Methods to Evaluate Subcolumn Profiles Based on Two‐Point Diagnostics

Journal of Geophysical Research Atmospheres, vol. 129

Abstract

Abstract In atmospheric models, stochastic generation of subgrid‐scale profiles or “subcolumns” has been used for a variety of purposes. Such subcolumns can be generated from subgrid probability density functions (PDFs) at different vertical levels, when such PDFs are available. To do so, the generator needs to decide how strongly points should be correlated in the vertical, that is, how much the values should be overlapped. This is sometimes called “PDF overlap.” To assess vertical correlation in a simplified, observable setting, here the vertical correlation of vertical velocity in subcloud layers is examined. Doppler lidar is used to evaluate the vertical profiles of vertical velocity produced by a large‐eddy simulation (LES) model and the Subgrid Importance Latin Hypercube Sampler (SILHS) subcolumn generator. In order to diagnose unrealistic features in subcolumn profiles, various statistical diagnostics are examined here, including the bivariate PDF of vertical velocity at two separated points (i.e., altitudes), the two‐point velocity correlation, the integral correlation length, the PDF of two‐point velocity differences, and the skewness and kurtosis of two‐point velocity differences. The profiles produced by LES match lidar well, except that they are too smooth at small scales. The profiles produced by SILHS exhibit sharp jumps from updraft to downdraft that are not observed in the lidar data. To reduce the generation of these unrealistically sharp jumps, the SILHS sampling method is revised. The diagnostics confirm that the revised sampling method reduces the overprediction of sharp jumps.

Authors 5

1. Benjamin A. Stephens corresponding

   NSF National Center for Atmospheric Research

   Affiliation as printed

   National Center for Atmospheric Research Boulder CO USA

2. Vincent E. Larson

   Pacific Northwest National Laboratory · University of Wisconsin–Milwaukee

   Affiliation as printed

   Department of Mathematical Sciences University of Wisconsin — Milwaukee Milwaukee WI USA

   Pacific Northwest National Laboratory Richland WA USA

3. Rob Newsom

   Pacific Northwest National Laboratory

   Affiliation as printed

   Pacific Northwest National Laboratory Richland WA USA

4. William I. Gustafson

   Pacific Northwest National Laboratory

   Affiliation as printed

   Pacific Northwest National Laboratory Richland WA USA

5. Gerhard Dikta Aachen

   FH Aachen

   Affiliation as printed

   Department of Medical Engineering and Technomathematics Fachhochschule Aachen Jülich Germany