Impact of humidity biases on light precipitation occurrence: observations versus simulations

Abstract

<p><strong>Abstract.</strong> This work uses a network of GPS stations over Europe from which a homogenised integrated water vapor (IWV) dataset has been retrieved, completed with colocated temperature and precipitation measurements over specific stations to i) estimate the biases of six regional climate models over Europe in terms of humidity; ii) understand their origins; iii) and finally assess the impact of these biases on the frequency of occurrence of precipitation. The evaluated simulations have been performed in the framework of HYMEX/Med-CORDEX programs and cover the Mediterranean area and part of Europe at horizontal resolutions of 50 to 12 km.</p> <p>The analysis shows that models tend to overestimate the low values of IWV and the use of the nudging technique reduces the differences between GPS and simulated IWV. Results suggest that physics of models mostly explain the mean biases, while dynamics affects the variability. The land surface/atmosphere exchanges affect the estimation of IWV over most part of Europe, especially in summer. The limitations of the models to represent these processes explain part of their baises in IWV. However, models correctly simulate the dependance between IWV and temperature, and specifically the deviation that this relationship experiences regarding the Clausius-Clapeyron law after a critical value of temperature (T<sub>break</sub>). The high spatial variability of T<sub>break</sub> indicates that it has a strong dependence on local processes which drive the local humidity sources. This explains why the maximum values of IWV are not necessarely observed over warmer area, that are often dry area.</p> <p>Finally, it is shown over SIRTA observatory (near Paris) that the frequency of occurrence of light precipitation is strongly conditioned by the biases in IWV and by the precision of the models to reproduce the distribution of IWV as a function of the temperature. The results of the models indicate that a similar dependence occurs in other areas of Europe, especially where precipitation has a predominantly convective character. According to the observations, for each range of temperature, there is a critical value of IWV from which precipitation picks up. The critical values and the probability to exceed them are simulated with a bias that depends on the model. Those models which present too often light precipitation generally show lower critical values and higher probability to exceed them.</p>

Recent trends in climate variability at the local scale using 40 years of observations: the case of the Paris region of France

“…In spring, the weather is drier with fewer rainy days, but slightly more extremes. This is consistent with the decrease in relative humidity that affects the triggering of precipitation (Rasmussen et al, 2017;Bastin et al, 2019).…”

“…The analysis of precipitation can be sensitive to these differences and to local effects. It is expected that the decrease in relative humidity observed in the Paris area affects some indices of precipitation, especially indices concerning occurrence (Bastin et al, 2019).…”

Consistency and representativeness of integrated water vapour from ground-based GPS observations and ERA-Interim reanalysis

“…More recently, the value of continuous long time series of GPS IWV data has been investigated for the purpose of studying global and regional climate variability and validating climate models (Nilsson and Elgered, 2008;Vey et al, 2009;Roman et al, 2012;Ning et al, 2013;Chen and Liu, 2016;Wang et al, 2016;Parracho, 2017;Bastin et al, 2018). These studies reported various levels of agreement between GPS and atmospheric models/reanalyses making it difficult to draw general conclusions on the consistency between products.…”

Sensitivity of land-atmosphere coupling strength to perturbations of early-morning temperature and moisture profiles in the European summer

“…The core set aimed at covering four possible combinations of differences in the climate conditions, namely, cooler and moister conditions, cooler and dryer conditions, warmer and moister conditions, as well as warmer and dryer conditions. Previous observational and global model studies suggest that temperature and moisture are considerably positively correlated in most regions around the globe and trends lie around 7% change in moisture per K change in temperature, reflecting the Clausius-Clapeyron rate for increases in moisture and maintains a quasi-constant relative humidity (Bastin et al, 2019;Willett et al, 2010). In Europe, the scaling of moisture to temperature is slightly higher (section 2.3.2).…”

“…The perturbations of moisture were implemented under consideration of the close relationship between temperature and water vapor in the atmosphere, thus, taking into account the respective temperature perturbation (e.g. Willett et al, 2010;Bastin et al, 2019).…”