John F. Mejia

Improved seasonal drought forecasts using reference evapotranspiration anomalies

A novel contiguous United States (CONUS) wide evaluation of reference evapotranspiration (ET0; a formulation of evaporative demand) anomalies is performed using the Climate Forecast System version 2 (CFSv2) reforecast data for 1982–2009. This evaluation was motivated by recent research showing ET0 anomalies can accurately represent drought through exploitation of the complementary relationship between actual evapotranspiration and ET0. Moderate forecast skill of ET0 was found up to leads of 5 months and was consistently better than precipitation skill over most of CONUS. Forecasts of ET0 during drought events revealed high categorical skill for notable warm-season droughts of 1988 and 1999 in the central and northeast CONUS, with precipitation skill being much lower or absent. Increased ET0 skill was found in several climate regions when CFSv2 forecasts were initialized during moderate-to-strong El Nino–Southern Oscillation events. Our findings suggest that ET0 anomaly forecasts can improve and complement existing seasonal drought forecasts.

Use of an Observation Network in the Great Basin to Evaluate Gridded Climate Data

AbstractPredicting sharp hydroclimatic gradients in the complex terrain of the Great Basin can prove to be challenging because of the lack of climate observations that are gradient focused. Furthermore, evaluating gridded data products (GDPs) of climate in such environments for use in local hydroclimatic assessments is also challenging and typically ignored because of the lack of observations. In this study, independent Nevada Climate-Ecohydrological Assessment Network (NevCAN) observations of temperature, relative humidity, and precipitation collected along large altitudinal gradients of the Snake and Sheep mountain ranges from water-year 2012 (October–September) are utilized to evaluate four GDPs of different spatial resolutions: Parameter–Elevation Regressions on Independent Slopes Model (PRISM) 4 km, PRISM 800 m, Daymet 1 km, and a North American Land Data Assimilation System (NLDAS)–PRISM hybrid 4-km product. Inconsistencies and biases in precipitation measurements due to station siting and gauge typ...

Synoptic Variability of Rainfall and Cloudiness along the Coasts of Northern Peru and Ecuador during the 1997/98 El Niño Event

Abstract This paper describes the meteorological conditions associated with large fluctuations in rainfall over the coastal regions of northern Peru and Ecuador during the 1997/98 El Nino event. Using data from a network of routine rain gauges and special gauges established just prior to the onset of heavy rains, it is shown that large variations in the daily rainfall on quasi-weekly time scales occurred during the period January–April 1998. These rainfall fluctuations were approximately in phase along the coast from near the equator to ∼7°S. The daily rainfall data was averaged to develop a subset of wet and dry days, and then these dates were used as the basis for compositing. Special pilot balloon observations were composited with respect to the wet and dry days, showing that westerly and northerly wind anomalies are associated with wet spells. Composites of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis and outgoing longwave radiation ...

Aircraft Observations of the 12–15 July 2004 Moisture Surge Event during the North American Monsoon Experiment

Abstract This paper describes aspects of a strong moisture surge over the Gulf of California that was observed during the 2004 North American Monsoon Experiment. Although a variety of special observation platforms aid the analyses, the authors focus on observations collected during two NOAA research aircraft flights made on 12 and 13 July. These flights sampled the initial and mature phases of a strong surge associated with Tropical Storm Blas. The first flight is identified by both a convective outflow and another feature, both deeper and with larger spatial scale, ahead of the outflow in association with the surge’s leading edge. The surge speed, ~18 m s−1, was identified from anomaly analysis of surface station pressure data. Observations show interesting multiscale features associated with the surge during its initial stages but do not allow for unambiguous identification of the surge’s forcing mechanism or dynamical properties. Data from the second flight were used to describe the along- and cross-gu...

A Climatology of the Vertical Structure of Water Vapor Transport to the Sierra Nevada in Cool Season Atmospheric River Precipitation Events

AbstractThis study presents the climatology of the vertical structure of water vapor flux above the Sierra Nevada during significant cool season (November–April) precipitation events. Atmospheric river (AR) and non-AR events are analyzed to better understand the effect of this structure on precipitation patterns. Daily measurements of cool season precipitation at seven weather stations around the Tahoe basin from 1974 to 2012 and NCEP/CPC gridded daily precipitation analysis along the Sierra crest for the period 1948–2012 are examined. NCEP–NCAR reanalysis and soundings from Oakland are used to look at upper atmospheric conditions, including the presence of vapor transport by low- and midlevel jets on storm days as well as upstream static stability in relation to significant precipitation events. Key findings are as follows: 1) ARs play a disproportionately large role in generating Tahoe basin precipitation during the cool season; 2) strong midlevel vapor transport needs to occur in tandem with low-level ...

Worldwide Marine Fog Occurrence and Climatology

Herein, an analysis is presented of the world’s marine fog distribution based upon the International Comprehensive Ocean-atmosphere Data Set (ICOADS) ship observations taken during 1950–2007. Fog, shallow fog, and mist are taken from routine weather reports that are encoded in an ICOADS ship observation with the “present weather” code. Occurrence is estimated by the number of observations of a type divided by the total present weather observations in a one-degree area. The bulk of the observations are in the northern temperate and tropical oceans, with decreasing numbers south of 20 °S and large data voids in the polar oceans. Marine fog is infrequent over most of the world’s oceans with the median occurrence 0.2 % while it is in isolated maxima for values larger than about 2 %. In a specific location, either fog or mist are the most frequent, followed with an order of magnitude lower occurrence by shallow fog.

WRF downscaling improves ERA-Interim representation of precipitation around a tropical Andean valley during El Niño: implications for GCM-scale simulation of precipitation over complex terrain

Precipitation in the tropical Andes is strongly influenced by the ENSO phases and orographic effects. In particular, precipitation can be drastically reduced during El Niño. Decision-making about water resources relies on modelling precipitation as the main source for water availability. Here we evaluate ERA-Interim´s capacity to represent precipitation in the mountainous central Colombian Andes, a strategic region for water supply and hydropower generation, for different phases of ENSO during 1998–2012. Our results show that ERA-Interim fails to reproduce important features of precipitation spatial and temporal variability during different ENSO phases. Most critical in these results is how ERA-Interim overestimates precipitation during the dry season in El Niño years, which corresponds to the most critical condition for water supply. We show that ERA-Interim limitations are likely related to its simplified representation of the complex topography in the region, which excludes the inter-Andean Cauca river valley. To improve this, we implement a dynamical downscaling experiment using the WRF regional climate model, including a sensitivity analysis that considers three convective parameterization schemes and a convection-permitting simulation. WRF downscaling outperforms ERA-Interim in the representation of precipitation during the dry season of El Niño years, especially through correcting positive precipitation biases. This improvement is related to a better representation of orographic effects in WRF simulations. Our results suggest that ERA-Interim and, more generally, climate simulations with comparable coarse resolutions, may produce misleading precipitation overestimations in the tropical Andes if they do not adequately represent inter-Andean valleys, with important implications for water resources management.

Climatology and Teleconnections of Mesoscale Convective Systems in an Andean Basin in Southern Ecuador: The Case of the Paute Basin

Mesoscale convective systems (MCSs) climatology, the thermodynamic and dynamical variables, and teleconnections influencing MCSs development are assessed for the Paute basin (PB) in the Ecuadorian Andes from 2000 to 2009. The seasonality of MCSs occurrence shows a bimodal pattern, with higher occurrence during March-April (MA) and October-November (ON), analogous to the regional rainfall seasonality. The diurnal cycle of MCSs shows a clear nocturnal occurrence, especially during the MA and ON periods. Interestingly, despite the higher occurrence of MCSs during the rainy seasons, the monthly size relative frequency remains fairly constant throughout the year. On the east of the PB, the persistent high convective available potential and low convective inhibition values from midday to nighttime are likely related to the nocturnal development of the MCSs. A significant positive correlation between the MCSs occurrence to the west of the PB and the Trans-Nino index was found, suggesting that ENSO is an important source of interannual variability of MCSs frequency with increasing development of MCSs during warm ENSO phases. On the east of the PB, the variability of MCSs is positively correlated to the tropical Atlantic sea surface temperature anomalies south of the equator, due to the variability of the Atlantic subtropical anticyclone, showing main departures from this relation when anomalous conditions occur in the tropical Pacific due to ENSO.

Developing MODIS-based cloud climatologies to aid species distribution modeling and conservation activities

Author(s): Douglas, Michael William; Beida, Rahama; Mejia, John F.; Fuentes, Marcia Vetromilla | Abstract: WorldClim (Hijmans et al. 2005) has been the de-facto source of basic climatological analyses for most species distribution modeling research and conservation science applications because of its global coverage and fine (l1 km) spatial resolution. However, it has been recognized since its development that there are limitations in data-poor regions, especially with regard to the precipitation analyses. Here we describe procedures to develop a satellite-based daytime cloudiness climatology that better reflects the variations in vegetation cover in many regions of the globe than do the WorldClim precipitation products. Moderate Resolution Imaging Spectroradiometer (MODIS) imagery from the National Aeronautics and Space Administration (NASA) Terra and Aqua sun-synchronous satellites have recently been used to develop multi-year climatologies of cloudiness. Several procedures exist for developing such climatologies. We first discuss a simple procedure that uses brightness thresholds to identify clouds. We compare these results with those from a more complex procedure: the MODIS Cloud Mask product, recently averaged into climatological products by Wilson and Jetz (2016). We discuss advantages and limitations of both approaches. We also speculate on further work that will be needed to improve the usefulness of these MODIS-based climatologies of cloudiness. Despite limitations of current MODIS-based climatology products, they have the potential to greatly improve our understanding of the distribution of biota across the globe. We show examples from oceanic islands and arid coastlines in the subtropics and tropics where the MODIS products should be of special value in predicting the observed vegetation cover. Some important applications of reliable climatologies based on MODIS imagery products will include 1) helping to restore long-degraded cloud-impacted environments; 2) improving estimations of the spatial distribution of cloud-impacted species; and 3) helping to identify areas for rapid biological assessments. The last application can even benefit from qualitative perusal of the current MODIS climatologies.