The Role of Convective Gustiness in Reducing Seasonal Precipitation Biases in the Tropical West Pacific. (10th April 2018)

• Record Type: Journal Article
• Authors: Harrop, Bryce E.; Ma, Po‐Lun; Rasch, Philip J.; Neale, Richard B.; Hannay, Cecile
• Is Part Of: Journal of advances in modeling earth systems. Volume 10:Number 4(2018)
• Page Start: 961
• Page End: 970
• Publication Date: 2018-04-10
• Subjects: tropical precipitation -- model parameterization -- gross moist stability; Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1002/2017MS001157
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: Precipitation is an important climate quantity that is critically relevant to society. In spite of intense efforts, significant precipitation biases remain in most climate models. One pervasive and persistent bias found in many general circulation models occurs in the Tropical West Pacific where northern hemisphere summer‐time precipitation is often underestimated compared to observations. Using the DOE‐E3SM model, the inclusion of a missing process, convective gustiness, is shown to reduce those biases through a net increase in surface evaporation. Gustiness in surface wind fields is assumed to arise empirically in proportion to the intensity of convective precipitation. The increased evaporation can be treated as an increase in the moist static energy forcing into the atmosphere. A Normalized Gross Moist Stability (NGMS) framework (which characterizes the relationship between convective forcing and convective response) is used to explore the processes responsible for the precipitation bias, and the impact of the gustiness parameterization in reducing that bias. Because the NGMS of the Tropical West Pacific is less than unity in the E3SMv1 model, the increase in energy forcing amplifies the increase in precipitation to exceed that of the evaporative flux. Convective gustiness favors increased precipitation in regions where the resolved surface winds are weak and convection is present. Plain Language Summary: The Tropical West Pacific is a region of very strong rainfall and weak surface winds. Climate models often underestimate rainfall in the Tropical West Pacific during the summer. The shortage of rain is related to the low amount of evaporation arising from the light surface winds in the region. General circulation models have typically neglected gusts that are generated below evaporating rain, but these gusts are known to create circulations that increase surface evaporation. Accounting for these "gusty winds" increases surface evaporation in the model, which acts as a local source of moist energy to the atmosphere, intensifying the circulation and surface rainfall amount. Including gustiness leads to a more realistic rainfall pattern when compared to the real world. Key Points: Convective gustiness is shown to reduce the low precipitation bias in the Tropical West Pacific during Boreal summer Convective gustiness increases precipitation in regions where resolved winds are weak and convective precipitation is strong Increases in surface evaporation invigorate the circulation, increasing divergence of moist static energy out of the Tropical West Pacific