
mm/hr) and light (≥1mm/hr) rainfall events at 1-6 hr lead
times by 5.7%, compared to DeepRaNE. For reproducibility,
we made the source code used in the paper publicly available
at https://github.com/jihoonko/ASOC.
In Section II, we briefly review related studies. In Sec-
tion III, we introduce the notations used in this paper and
define the precipitation nowcasting problem. In Section IV, we
present ASOC and ASOC+. In Section V, we provide experi-
mental results. Lastly, in Section VI, we provide conclusions.
II. RELATED WORK
In the machine-learning literature, precipitation nowcasting
is often formulated as pixel-wise classification of precipitation
levels in the near future from input radar-reflectivity images,
and satellite images are often used additionally as inputs.
Among convolutional neural networks (CNNs), U-Net [11]
has been widely used for precipitation nowcasting [3], [4], [6],
[16]. U-Net was originally designed for an image segmentation
task, i.e., pixel-wise classification. For example, Lebedev et al.
[6] used U-Net for precipitation detection, which is formulated
as a pixel-wise binary-classification problem. Agrawal et al.
[4] divided precipitation levels into four classes and used U-
Net for pixel-wise multiclass classification. Based on a similar
multiclass classification formulation, Ko et al. [3] proposed
training strategies for precipitation nowcasting (spec., a pre-
training scheme and a loss function) and demonstrated their
effectiveness using a U-Net-based model.
Moreover, in order to aggregate both spatial and temporal
information, there have been several attempts to combine
recurrent neural networks (RNNs) (e.g., LSTM [14]) into
CNNs [1], [2], [5]. For example, Shi et al. [1] proposed
ConvLSTM, which has convolutional structures in the input-
to-state and state-to-state transitions in LSTM. Shi et al. [2]
extended ConvLSTM to TrajGRU, which can learn location-
variant connections between RNN layers. Sønderby et al. [5]
proposed MetNet, which uses ConvLSTM as its temporal
encoder and adapts axial attention structure for its spatial
encoder. Ravuri et al. [16] pointed out that deep-learning
based approaches tend to provide blurry predictions, especially
at long lead times, and they used a conditional generative
adversarial network [17] consists of ConvGRU-based [18]
generator and the spatial and temporal discriminators to ad-
dress this limitation. Espeholt et al. [9] extended ConvLSTM
with exponentially dilated convolution blocks, which enhance
expressive power by capturing additional spatial information.
Several studies utilized meteorological observations from
multiple weather stations as inputs to predict weather-related
events. For example, Seo et al. [19] considered temperature
forecasting. They generated a graph, where each node corre-
sponds to a weather station, and inferred the data quality of
each station, during training, by applying the graph convo-
lutional network (GCN) to the generated graph. Wang et al.
[20] focused on short-term intense precipitation (SIP) now-
casting. They generated a graph and its feature, by identifying
and clustering convective cells from radar-reflectivity images,
and used them, together with ground-based observations, as
TABLE I
FREQUENTLY USED NOTATION.
Notation Description
ttime (unit: minutes)
R(t)
x∈Rradar reflectivity at time tin each region x(unit: dbZ)
R(t)radar reflectivity image at time tin all regions
Iset of regions where ground weather stations are located
O(t)
x∈Rdground-based observations at time tin each region x
O(t)ground-based observations at time tin all regions
C(t)
xground-truth precipitation class at time tin each region x
ˆ
C(t)
xpredicted probability distribution over
precipitation classes at time tin each region x
the inputs of a random forest classifier. In contrast to our
deep-learning-based approach, they did not employ any deep-
learning techniques to process radar images and ground-based
observations together.
III. BASIC NOTATIONS & PROBLEM DEFINITION
In this section, we introduce basic notations and formulate
the precipitation nowcasting problem.
A. Basic Notations
The frequently-used symbols are listed in Table I. We use
R(t)
x∈Rto indicate the radar reflectivity in dBZ at time
tin each region x, and we use R(t)to indicate the whole
radar-reflectivity image at time t. We use Ito denote the set
of regions where ground weather stations are located. Then,
O(t)
x∈Rddenotes the ground-based observations in each
region x∈Iat time t, and O(t)denotes the ground-based
observations at time tfrom all regions in I. Lastly, C(t)
x
indicates the ground-truth precipitation class (see the following
subsection for precipitation classes) in each region x∈Iat
time t, and ˆ
C(t)
xindicates the predicted probability distribution
over all precipitation classes for each region xat time t.
B. Problem Definition
The goal of precipitation nowcasting is to predict precipita-
tion levels and locations at very short lead times. In this paper,
we formulate the problem as a location-wise classification
problem, as in [3]. Specifically, we split precipitation levels
into three classes: (a) HEAVY for precipitation at least 10
mm/hr, (b) LIGHT for precipitation at least 1 mm/hr but
less than 10 mm/hr, and (c) OTHERS for precipitation less
than 1 mm/hr. We also frequently use a combined class
named RAIN (=HEAVY+LIGHT) for precipitation at least 1
mm/hr. As inputs, we use ground-based observations collected
from multiple weather stations and radar-reflectivity images
collected for an hour. We assume that both are collected
every 10 minutes. For example, if we perform prediction at
time tin minutes, the inputs are (a) seven radar reflectivity
images at times {t−60, t −50,· · · , t}, i.e., R(t−60),R(t−50),
· · · ,R(t), and (b) seven snapshots of ground observations at
times {t−60, t −50,· · · , t}, i.e., O(t−60), O(t−50),· · · , O(t).