Local land-atmosphere coupling at Southern Great Plains: ARM data and climate model simulations
Submitter
Zhang, Yunyan — Lawrence Livermore National Laboratory
Area of research
Cloud Processes
Journal Reference
Science
Accurate representations of the land-atmosphere coupling processes are critical for weather forecasts and climate predictions. Using the 9-year warm-season observations at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site, we assess the land-atmosphere coupling in North American Regional Reanalysis (NARR) and two climate models: hindcasts with the Community Atmosphere Model version 5.1 by Cloud-Associated Parameterizations Testbed (CAM5-CAPT) and nudged runs with the Energy Exascale Earth System Model Atmosphere version 1 Regionally Refined Model (EAMv1-RRM).
Impact
This study provides a framework to systematically diagnose the model performance in representing land-atmosphere interactions by focusing on locally generated convection regimes. These convection regimes include clear-sky, fair-weather shallow cumulus, and late-afternoon deep convection days, in which the land-atmosphere coupling is the strongest and the impacts from synoptic weather patterns are minimized. Through local coupling metrics including mixing diagrams, the lifting condensation level deficit, and the framework of convective triggering potential and humidity index, this study advances process-oriented diagnoses and provides insights into the sources of model biases in the land-atmospheric boundary-layer cloud/precipitation interaction.
Summary
We use long-term warm-season observations at the ARM SGP site to assess the performance of NARR and two climate model simulations (CAM5-CAPT hindcasts and EAMv1-RRM nudged runs). An analysis framework based on locally generated convective regimes is established to attribute model biases in land-atmosphere coupling to initial morning conditions, large-scale advections, and parameterized boundary-layer and convective processes. In general, NARR agrees well with observations on the land-atmosphere coupling processes, the daily atmospheric moisture budget, and surface energy budget except a slightly warmer and drier surface with higher downwelling shortwave radiation and lower evaporative fraction. Compared with ARM observations, CAM5 overestimates entrainment drying while E3SMv1-RRM underestimates surface evaporation on clear-sky days. On shallow cumulus days, clouds are formed with a weaker PBL growth and more humid morning conditions in both models. On deep convection days, simulated convections are triggered more often by elevated instabilities with a relatively stable lower troposphere, especially in E3SMv1 RRM runs. In general, CAM5-CAPT can reproduce the local L-A coupling processes to some extent due to the constrained early morning conditions and large-scale winds. EAMv1-RRM exhibits large precipitation deficits and warm and dry biases towards mid-to-late summers, which may be an amplification through a positive feedback among initial atmosphere and land states in the early morning, convection triggering, and large-scale circulations.
Figure 1: Clear-sky-day mixing diagram shows the day-time evolution of the conservative variables in the atmospheric boundary layer: total water vapor specific humidity (Lvq) versus potential temperature (Cpq) from ARM observations (black), NARR (green), CAM5-CAPT hindcasts (red) and EAMv1-RRM nudged runs (blue). Dots denote the composite hourly means from 0730 to 1730 local time. The text annotations depict the vector component contributions from surface (Vsfc), advection (Vadv), and entrainment fluxes (Vent).
Figure 2: Comparison of the composite daytime evolution of (a) planetary boundary-layer height (PBL), (b) lifting condensation level (LCL), and (c) LCL deficit (PBL minus LCL) on clear-sky days among ARM observations (black), NARR (green), CAM5-CAPT hindcasts (red) and EAMv1-RRM nudged runs (blue). (d)-(f) are the same as (a)-(c) but on fair-weather shallow cumulus days. In (a) and (d), the pink triangles and dark green inverted triangles denote the PBL heights derived from the balloon sounding data by a method using the bulk Richardson number with a critical threshold of 0.25 and 0.50 respectively. The length of the vertical lines denotes two standard errors.