Soil Temperature and Moisture Errors in Operational Eta Model Analyses

Abstract

Proper partitioning of the surface heat fluxes that drive the evolution of the planetary boundary layer in numerical weather prediction models requires an accurate specification of the initial state of the land surface. The National Centers for Environmental Prediction (NCEP) operational Eta Model is used to produce land surface analyses by continuously cycling soil temperature and moisture fields. These fields previously evolved only in response to radiation budget constraints and modeled precipitation, but NCEP recently upgraded the self-cycling process so that soil fields respond instead to the radiation budget and observed precipitation. A comparison of 0000 and 1200 UTC Eta Model analyses of soil temperature and moisture at several soil depths with observations from the Oklahoma Mesonet during 2004 and 2005 shows that there are strong biases in soil temperature and a severe underestimation of soil moisture at all depths. After the change to a new assimilation scheme, there is notable improvement in the magnitude of the analyzed soil moisture fields, although a strong dry bias persists in the soil moisture field. A simple one-layer slab soil model quantifies the effect of such soil moisture errors on the diurnal cycle of soil temperature and reveals that these soil moisture errors alone may account for only 1.6°C increases in predicted maximum soil temperatures during the day and temperature reductions of the same magnitude at night. The much larger remaining soil temperature errors possibly stem from documented problems with the solar radiation and longwave parameterizations within the Eta Model.

Validation of Noah-Simulated Soil Temperature in the North American Land Data Assimilation System Phase 2

“…All these errors can interact together, making this issue complex. In general, a soil moisture error of 0.1 m 3 m 23 may lead to an error of more than 1.6 K for maximum or minimum daily soil temperature (Godfrey and Stensrud 2008). It should be noted that soil texture and vegetation classification problems definitely contributed errors to Noah-simulated soil temperature as well as to Noahsimulated soil moisture (Robock et al 2003) as they could not correctly represent the vegetation and soil texture conditions at some validation sites.…”

“…It provides a key link between the atmosphere and land surface moisture and energy partitioning through soil evaporation and transpiration processes (Robock et al 2000). However, the role of soil temperature and its influence on weather and climate, especially its effect on short-range weather processes, have been underestimated in the past (Godfrey and Stensrud 2008). Soil temperature directly affects the surface radiation budget through upward longwave radiation and ground heat flux as both depend on soil temperature.…”

“…In addition, ground heat flux also affects sensible heat flux, boundary layer dynamics, turbulence, and air temperature. Recent studies from the National Centers for Environmental Prediction (NCEP) operational Eta model (Godfrey and Stensrud 2008) and Weather Research and Forecasting Model (Fan 2009) show that soil temperature has significant effects on short-term model forecasts of near-surface variables such as precipitation and lower-atmospheric circulation fields. A modeling study from Xue et al (2001) demonstrated that subsurface soil temperature over the western United States in late spring has an impact on summer precipitation.…”

An Empirical Latent Heat Flux Parameterization for the Noah Land Surface Model

“…For the cases under consideration, the observed soil moisture is considerably wetter and the observed soil temperatures are generally cooler at the 1200 UTC start time than the Eta analyses (Godfrey and Stensrud 2008). The climatological s f and constant LAI values stand in stark contrast to the s f and LAI observations.…”

“…Soil temperature modulates the distribution of heat near the soil surface (Dudhia 1996) and affects the surface radiation budget through its influence on the ground heat flux (Brotzge and Crawford 2003). Unfortunately, NCEP operational Eta Model analyses (Black 1994), which provide initial conditions to a variety of operational and research models, exhibit strong biases in soil temperature and soil moisture (Marshall et al 2003;Godfrey and Stensrud 2008). This finding necessitates the inclusion of soil temperature and soil moisture observations in model initializations in order to address inaccuracies in LSM parameterizations.…”