CAPSULE SUMMARY A regional-scale observational experiment designed to address how the atmospheric boundary layer responds to spatial heterogeneity in surface energy fluxes.
This study utilizes six commonly used reanalysis products, including the NCEP–Department of Energy Reanalysis 2 (NCEP2), NCEP Climate Forecast System Reanalysis (CFSR), ECMWF interim reanalysis (ERA-Interim), Japanese 25-year Reanalysis Project (JRA-25), Modern-Era Retrospective Analysis for Research and Applications (MERRA), and North American Regional Reanalysis (NARR), to evaluate features of the southern Great Plains low-level jet (LLJ) above the U.S. Department of Energy’s Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF) Southern Great Plains site. Two sets of radiosonde data are utilized: the six-week Midlatitude Continental Convective Clouds Experiment (MC3E) and a 10-yr period spanning 2001 through 2010. All six reanalyses are compared to MC3E data, while only the NARR, MERRA, and CFSR are compared to the 10-yr data. The reanalyses are able to represent most aspects of the composite LLJ profile, although there is a tendency for each reanalysis to overestimate the wind speed between the nose of the LLJ (at approximately 900 mb) and a pressure level of 700 mb. There are large discrepancies in the number of LLJs observed and derived from the reanalysis, particularly for strong LLJs, leading to an underestimate of the moisture transport associated with LLJs. When the 10-yr period is considered, the NARR and CFSR overestimate and MERRA underestimates the total moisture transport, but all three underestimate the transport associated with strong LLJs by factors of 1.4, 2.0, and 2.7 for CFSR, NARR, and MERRA, respectively. During MC3E there were differences in the patterns of moisture convergence and divergence, but the patterns are more consistent during the 10-yr period.
[1] Tropical tropopause layer cirrus (TTLC) profiles identified from CALIPSO lidar measurements are grouped into cloud objects and classified according to whether or not they are directly connected to deep convection. TTLC objects directly connected to deep convection are optically and physically thicker than isolated objects, consistent with what would be expected if convective objects were formed from convective detrainment and isolated objects formed in situ. The frequency of occurrence of these different classifications of TTLC varies depending on height and region. Thirty-six percent of TTLC profiles contain convective TTLC when TTLC is defined with cloud base height greater than 14 km (±20°Latitude). This estimate of convective TTLC is thought to be a lower limit on the percentage of TTLC formed by convective detrainment. Aged anvil that has persisted longer than the thick regions of the convective cloud would not be classified as convective in our classification. Regions with higher occurrence of deep convection also have higher occurrence of TTLC, and a greater percentage of those TTLC are classified as convective. Definitions of TTLC using higher cloud base height thresholds contain a smaller percentage of TTLC that are classified as convective. Cloud top heights of both isolated and convective TTLC are distributed similarly with respect to the height of the cold point tropopause. Cloud optical depths were found to be higher in TTLC over thick clouds than in TTLC over clear sky during the day but not in night measurements. It is not clear whether this has a physical cause or is due to daytime measurement uncertainties. Both cloud base and top heights are higher over deep convection than over clear sky.
A 4-yr climatology of midlevel clouds is presented from vertically pointing cloud lidar and radar measurements at the Atmospheric Radiation Measurement Program (ARM) site at Darwin, Australia. Few studies exist of tropical midlevel clouds using a dataset of this length. Seventy percent of clouds with top heights between 4 and 8 km are less than 2 km thick. These thin layer clouds have a peak in cloud-top temperature around the melting level (08C) and also a second peak around 212.58C. The diurnal frequency of thin clouds is highest during the night and reaches a minimum around noon, consistent with variation caused by solar heating. Using a 1.5-yr subset of the observations, the authors found that thin clouds have a high probability of containing supercooled liquid water at low temperatures: ;20% of clouds at 2308C, ;50% of clouds at 2208C, and ;65% of clouds at 2108C contain supercooled liquid water. The authors hypothesize that thin midlevel clouds formed at the melting level are formed differently during active and break monsoon periods and test this over three monsoon seasons. A greater frequency of thin midlevel clouds are likely formed by increased condensation following the latent cooling of melting during active monsoon periods when stratiform precipitation is most frequent. This is supported by the high percentage (65%) of midlevel clouds with preceding stratiform precipitation and the high frequency of stable layers slightly warmer than 08C. In the break monsoon, a distinct peak in the frequency of stable layers at 08C matches the peak in thin midlevel cloudiness, consistent with detrainment from convection.
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