Physical controls on frost events in the central Andes of Peru using in situ observations and energy flux models
Introduction
The Mantaro valley in the Peruvian Andes is arguably the main agricultural region in the Peruvian Andes and is the main provider of produce of Lima, the Peruvian capital. However, the agriculture is extensive, largely dependent on rainfall, and is particularly vulnerable to frosts, which can destroy the crops and are, therefore, considered the most damaging among extreme hydrometeorological events (Trasmonte et al., 2008, Trasmonte, 2009, Núñez et al., 2012). As reported by Garcilaso de la Vega (1609) in his “Comentarios Reales”, the Inca farmers predicted frosts when they observed cloudless skies at night and produced smoke as a way to mitigate the frost damage to the crops. These prediction and mitigation practices persist in rural communities in the Mantaro valley (Martínez et al., 2012), where the absence of late afternoon cloudiness has been verified to increase the probability of freezing temperatures by more than four times relative to cloudy skies (Saavedra, 2012).
The strong relation between cloudiness and minimum temperature indicates that the net radiative loss in the nocturnal surface energy balance is a key process for the occurrence of frosts in the Andes (Lhomme et al., 2007, Sanabria, 2009) and different methods have been tested to mitigate the impacts on the crops (Morlon, 1991, Lhomme and Vacher, 2002). Sicart et al. (2010) indicate that clouds increase downward longwave by up to 55% near the Zongo glacier in Bolivia. Furthermore, low clouds are expected to emit more downward longwave radiation (e.g. Geiger et al., 2003, Dai et al., 1999). Using world-wide station data, Dai et al. (1999) concluded that the land diurnal temperature range (DTR), i.e. the difference between the maximum and minimum temperature, can be reduced in 25–50% by the presence of clouds and that their spatial distribution determines that of DTR. They found a strong correlation between minimum temperature and longwave radiation, and at the same time with specific humidity, but with cloud cover the correlation was weak. On the other hand, Huang et al. (2006) used models to estimate that a cloud radiative forcing of 8.5 W m−2 is associated with an increase of 0.68 °C in nocturnal temperature. Studies developed in the Andes of Peru are scant. Villegas (1991) indicate that frost in Mantaro basin, in the Andes of Peru, is associated with clear skies and dry air. The occurrence of frost in this region is mainly between April and August, and the lowest temperatures during June and July (Instituto Geofísico del Perú, 2005).
Sanabria (2009) made use of two models (Lhomme and Guilioni, 2004, Cellier, 1993) built taking into account physical processes to model minimum temperatures. Sanabria (2009) used longwave radiation and empirical methods (Swinbank, 1963, Brutsaert, 1975), but based on their results, they recommended that measurements be made with pyrgeometers. Sicart et al. (2010) implemented a parameterization of this longwave radiation in tropical mountains using surface air temperature and vapor pressure, as well as solar radiation, to estimate the cloud effects. Although clouds are arguably the primary control on longwave radiation, the air humidity and temperature also play a role in the clear-sky emission and are key parameters used in empirical models (e.g. Lhomme et al., 2007, Sicart et al., 2010). Another physical variable that affects the minimum temperature is the soil thermal conductivity, which allows thermal exchange between the surface and the deeper layers, damping the temperature changes at the surface, and increases with the soil moisture, which implies that precipitation in previous days can prevent the occurrence of a frost event (e.g. Geiger et al., 2003).
To better understand and quantify the roles of different atmospheric variables that control the minimum temperature at a site in the Mantaro valley in the central Andes of Peru, in this study we used a combination of in situ observations with numerical 1D models. We first provide a description and climatic characterization of the region of study. Then, we present an empirical analysis of the relationship between the minimum temperatures and variables such as specific humidity, cloud cover and precipitation using long-term meteorological data. After this, we use a soil heat diffusion model and an atmospheric radiative transfer model to model the minimum temperatures at the surface and LW↓, respectively, validated with data from a field experiment. Experiments with the models are then used to assess the sensitivity of the minimum temperature and LW↓ to changes in other atmospheric variables.
Section snippets
Study area and data
All the measurements in this study were made in the Huancayo Observatory of the Geophysical Institute of Peru (IGP Observatory, 12.04°S, 75.32°W, 3350 m.a.s.l.), which is surrounded by non-irrigated agricultural land in the Mantaro valley, in the central Andes of Peru, located 12 km from the city of Huancayo and 7 km from the Mantaro river (Fig. 1), between the western Andes and the Huaytapallana Cordillera to the east, between the Pacific ocean and the Amazon. In this research, we used three
Climatology in the IGP observatory
In this section we report the thirty-three year (1973–2006) climatology for the IGP Observatory. The seasonal cycle of precipitation (Fig. 2a) is primarily uni-modal with a wet season from September to March (austral spring-fall) and a dry season between May and July (austral fall-winter), with values below 0.5 mm d−1. The onset of the rainy season starts in August and reaches an initial peak in early October of around 2.3 mm d−1, after which it decreases slightly and starts increasing from the end
Case studies
We have selected four cases of frost events that produced important damage to crops in the region: January 17, 2005, November 23, 2005, October 15, 2006, and February 17, 2007 (Trasmonte, 2009). These occurred within the frost-unfavorable wet and warm season, between October and February, and therefore they were exceptional events. In order to learn what the conditions were before and during the frosts, we analyzed the specific humidity and air temperature at 2 m, cloud type and cloud cover, and
Heat diffusion/energy balance modeling of Tmin
We have argued in Section 4.3 that the amplitude and propagation of the nocturnal thermal wave matches well the theoretical prediction of a diurnally-forced model of heat diffusion in homogeneous soil. Here we implement a numerical version of the ground heat diffusion model in which the surface energy balance is given by the forcing by LW↓, the response of LW↑ and heat diffusion into the ground. We then test the sensitivity of Tmin to changes in LW↓, initial conditions in temperature, θ, and
Conclusions
Radiative frosts are one of the main hazards for agriculture in the tropical Andes of Peru but they are poorly understood from a physical perspective. This study was focused on characterizing and understanding the physical controls on the minimum temperature in this region, combining case studies, field measurements, statistical analysis, and theoretical and numerical modeling.
Firstly, we characterized the climatology in the IGP Observatory in the central Andes of Peru (3330 m.a.s.l.), focusing
Acknowledgements
This work was supported by the project “Enhancing Resilience of Rural Communities to Drought, Floods and Frost in the Mantaro Valley, Peru (MAREMEX Mantaro)” (IDRC 105567).
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