70˚W can help explain the unexpected positive correlations between ω and Z_top. The relationships described between ω and Z_top may have implications for diagnosing low cloud feedbacks in other subtropical Sc areas as well, especially where baroclinicity is important.