An Assessment of Planetary Boundary Layer Lapse Rate Variations and New MODIS Low Cloud Top Height Retrieval over the Northeast Pacific Ocean

The prevailing low clouds over the northeast Pacific ocean are mostly trapped below the planetary boundary layer (PBL) inversion. The Moderate Resolution Imaging Spectroradiometer (MODIS) window channel offers long-term, high spatial resolution cloud-top-temperatures (CTT) measurements. Combining the CTT with the collocated sea surface temperature (SST) measurement along with a simple fixed lapse rate (LR) PBL model, the cloud-top-height (CTH) can be easily retrieved. The current MODIS Collection 6 only considers the latitudinal variation of the lapse rate. Over the subtropical eastern ocean where the PBL lapse rate shows significant temporal and longitudinal variation from the stratocumulus regime to the trade cumulus regime. The unaccounted longitudinal variation in LR leads to a positive bias of about 250 – 350 m in Collection 6 CTH. In this study, the day-night separated monthly LR lookup table (2.5° longitude x 2.5° latitude) is derived from MODIS CTT and collocated AMSR-E SST and CALIPSO lidar CTH measurements over the northeast Pacific from 2006 to 2010. The LR decreases from ~8.5 K/km near the south California coast to ~6.5 K/km near Hawaii. Diurnal variations of LR are small over the coastal regions, but nighttime LR are 1-2 K/km steeper than daytime LR west of 140°W meridian. The new MODIS CTH retrievals are then derived based on the LR look-up table from October 2012 through September 2013, when the Marine ARM GPCI Investigation of Clouds (MAGIC) campaign took place. The new MODIS CTH product is then validated by the MAGIC radiosonde soundings and the CALIPSO CTH measurements. The new MODIS CTH significantly reduced the positive biases in Collection 6, and captures the fine spatial and seasonal variation of the CTH. Such long-term high spatial resolution MODIS CTH measurements will be very valuable for PBL and low cloud study.