Assessment of warm and dry bias over ARM SGP site in E3SMv2 and E3SM-MMF
Xie, Shaocheng — Lawrence Livermore National Laboratory
Area of research:
General Circulation and Single Column Models/Parameterizations
Many climate models exhibit a dry and warm bias over the central U.S. during the summer months. Understanding the causes of this bias is important to shine a light on this common model error and reduce the uncertainty in future projections. In this study, we diagnosed the summertime warm and dry bias at the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement user facility's Southern Great Plains observatory in version 2 of DOE's global climate model E3SM (E3SMv2) and multiscale modeling framework version of E3SMv2 (E3SM-MMF).
Our results confirm that the deficit of precipitation leads to the excessive warming near surface. However, the reason for the underestimate of precipitation is different in both configurations of E3SMv2. In E3SMv2 the underestimation of precipitation is due to precipitating events that are “too frequent, too weak” while in E3SM-MMF precipitation events are “too rare, too intense” relative to the observations.
In this study, we use E3SMv2 and E3SM-MMF to assess how parameterized and resolved convection affect temperature and precipitation biases over the Southern Great Plains site of the ARM facility. Both configurations overestimate near-surface temperature and underestimate precipitation at the SGP site. The bias is associated with a lack of low-level clouds during days without precipitation and too much incoming solar radiation causing the surface to warm. Low-level cloud fraction in E3SM-MMF during the non-precipitating days is lower in comparison to E3SMv2 and observation, consistent with the larger warm bias. We also find that the underestimated precipitation can be characterized as “too frequent, too weak” in E3SMv2 and “too rare, too intense” in E3SM-MMF. These deficiencies conspire to sustain the warm and dry bias over the central U.S.